* For review: user_namespace(7) man page
@ 2014-08-20 23:36 Michael Kerrisk (man-pages)
2014-08-22 21:12 ` Serge E. Hallyn
` (2 more replies)
0 siblings, 3 replies; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-08-20 23:36 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
[-- Attachment #1: Type: text/plain, Size: 31172 bytes --]
Hello Eric et al.,
For various reasons, my work on the namespaces man pages
fell off the table a while back. Nevertheless, the pages have
been close to completion for a while now, and I recently restarted,
in an effort to finish them. As you also noted to me f2f, there have
been recently been some small namespace changes that you may affect
the content of the pages. Therefore, I'll take the opportunity to
send the namespace-related pages out for further (final?) review.
So, here, I start with the user_namespaces(7) page, which is shown
in rendered form below, with source attached to this mail. I'll
send various other pages in follow-on mails.
Review comments/suggestions for improvements / bug fixes welcome.
Cheers,
Michael
==
NAME
user_namespaces - overview of Linux user_namespaces
DESCRIPTION
For an overview of namespaces, see namespaces(7).
User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐
tials(7), the root directory, keys (see keyctl(2)), and capabili‐
ties (see capabilities(7)). A process's user and group IDs can
be different inside and outside a user namespace. In particular,
a process can have a normal unprivileged user ID outside a user
namespace while at the same time having a user ID of 0 inside the
namespace; in other words, the process has full privileges for
operations inside the user namespace, but is unprivileged for
operations outside the namespace.
Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace—
except the initial ("root") namespace—has a parent user names‐
pace, and can have zero or more child user namespaces. The par‐
ent user namespace is the user namespace of the process that cre‐
ates the user namespace via a call to unshare(2) or clone(2) with
the CLONE_NEWUSER flag.
The kernel imposes (since version 3.11) a limit of 32 nested lev‐
els of user namespaces. Calls to unshare(2) or clone(2) that
would cause this limit to be exceeded fail with the error EUSERS.
Each process is a member of exactly one user namespace. A
process created via fork(2) or clone(2) without the CLONE_NEWUSER
flag is a member of the same user namespace as its parent. A
process can join another user namespace with setns(2) if it has
the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
full set of capabilities in that namespace.
A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
makes the new child process (for clone(2)) or the caller (for
unshare(2)) a member of the new user namespace created by the
call.
Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag
starts out with a complete set of capabilities in the new user
namespace. Likewise, a process that creates a new user namespace
using unshare(2) or joins an existing user namespace using
setns(2) gains a full set of capabilities in that namespace. On
the other hand, that process has no capabilities in the parent
(in the case of clone(2)) or previous (in the case of unshare(2)
and setns(2)) user namespace, even if the new namespace is cre‐
ated or joined by the root user (i.e., a process with user ID 0
in the root namespace).
Note that a call to execve(2) will cause a process to lose any
capabilities that it has, unless it has a user ID of 0 within the
namespace. See the discussion of user and group ID mappings,
below.
A call to clone(2), unshare(2), or setns(2) using the
CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐
ties(7)) to their default values (all flags disabled) in the
child (for clone(2)) or caller (for unshare(2), or setns(2)).
Note that because the caller no longer has capabilities in its
original user namespace after a call to setns(2), it is not pos‐
sible for a process to reset its "securebits" flags while retain‐
ing its user namespace membership by using a pair of setns(2)
calls to move to another user namespace and then return to its
original user namespace.
Having a capability inside a user namespace permits a process to
perform operations (that require privilege) only on resources
governed by that namespace. The rules for determining whether or
not a process has a capability in a particular user namespace are
as follows:
1. A process has a capability inside a user namespace if it is a
member of that namespace and it has the capability in its
effective capability set. A process can gain capabilities in
its effective capability set in various ways. For example, it
may execute a set-user-ID program or an executable with asso‐
ciated file capabilities. In addition, a process may gain
capabilities via the effect of clone(2), unshare(2), or
setns(2), as already described.
2. If a process has a capability in a user namespace, then it has
that capability in all child (and further removed descendant)
namespaces as well.
3. When a user namespace is created, the kernel records the
effective user ID of the creating process as being the "owner"
of the namespace. A process that resides in the parent of the
user namespace and whose effective user ID matches the owner
of the namespace has all capabilities in the namespace. By
virtue of the previous rule, this means that the process has
all capabilities in all further removed descendant user names‐
paces as well.
Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user
namespaces, and mount, PID, IPC, network, and UTS namespaces can
be created with just the CAP_SYS_ADMIN capability in the caller's
user namespace.
If CLONE_NEWUSER is specified along with other CLONE_NEW* flags
in a single clone(2) or unshare(2) call, the user namespace is
guaranteed to be created first, giving the child (clone(2)) or
caller (unshare(2)) privileges over the remaining namespaces cre‐
ated by the call. Thus, it is possible for an unprivileged call‐
er to specify this combination of flags.
When a new IPC, mount, network, PID, or UTS namespace is created
via clone(2) or unshare(2), the kernel records the user namespace
of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on
global resources isolated by the namespace, the permission checks
are performed according to the process's capabilities in the user
namespace that the kernel associated with the new namespace.
User and group ID mappings: uid_map and gid_map
When a user namespace is created, it starts out without a mapping
of user IDs (group IDs) to the parent user namespace. The
/proc/[pid]/uid_map and /proc/[pid]/gid_map files (available
since Linux 3.5) expose the mappings for user and group IDs
inside the user namespace for the process pid. These files can
be read to view the mappings in a user namespace and written to
(once) to define the mappings.
The description in the following paragraphs explains the details
for uid_map; gid_map is exactly the same, but each instance of
"user ID" is replaced by "group ID".
The uid_map file exposes the mapping of user IDs from the user
namespace of the process pid to the user namespace of the process
that opened uid_map (but see a qualification to this point
below). In other words, processes that are in different user
namespaces will potentially see different values when reading
from a particular uid_map file, depending on the user ID mappings
for the user namespaces of the reading processes.
Each line in the uid_map file specifies a 1-to-1 mapping of a
range of contiguous user IDs between two user namespaces. (When
a user namespace is first created, this file is empty.) The
specification in each line takes the form of three numbers delim‐
ited by white space. The first two numbers specify the starting
user ID in each of the two user namespaces. The third number
specifies the length of the mapped range. In detail, the fields
are interpreted as follows:
(1) The start of the range of user IDs in the user namespace of
the process pid.
(2) The start of the range of user IDs to which the user IDs
specified by field one map. How field two is interpreted
depends on whether the process that opened uid_map and the
process pid are in the same user namespace, as follows:
a) If the two processes are in different user namespaces:
field two is the start of a range of user IDs in the user
namespace of the process that opened uid_map.
b) If the two processes are in the same user namespace: field
two is the start of the range of user IDs in the parent
user namespace of the process pid. This case enables the
opener of uid_map (the common case here is opening
/proc/self/uid_map) to see the mapping of user IDs into
the user namespace of the process that created this user
namespace.
(3) The length of the range of user IDs that is mapped between
the two user namespaces.
System calls that return user IDs (group IDs)—for example,
getuid(2), getgid(2), and the credential fields in the structure
returned by stat(2)—return the user ID (group ID) mapped into the
caller's user namespace.
When a process accesses a file, its user and group IDs are mapped
into the initial user namespace for the purpose of permission
checking and assigning IDs when creating a file. When a process
retrieves file user and group IDs via stat(2), the IDs are mapped
in the opposite direction, to produce values relative to the
process user and group ID mappings.
The initial user namespace has no parent namespace, but, for con‐
sistency, the kernel provides dummy user and group ID mapping
files for this namespace. Looking at the uid_map file (gid_map
is the same) from a shell in the initial namespace shows:
$ cat /proc/$$/uid_map
0 0 4294967295
This mapping tells us that the range starting at user ID 0 in
this namespace maps to a range starting at 0 in the (nonexistent)
parent namespace, and the length of the range is the largest
32-bit unsigned integer.
Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of
one of the processes in the namespace may be written to once to
define the mapping of user IDs in the new user namespace. An
attempt to write more than once to a uid_map file in a user
namespace fails with the error EPERM. Similar rules apply for
gid_map files.
The lines written to uid_map (gid_map) must conform to the fol‐
lowing rules:
* The three fields must be valid numbers, and the last field
must be greater than 0.
* Lines are terminated by newline characters.
* There is an (arbitrary) limit on the number of lines in the
file. As at Linux 3.8, the limit is five lines. In addition,
the number of bytes written to the file must be less than the
system page size, and the write must be performed at the start
of the file (i.e., lseek(2) and pwrite(2) can't be used to
write to nonzero offsets in the file).
* The range of user IDs (group IDs) specified in each line can‐
not overlap with the ranges in any other lines. In the ini‐
tial implementation (Linux 3.8), this requirement was satis‐
fied by a simplistic implementation that imposed the further
requirement that the values in both field 1 and field 2 of
successive lines must be in ascending numerical order, which
prevented some otherwise valid maps from being created. Linux
3.9 and later fix this limitation, allowing any valid set of
nonoverlapping maps.
* At least one line must be written to the file.
Writes that violate the above rules fail with the error EINVAL.
In order for a process to write to the /proc/[pid]/uid_map
(/proc/[pid]/gid_map) file, all of the following requirements
must be met:
1. The writing process must have the CAP_SETUID (CAP_SETGID)
capability in the user namespace of the process pid.
2. The writing process must be in either the user namespace of
the process pid or inside the parent user namespace of the
process pid.
3. The mapped user IDs (group IDs) must in turn have a mapping in
the parent user namespace.
4. One of the following is true:
* The data written to uid_map (gid_map) consists of a single
line that maps the writing process's filesystem user ID
(group ID) in the parent user namespace to a user ID (group
ID) in the user namespace. The usual case here is that
this single line provides a mapping for user ID of the
process that created the namespace.
* The process has the CAP_SETUID (CAP_SETGID) capability in
the parent user namespace. Thus, a privileged process can
make mappings to arbitrary user IDs (group IDs) in the par‐
ent user namespace.
Writes that violate the above rules fail with the error EPERM.
Unmapped user and group IDs
There are various places where an unmapped user ID (group ID) may
be exposed to user space. For example, the first process in a
new user namespace may call getuid() before a user ID mapping has
been defined for the namespace. In most such cases, an unmapped
user ID is converted to the overflow user ID (group ID); the
default value for the overflow user ID (group ID) is 65534. See
the descriptions of /proc/sys/kernel/overflowuid and
/proc/sys/kernel/overflowgid in proc(5).
The cases where unmapped IDs are mapped in this fashion include
system calls that return user IDs (getuid(2) getgid(2), and simi‐
lar), credentials passed over a UNIX domain socket, credentials
returned by stat(2), waitid(2), and the System V IPC "ctl"
IPC_STAT operations, credentials exposed by /proc/PID/status and
the files in /proc/sysvipc/*, credentials returned via the si_uid
field in the siginfo_t received with a signal (see sigaction(2)),
credentials written to the process accounting file (see acct(5)),
and credentials returned with POSIX message queue notifications
(see mq_notify(3)).
There is one notable case where unmapped user and group IDs are
not converted to the corresponding overflow ID value. When view‐
ing a uid_map or gid_map file in which there is no mapping for
the second field, that field is displayed as 4294967295 (-1 as an
unsigned integer);
Set-user-ID and set-group-ID programs
When a process inside a user namespace executes a set-user-ID
(set-group-ID) program, the process's effective user (group) ID
inside the namespace is changed to whatever value is mapped for
the user (group) ID of the file. However, if either the user or
the group ID of the file has no mapping inside the namespace, the
set-user-ID (set-group-ID) bit is silently ignored: the new pro‐
gram is executed, but the process's effective user (group) ID is
left unchanged. (This mirrors the semantics of executing a set-
user-ID or set-group-ID program that resides on a filesystem that
was mounted with the MS_NOSUID flag, as described in mount(2).)
Miscellaneous
When a process's user and group IDs are passed over a UNIX domain
socket to a process in a different user namespace (see the
description of SCM_CREDENTIALS in unix(7)), they are translated
into the corresponding values as per the receiving process's user
and group ID mappings.
CONFORMING TO
Namespaces are a Linux-specific feature.
NOTES
Over the years, there have been a lot of features that have been
added to the Linux kernel that have been made available only to
privileged users because of their potential to confuse set-user-
ID-root applications. In general, it becomes safe to allow the
root user in a user namespace to use those features because it is
impossible, while in a user namespace, to gain more privilege
than the root user of a user namespace has.
Availability
Use of user namespaces requires a kernel that is configured with
the CONFIG_USER_NS option. User namespaces require support in a
range of subsystems across the kernel. When an unsupported sub‐
system is configured into the kernel, it is not possible to con‐
figure user namespaces support.
As at Linux 3.8, most relevant subsystems supported user names‐
paces, but a number of filesystems did not have the infrastruc‐
ture needed to map user and group IDs between user namespaces.
Linux 3.9 added the required infrastructure support for many of
the remaining unsupported filesystems (Plan 9 (9P), Andrew File
System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2). Linux 3.11
added support the last of the unsupported major filesystems, XFS.
EXAMPLE
The program below is designed to allow experimenting with user
namespaces, as well as other types of namespaces. It creates
namespaces as specified by command-line options and then executes
a command inside those namespaces. The comments and usage()
function inside the program provide a full explanation of the
program. The following shell session demonstrates its use.
First, we look at the run-time environment:
$ uname -rs # Need Linux 3.8 or later
Linux 3.8.0
$ id -u # Running as unprivileged user
1000
$ id -g
1000
Now start a new shell in new user (-U), mount (-m), and PID (-p)
namespaces, with user ID (-M) and group ID (-G) 1000 mapped to 0
inside the user namespace:
$ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
The shell has PID 1, because it is the first process in the new
PID namespace:
bash$ echo $$
1
Inside the user namespace, the shell has user and group ID 0, and
a full set of permitted and effective capabilities:
bash$ cat /proc/$$/status | egrep '^[UG]id'
Uid: 0 0 0 0
Gid: 0 0 0 0
bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff
Mounting a new /proc filesystem and listing all of the processes
visible in the new PID namespace shows that the shell can't see
any processes outside the PID namespace:
bash$ mount -t proc proc /proc
bash$ ps ax
PID TTY STAT TIME COMMAND
1 pts/3 S 0:00 bash
22 pts/3 R+ 0:00 ps ax
Program source
/* userns_child_exec.c
Licensed under GNU General Public License v2 or later
Create a child process that executes a shell command in new
namespace(s); allow UID and GID mappings to be specified when
creating a user namespace.
*/
#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <errno.h>
/* A simple error-handling function: print an error message based
on the value in 'errno' and terminate the calling process */
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
struct child_args {
char **argv; /* Command to be executed by child, with args */
int pipe_fd[2]; /* Pipe used to synchronize parent and child */
};
static int verbose;
static void
usage(char *pname)
{
fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
fprintf(stderr, "Create a child process that executes a shell "
"command in a new user namespace,\n"
"and possibly also other new namespace(s).\n\n");
fprintf(stderr, "Options can be:\n\n");
#define fpe(str) fprintf(stderr, " %s", str);
fpe("-i New IPC namespace\n");
fpe("-m New mount namespace\n");
fpe("-n New network namespace\n");
fpe("-p New PID namespace\n");
fpe("-u New UTS namespace\n");
fpe("-U New user namespace\n");
fpe("-M uid_map Specify UID map for user namespace\n");
fpe("-G gid_map Specify GID map for user namespace\n");
fpe("-z Map user's UID and GID to 0 in user namespace\n");
fpe(" (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
fpe("-v Display verbose messages\n");
fpe("\n");
fpe("If -z, -M, or -G is specified, -U is required.\n");
fpe("It is not permitted to specify both -z and either -M or -G.\n");
fpe("\n");
fpe("Map strings for -M and -G consist of records of the form:\n");
fpe("\n");
fpe(" ID-inside-ns ID-outside-ns len\n");
fpe("\n");
fpe("A map string can contain multiple records, separated"
" by commas;\n");
fpe("the commas are replaced by newlines before writing"
" to map files.\n");
exit(EXIT_FAILURE);
}
/* Update the mapping file 'map_file', with the value provided in
'mapping', a string that defines a UID or GID mapping. A UID or
GID mapping consists of one or more newline-delimited records
of the form:
ID_inside-ns ID-outside-ns length
Requiring the user to supply a string that contains newlines is
of course inconvenient for command-line use. Thus, we permit the
use of commas to delimit records in this string, and replace them
with newlines before writing the string to the file. */
static void
update_map(char *mapping, char *map_file)
{
int fd, j;
size_t map_len; /* Length of 'mapping' */
/* Replace commas in mapping string with newlines */
map_len = strlen(mapping);
for (j = 0; j < map_len; j++)
if (mapping[j] == ',')
mapping[j] = '\n';
fd = open(map_file, O_RDWR);
if (fd == -1) {
fprintf(stderr, "ERROR: open %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
if (write(fd, mapping, map_len) != map_len) {
fprintf(stderr, "ERROR: write %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
close(fd);
}
static int /* Start function for cloned child */
childFunc(void *arg)
{
struct child_args *args = (struct child_args *) arg;
char ch;
/* Wait until the parent has updated the UID and GID mappings.
See the comment in main(). We wait for end of file on a
pipe that will be closed by the parent process once it has
updated the mappings. */
close(args->pipe_fd[1]); /* Close our descriptor for the write
end of the pipe so that we see EOF
when parent closes its descriptor */
if (read(args->pipe_fd[0], &ch, 1) != 0) {
fprintf(stderr,
"Failure in child: read from pipe returned != 0\n");
exit(EXIT_FAILURE);
}
/* Execute a shell command */
printf("About to exec %s\n", args->argv[0]);
execvp(args->argv[0], args->argv);
errExit("execvp");
}
#define STACK_SIZE (1024 * 1024)
static char child_stack[STACK_SIZE]; /* Space for child's stack */
int
main(int argc, char *argv[])
{
int flags, opt, map_zero;
pid_t child_pid;
struct child_args args;
char *uid_map, *gid_map;
const int MAP_BUF_SIZE = 100;
char map_buf[MAP_BUF_SIZE];
char map_path[PATH_MAX];
/* Parse command-line options. The initial '+' character in
the final getopt() argument prevents GNU-style permutation
of command-line options. That's useful, since sometimes
the 'command' to be executed by this program itself
has command-line options. We don't want getopt() to treat
those as options to this program. */
flags = 0;
verbose = 0;
gid_map = NULL;
uid_map = NULL;
map_zero = 0;
while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
switch (opt) {
case 'i': flags |= CLONE_NEWIPC; break;
case 'm': flags |= CLONE_NEWNS; break;
case 'n': flags |= CLONE_NEWNET; break;
case 'p': flags |= CLONE_NEWPID; break;
case 'u': flags |= CLONE_NEWUTS; break;
case 'v': verbose = 1; break;
case 'z': map_zero = 1; break;
case 'M': uid_map = optarg; break;
case 'G': gid_map = optarg; break;
case 'U': flags |= CLONE_NEWUSER; break;
default: usage(argv[0]);
}
}
/* -M or -G without -U is nonsensical */
if (((uid_map != NULL || gid_map != NULL || map_zero) &&
!(flags & CLONE_NEWUSER)) ||
(map_zero && (uid_map != NULL || gid_map != NULL)))
usage(argv[0]);
args.argv = &argv[optind];
/* We use a pipe to synchronize the parent and child, in order to
ensure that the parent sets the UID and GID maps before the child
calls execve(). This ensures that the child maintains its
capabilities during the execve() in the common case where we
want to map the child's effective user ID to 0 in the new user
namespace. Without this synchronization, the child would lose
its capabilities if it performed an execve() with nonzero
user IDs (see the capabilities(7) man page for details of the
transformation of a process's capabilities during execve()). */
if (pipe(args.pipe_fd) == -1)
errExit("pipe");
/* Create the child in new namespace(s) */
child_pid = clone(childFunc, child_stack + STACK_SIZE,
flags | SIGCHLD, &args);
if (child_pid == -1)
errExit("clone");
/* Parent falls through to here */
if (verbose)
printf("%s: PID of child created by clone() is %ld\n",
argv[0], (long) child_pid);
/* Update the UID and GID maps in the child */
if (uid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
(long) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid());
uid_map = map_buf;
}
update_map(uid_map, map_path);
}
if (gid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
(long) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid());
gid_map = map_buf;
}
update_map(gid_map, map_path);
}
/* Close the write end of the pipe, to signal to the child that we
have updated the UID and GID maps */
close(args.pipe_fd[1]);
if (waitpid(child_pid, NULL, 0) == -1) /* Wait for child */
errExit("waitpid");
if (verbose)
printf("%s: terminating\n", argv[0]);
exit(EXIT_SUCCESS);
}
SEE ALSO
newgidmap(1), newuidmap(1), clone(2), setns(2), unshare(2),
proc(5), subgid(5), subuid(5), credentials(7), capabilities(7),
namespaces(7), pid_namespaces(7)
The kernel source file Documentation/namespaces/resource-con‐
trol.txt.
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-08-20 23:36 For review: user_namespace(7) man page Michael Kerrisk (man-pages)
@ 2014-08-22 21:12 ` Serge E. Hallyn
2014-09-01 16:58 ` Michael Kerrisk (man-pages)
2014-08-30 21:53 ` Eric W. Biederman
2014-09-01 20:57 ` Andy Lutomirski
2 siblings, 1 reply; 26+ messages in thread
From: Serge E. Hallyn @ 2014-08-22 21:12 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: Eric W. Biederman, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
Quoting Michael Kerrisk (man-pages) (mtk.manpages@gmail.com):
> Hello Eric et al.,
>
> For various reasons, my work on the namespaces man pages
> fell off the table a while back. Nevertheless, the pages have
> been close to completion for a while now, and I recently restarted,
> in an effort to finish them. As you also noted to me f2f, there have
> been recently been some small namespace changes that you may affect
> the content of the pages. Therefore, I'll take the opportunity to
> send the namespace-related pages out for further (final?) review.
>
> So, here, I start with the user_namespaces(7) page, which is shown
> in rendered form below, with source attached to this mail. I'll
> send various other pages in follow-on mails.
>
> Review comments/suggestions for improvements / bug fixes welcome.
>
> Cheers,
>
> Michael
>
> ==
>
> NAME
> user_namespaces - overview of Linux user_namespaces
>
> DESCRIPTION
> For an overview of namespaces, see namespaces(7).
>
> User namespaces isolate security-related identifiers and
> attributes, in particular, user IDs and group IDs (see creden‐
> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
> ties (see capabilities(7)). A process's user and group IDs can
> be different inside and outside a user namespace. In particular,
> a process can have a normal unprivileged user ID outside a user
> namespace while at the same time having a user ID of 0 inside the
> namespace; in other words, the process has full privileges for
> operations inside the user namespace, but is unprivileged for
> operations outside the namespace.
>
> Nested namespaces, namespace membership
> User namespaces can be nested; that is, each user namespace—
> except the initial ("root") namespace—has a parent user names‐
> pace, and can have zero or more child user namespaces. The par‐
> ent user namespace is the user namespace of the process that cre‐
> ates the user namespace via a call to unshare(2) or clone(2) with
> the CLONE_NEWUSER flag.
>
> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
> els of user namespaces. Calls to unshare(2) or clone(2) that
> would cause this limit to be exceeded fail with the error EUSERS.
>
> Each process is a member of exactly one user namespace. A
> process created via fork(2) or clone(2) without the CLONE_NEWUSER
> flag is a member of the same user namespace as its parent. A
> process can join another user namespace with setns(2) if it has
> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
> full set of capabilities in that namespace.
>
> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
> makes the new child process (for clone(2)) or the caller (for
> unshare(2)) a member of the new user namespace created by the
> call.
>
> Capabilities
> The child process created by clone(2) with the CLONE_NEWUSER flag
> starts out with a complete set of capabilities in the new user
> namespace. Likewise, a process that creates a new user namespace
> using unshare(2) or joins an existing user namespace using
> setns(2) gains a full set of capabilities in that namespace. On
> the other hand, that process has no capabilities in the parent
> (in the case of clone(2)) or previous (in the case of unshare(2)
> and setns(2)) user namespace, even if the new namespace is cre‐
> ated or joined by the root user (i.e., a process with user ID 0
> in the root namespace).
>
> Note that a call to execve(2) will cause a process to lose any
> capabilities that it has, unless it has a user ID of 0 within the
> namespace. See the discussion of user and group ID mappings,
> below.
The above is an approximation, but a bit misleading. On exec, the task
capability set is recalculated according to the usual rules. So if the
file being executed has file capabilities, the result task may end up
with capabilities even if it is not root (even if it is uid -1).
Perhaps it should be phrased as:
Note that a call to execve(2) will cause a process' capabilities
to be recalculated (see capabilities(7)), so that usually, unless
it has a user ID of 0 within the namespace, it will lose all
capabilities. See the discussion of user and group ID mappings,
below.
-serge
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-08-20 23:36 For review: user_namespace(7) man page Michael Kerrisk (man-pages)
2014-08-22 21:12 ` Serge E. Hallyn
@ 2014-08-30 21:53 ` Eric W. Biederman
2014-09-01 17:31 ` Michael Kerrisk (man-pages)
` (2 more replies)
2014-09-01 20:57 ` Andy Lutomirski
2 siblings, 3 replies; 26+ messages in thread
From: Eric W. Biederman @ 2014-08-30 21:53 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: lkml, linux-man, containers, Andy Lutomirski, richard.weinberger,
Serge E. Hallyn
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> Hello Eric et al.,
>
> For various reasons, my work on the namespaces man pages
> fell off the table a while back. Nevertheless, the pages have
> been close to completion for a while now, and I recently restarted,
> in an effort to finish them. As you also noted to me f2f, there have
> been recently been some small namespace changes that you may affect
> the content of the pages. Therefore, I'll take the opportunity to
> send the namespace-related pages out for further (final?) review.
>
> So, here, I start with the user_namespaces(7) page, which is shown
> in rendered form below, with source attached to this mail. I'll
> send various other pages in follow-on mails.
>
> Review comments/suggestions for improvements / bug fixes welcome.
>
> Cheers,
>
> Michael
>
> ==
>
> NAME
> user_namespaces - overview of Linux user_namespaces
>
> DESCRIPTION
> For an overview of namespaces, see namespaces(7).
>
> User namespaces isolate security-related identifiers and
> attributes, in particular, user IDs and group IDs (see creden‐
> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
> ties (see capabilities(7)). A process's user and group IDs can
> be different inside and outside a user namespace. In particular,
> a process can have a normal unprivileged user ID outside a user
> namespace while at the same time having a user ID of 0 inside the
> namespace; in other words, the process has full privileges for
> operations inside the user namespace, but is unprivileged for
> operations outside the namespace.
>
> Nested namespaces, namespace membership
> User namespaces can be nested; that is, each user namespace—
> except the initial ("root") namespace—has a parent user names‐
> pace, and can have zero or more child user namespaces. The par‐
> ent user namespace is the user namespace of the process that cre‐
> ates the user namespace via a call to unshare(2) or clone(2) with
> the CLONE_NEWUSER flag.
>
> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
> els of user namespaces. Calls to unshare(2) or clone(2) that
> would cause this limit to be exceeded fail with the error EUSERS.
>
> Each process is a member of exactly one user namespace. A
> process created via fork(2) or clone(2) without the CLONE_NEWUSER
> flag is a member of the same user namespace as its parent.
> A
^ single-threaded
Because of chroot and other things multi-threaded processes are not
allowed to join a user namespace. For the documentation just saying
single-threaded sounds like enough here.
> process can join another user namespace with setns(2) if it has
> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
> full set of capabilities in that namespace.
>
> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
> makes the new child process (for clone(2)) or the caller (for
> unshare(2)) a member of the new user namespace created by the
> call.
>
> Capabilities
> The child process created by clone(2) with the CLONE_NEWUSER flag
> starts out with a complete set of capabilities in the new user
> namespace. Likewise, a process that creates a new user namespace
> using unshare(2) or joins an existing user namespace using
> setns(2) gains a full set of capabilities in that namespace. On
> the other hand, that process has no capabilities in the parent
> (in the case of clone(2)) or previous (in the case of unshare(2)
> and setns(2)) user namespace, even if the new namespace is cre‐
> ated or joined by the root user (i.e., a process with user ID 0
> in the root namespace).
>
> Note that a call to execve(2) will cause a process to lose any
> capabilities that it has, unless it has a user ID of 0 within the
> namespace. See the discussion of user and group ID mappings,
> below.
>
> A call to clone(2), unshare(2), or setns(2) using the
> CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐
> ties(7)) to their default values (all flags disabled) in the
> child (for clone(2)) or caller (for unshare(2), or setns(2)).
> Note that because the caller no longer has capabilities in its
> original user namespace after a call to setns(2), it is not pos‐
> sible for a process to reset its "securebits" flags while retain‐
> ing its user namespace membership by using a pair of setns(2)
> calls to move to another user namespace and then return to its
> original user namespace.
>
> Having a capability inside a user namespace permits a process to
> perform operations (that require privilege) only on resources
> governed by that namespace. The rules for determining whether or
> not a process has a capability in a particular user namespace are
> as follows:
>
> 1. A process has a capability inside a user namespace if it is a
> member of that namespace and it has the capability in its
> effective capability set. A process can gain capabilities in
> its effective capability set in various ways. For example, it
> may execute a set-user-ID program or an executable with asso‐
> ciated file capabilities. In addition, a process may gain
> capabilities via the effect of clone(2), unshare(2), or
> setns(2), as already described.
>
> 2. If a process has a capability in a user namespace, then it has
> that capability in all child (and further removed descendant)
> namespaces as well.
>
> 3. When a user namespace is created, the kernel records the
> effective user ID of the creating process as being the "owner"
> of the namespace. A process that resides in the parent of the
> user namespace and whose effective user ID matches the owner
> of the namespace has all capabilities in the namespace. By
> virtue of the previous rule, this means that the process has
> all capabilities in all further removed descendant user names‐
> paces as well.
>
> Interaction of user namespaces and other types of namespaces
> Starting in Linux 3.8, unprivileged processes can create user
> namespaces, and mount, PID, IPC, network, and UTS namespaces can
> be created with just the CAP_SYS_ADMIN capability in the caller's
> user namespace.
>
> If CLONE_NEWUSER is specified along with other CLONE_NEW* flags
> in a single clone(2) or unshare(2) call, the user namespace is
> guaranteed to be created first, giving the child (clone(2)) or
> caller (unshare(2)) privileges over the remaining namespaces cre‐
> ated by the call. Thus, it is possible for an unprivileged call‐
> er to specify this combination of flags.
>
> When a new IPC, mount, network, PID, or UTS namespace is created
> via clone(2) or unshare(2), the kernel records the user namespace
> of the creating process against the new namespace. (This associ‐
> ation can't be changed.) When a process in the new namespace
> subsequently performs privileged operations that operate on
> global resources isolated by the namespace, the permission checks
> are performed according to the process's capabilities in the user
> namespace that the kernel associated with the new namespace.
Restrictions on mount namespaces.
- A mount namespace has a owner user namespace. A mount namespace whose
owner user namespace is different than the owerner user namespace of
it's parent mount namespace is considered a less privileged mount
namespace.
- When creating a less privileged mount namespace shared mounts are
reduced to slave mounts. This ensures that mappings performed in less
privileged mount namespaces will not propogate to more privielged
mount namespaces.
- Mounts that come as a single unit from more privileged mount are
locked together and may not be separated in a less privielged mount
namespace.
- The mount flags readonly, nodev, nosuid, noexec, and the mount atime
settings when propogated from a more privielged to a less privileged
mount namespace become locked, and may not be changed in the less
privielged mount namespace.
- (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or
directory that is a mountpoint in one namespace that is not a mount
point in another namespace, may be renamed, unlinked, or rmdired in
the mount namespace in which it is not a mount namespace if the
ordinary permission checks pass.
Previously attemping to rmdir, unlink or rename a file or directory
that was a mount point in another mount namespace would result in
-EBUSY. This behavior had technical problems of enforcement (nfs)
and resulted in a nice denial of servial attack against more
privileged users. (Aka preventing individual files from being updated
by bind mounting on top of them).
> User and group ID mappings: uid_map and gid_map
> When a user namespace is created, it starts out without a mapping
> of user IDs (group IDs) to the parent user namespace. The
> /proc/[pid]/uid_map and /proc/[pid]/gid_map files (available
> since Linux 3.5) expose the mappings for user and group IDs
> inside the user namespace for the process pid. These files can
> be read to view the mappings in a user namespace and written to
> (once) to define the mappings.
>
> The description in the following paragraphs explains the details
> for uid_map; gid_map is exactly the same, but each instance of
> "user ID" is replaced by "group ID".
>
> The uid_map file exposes the mapping of user IDs from the user
> namespace of the process pid to the user namespace of the process
> that opened uid_map (but see a qualification to this point
> below). In other words, processes that are in different user
> namespaces will potentially see different values when reading
> from a particular uid_map file, depending on the user ID mappings
> for the user namespaces of the reading processes.
>
> Each line in the uid_map file specifies a 1-to-1 mapping of a
> range of contiguous user IDs between two user namespaces. (When
> a user namespace is first created, this file is empty.) The
> specification in each line takes the form of three numbers delim‐
> ited by white space. The first two numbers specify the starting
> user ID in each of the two user namespaces. The third number
> specifies the length of the mapped range. In detail, the fields
> are interpreted as follows:
>
> (1) The start of the range of user IDs in the user namespace of
> the process pid.
>
> (2) The start of the range of user IDs to which the user IDs
> specified by field one map. How field two is interpreted
> depends on whether the process that opened uid_map and the
> process pid are in the same user namespace, as follows:
>
> a) If the two processes are in different user namespaces:
> field two is the start of a range of user IDs in the user
> namespace of the process that opened uid_map.
>
> b) If the two processes are in the same user namespace: field
> two is the start of the range of user IDs in the parent
> user namespace of the process pid. This case enables the
> opener of uid_map (the common case here is opening
> /proc/self/uid_map) to see the mapping of user IDs into
> the user namespace of the process that created this user
> namespace.
>
> (3) The length of the range of user IDs that is mapped between
> the two user namespaces.
>
> System calls that return user IDs (group IDs)—for example,
> getuid(2), getgid(2), and the credential fields in the structure
> returned by stat(2)—return the user ID (group ID) mapped into the
> caller's user namespace.
>
> When a process accesses a file, its user and group IDs are mapped
> into the initial user namespace for the purpose of permission
> checking and assigning IDs when creating a file. When a process
> retrieves file user and group IDs via stat(2), the IDs are mapped
> in the opposite direction, to produce values relative to the
> process user and group ID mappings.
>
> The initial user namespace has no parent namespace, but, for con‐
> sistency, the kernel provides dummy user and group ID mapping
> files for this namespace. Looking at the uid_map file (gid_map
> is the same) from a shell in the initial namespace shows:
>
> $ cat /proc/$$/uid_map
> 0 0 4294967295
>
> This mapping tells us that the range starting at user ID 0 in
> this namespace maps to a range starting at 0 in the (nonexistent)
> parent namespace, and the length of the range is the largest
> 32-bit unsigned integer.
Which deliberately leaves 4294967295 32bit (-1) unmapped. (uid_t)-1 is
used in several interfaces (like setreuid) as a way to specify no uid
leaving it unmapped and unusuable guarantees that there will be no
confusion when using those kernel methods.
> Defining user and group ID mappings: writing to uid_map and gid_map
> After the creation of a new user namespace, the uid_map file of
> one of the processes in the namespace may be written to once to
> define the mapping of user IDs in the new user namespace. An
> attempt to write more than once to a uid_map file in a user
> namespace fails with the error EPERM. Similar rules apply for
> gid_map files.
>
> The lines written to uid_map (gid_map) must conform to the fol‐
> lowing rules:
>
> * The three fields must be valid numbers, and the last field
> must be greater than 0.
>
> * Lines are terminated by newline characters.
>
> * There is an (arbitrary) limit on the number of lines in the
> file. As at Linux 3.8, the limit is five lines. In addition,
> the number of bytes written to the file must be less than the
> system page size, and the write must be performed at the start
> of the file (i.e., lseek(2) and pwrite(2) can't be used to
> write to nonzero offsets in the file).
>
> * The range of user IDs (group IDs) specified in each line can‐
> not overlap with the ranges in any other lines. In the ini‐
> tial implementation (Linux 3.8), this requirement was satis‐
> fied by a simplistic implementation that imposed the further
> requirement that the values in both field 1 and field 2 of
> successive lines must be in ascending numerical order, which
> prevented some otherwise valid maps from being created. Linux
> 3.9 and later fix this limitation, allowing any valid set of
> nonoverlapping maps.
>
> * At least one line must be written to the file.
>
> Writes that violate the above rules fail with the error EINVAL.
>
> In order for a process to write to the /proc/[pid]/uid_map
> (/proc/[pid]/gid_map) file, all of the following requirements
> must be met:
>
> 1. The writing process must have the CAP_SETUID (CAP_SETGID)
> capability in the user namespace of the process pid.
>
> 2. The writing process must be in either the user namespace of
> the process pid or inside the parent user namespace of the
> process pid.
>
> 3. The mapped user IDs (group IDs) must in turn have a mapping in
> the parent user namespace.
>
> 4. One of the following is true:
>
> * The data written to uid_map (gid_map) consists of a single
> line that maps the writing process's filesystem user ID
> (group ID) in the parent user namespace to a user ID (group
> ID) in the user namespace. The usual case here is that
> this single line provides a mapping for user ID of the
> process that created the namespace.
>
> * The process has the CAP_SETUID (CAP_SETGID) capability in
> the parent user namespace. Thus, a privileged process can
> make mappings to arbitrary user IDs (group IDs) in the par‐
> ent user namespace.
>
> Writes that violate the above rules fail with the error EPERM.
>
> Unmapped user and group IDs
> There are various places where an unmapped user ID (group ID) may
> be exposed to user space. For example, the first process in a
> new user namespace may call getuid() before a user ID mapping has
> been defined for the namespace. In most such cases, an unmapped
> user ID is converted to the overflow user ID (group ID); the
> default value for the overflow user ID (group ID) is 65534. See
> the descriptions of /proc/sys/kernel/overflowuid and
> /proc/sys/kernel/overflowgid in proc(5).
>
> The cases where unmapped IDs are mapped in this fashion include
> system calls that return user IDs (getuid(2) getgid(2), and simi‐
> lar), credentials passed over a UNIX domain socket, credentials
> returned by stat(2), waitid(2), and the System V IPC "ctl"
> IPC_STAT operations, credentials exposed by /proc/PID/status and
> the files in /proc/sysvipc/*, credentials returned via the si_uid
> field in the siginfo_t received with a signal (see sigaction(2)),
> credentials written to the process accounting file (see acct(5)),
> and credentials returned with POSIX message queue notifications
> (see mq_notify(3)).
>
> There is one notable case where unmapped user and group IDs are
> not converted to the corresponding overflow ID value. When view‐
> ing a uid_map or gid_map file in which there is no mapping for
> the second field, that field is displayed as 4294967295 (-1 as an
> unsigned integer);
>
> Set-user-ID and set-group-ID programs
> When a process inside a user namespace executes a set-user-ID
> (set-group-ID) program, the process's effective user (group) ID
> inside the namespace is changed to whatever value is mapped for
> the user (group) ID of the file. However, if either the user or
> the group ID of the file has no mapping inside the namespace, the
> set-user-ID (set-group-ID) bit is silently ignored: the new pro‐
> gram is executed, but the process's effective user (group) ID is
> left unchanged. (This mirrors the semantics of executing a set-
> user-ID or set-group-ID program that resides on a filesystem that
> was mounted with the MS_NOSUID flag, as described in mount(2).)
>
> Miscellaneous
> When a process's user and group IDs are passed over a UNIX domain
> socket to a process in a different user namespace (see the
> description of SCM_CREDENTIALS in unix(7)), they are translated
> into the corresponding values as per the receiving process's user
> and group ID mappings.
>
> CONFORMING TO
> Namespaces are a Linux-specific feature.
>
> NOTES
> Over the years, there have been a lot of features that have been
> added to the Linux kernel that have been made available only to
> privileged users because of their potential to confuse set-user-
> ID-root applications. In general, it becomes safe to allow the
> root user in a user namespace to use those features because it is
> impossible, while in a user namespace, to gain more privilege
> than the root user of a user namespace has.
>
> Availability
> Use of user namespaces requires a kernel that is configured with
> the CONFIG_USER_NS option. User namespaces require support in a
> range of subsystems across the kernel. When an unsupported sub‐
> system is configured into the kernel, it is not possible to con‐
> figure user namespaces support.
>
> As at Linux 3.8, most relevant subsystems supported user names‐
> paces, but a number of filesystems did not have the infrastruc‐
> ture needed to map user and group IDs between user namespaces.
> Linux 3.9 added the required infrastructure support for many of
> the remaining unsupported filesystems (Plan 9 (9P), Andrew File
> System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2). Linux 3.11
> added support the last of the unsupported major filesystems, XFS.
>
> EXAMPLE
> The program below is designed to allow experimenting with user
> namespaces, as well as other types of namespaces. It creates
> namespaces as specified by command-line options and then executes
> a command inside those namespaces. The comments and usage()
> function inside the program provide a full explanation of the
> program. The following shell session demonstrates its use.
>
> First, we look at the run-time environment:
>
> $ uname -rs # Need Linux 3.8 or later
> Linux 3.8.0
> $ id -u # Running as unprivileged user
> 1000
> $ id -g
> 1000
>
> Now start a new shell in new user (-U), mount (-m), and PID (-p)
> namespaces, with user ID (-M) and group ID (-G) 1000 mapped to 0
> inside the user namespace:
>
> $ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
>
> The shell has PID 1, because it is the first process in the new
> PID namespace:
>
> bash$ echo $$
> 1
>
> Inside the user namespace, the shell has user and group ID 0, and
> a full set of permitted and effective capabilities:
>
> bash$ cat /proc/$$/status | egrep '^[UG]id'
> Uid: 0 0 0 0
> Gid: 0 0 0 0
> bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
> CapInh: 0000000000000000
> CapPrm: 0000001fffffffff
> CapEff: 0000001fffffffff
>
> Mounting a new /proc filesystem and listing all of the processes
> visible in the new PID namespace shows that the shell can't see
> any processes outside the PID namespace:
>
> bash$ mount -t proc proc /proc
> bash$ ps ax
> PID TTY STAT TIME COMMAND
> 1 pts/3 S 0:00 bash
> 22 pts/3 R+ 0:00 ps ax
>
> Program source
>
> /* userns_child_exec.c
>
> Licensed under GNU General Public License v2 or later
>
> Create a child process that executes a shell command in new
> namespace(s); allow UID and GID mappings to be specified when
> creating a user namespace.
> */
> #define _GNU_SOURCE
> #include <sched.h>
> #include <unistd.h>
> #include <stdlib.h>
> #include <sys/wait.h>
> #include <signal.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <string.h>
> #include <limits.h>
> #include <errno.h>
>
> /* A simple error-handling function: print an error message based
> on the value in 'errno' and terminate the calling process */
>
> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
> } while (0)
>
> struct child_args {
> char **argv; /* Command to be executed by child, with args */
> int pipe_fd[2]; /* Pipe used to synchronize parent and child */
> };
>
> static int verbose;
>
> static void
> usage(char *pname)
> {
> fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
> fprintf(stderr, "Create a child process that executes a shell "
> "command in a new user namespace,\n"
> "and possibly also other new namespace(s).\n\n");
> fprintf(stderr, "Options can be:\n\n");
> #define fpe(str) fprintf(stderr, " %s", str);
> fpe("-i New IPC namespace\n");
> fpe("-m New mount namespace\n");
> fpe("-n New network namespace\n");
> fpe("-p New PID namespace\n");
> fpe("-u New UTS namespace\n");
> fpe("-U New user namespace\n");
> fpe("-M uid_map Specify UID map for user namespace\n");
> fpe("-G gid_map Specify GID map for user namespace\n");
> fpe("-z Map user's UID and GID to 0 in user namespace\n");
> fpe(" (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
> fpe("-v Display verbose messages\n");
> fpe("\n");
> fpe("If -z, -M, or -G is specified, -U is required.\n");
> fpe("It is not permitted to specify both -z and either -M or -G.\n");
> fpe("\n");
> fpe("Map strings for -M and -G consist of records of the form:\n");
> fpe("\n");
> fpe(" ID-inside-ns ID-outside-ns len\n");
> fpe("\n");
> fpe("A map string can contain multiple records, separated"
> " by commas;\n");
> fpe("the commas are replaced by newlines before writing"
> " to map files.\n");
>
> exit(EXIT_FAILURE);
> }
>
> /* Update the mapping file 'map_file', with the value provided in
> 'mapping', a string that defines a UID or GID mapping. A UID or
> GID mapping consists of one or more newline-delimited records
> of the form:
>
> ID_inside-ns ID-outside-ns length
>
> Requiring the user to supply a string that contains newlines is
> of course inconvenient for command-line use. Thus, we permit the
> use of commas to delimit records in this string, and replace them
> with newlines before writing the string to the file. */
>
> static void
> update_map(char *mapping, char *map_file)
> {
> int fd, j;
> size_t map_len; /* Length of 'mapping' */
>
> /* Replace commas in mapping string with newlines */
>
> map_len = strlen(mapping);
> for (j = 0; j < map_len; j++)
> if (mapping[j] == ',')
> mapping[j] = '\n';
>
> fd = open(map_file, O_RDWR);
> if (fd == -1) {
> fprintf(stderr, "ERROR: open %s: %s\n", map_file,
> strerror(errno));
> exit(EXIT_FAILURE);
> }
>
> if (write(fd, mapping, map_len) != map_len) {
> fprintf(stderr, "ERROR: write %s: %s\n", map_file,
> strerror(errno));
> exit(EXIT_FAILURE);
> }
>
> close(fd);
> }
>
> static int /* Start function for cloned child */
> childFunc(void *arg)
> {
> struct child_args *args = (struct child_args *) arg;
> char ch;
>
> /* Wait until the parent has updated the UID and GID mappings.
> See the comment in main(). We wait for end of file on a
> pipe that will be closed by the parent process once it has
> updated the mappings. */
>
> close(args->pipe_fd[1]); /* Close our descriptor for the write
> end of the pipe so that we see EOF
> when parent closes its descriptor */
> if (read(args->pipe_fd[0], &ch, 1) != 0) {
> fprintf(stderr,
> "Failure in child: read from pipe returned != 0\n");
> exit(EXIT_FAILURE);
> }
>
> /* Execute a shell command */
>
> printf("About to exec %s\n", args->argv[0]);
> execvp(args->argv[0], args->argv);
> errExit("execvp");
> }
>
> #define STACK_SIZE (1024 * 1024)
>
> static char child_stack[STACK_SIZE]; /* Space for child's stack */
>
> int
> main(int argc, char *argv[])
> {
> int flags, opt, map_zero;
> pid_t child_pid;
> struct child_args args;
> char *uid_map, *gid_map;
> const int MAP_BUF_SIZE = 100;
> char map_buf[MAP_BUF_SIZE];
> char map_path[PATH_MAX];
>
> /* Parse command-line options. The initial '+' character in
> the final getopt() argument prevents GNU-style permutation
> of command-line options. That's useful, since sometimes
> the 'command' to be executed by this program itself
> has command-line options. We don't want getopt() to treat
> those as options to this program. */
>
> flags = 0;
> verbose = 0;
> gid_map = NULL;
> uid_map = NULL;
> map_zero = 0;
> while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
> switch (opt) {
> case 'i': flags |= CLONE_NEWIPC; break;
> case 'm': flags |= CLONE_NEWNS; break;
> case 'n': flags |= CLONE_NEWNET; break;
> case 'p': flags |= CLONE_NEWPID; break;
> case 'u': flags |= CLONE_NEWUTS; break;
> case 'v': verbose = 1; break;
> case 'z': map_zero = 1; break;
> case 'M': uid_map = optarg; break;
> case 'G': gid_map = optarg; break;
> case 'U': flags |= CLONE_NEWUSER; break;
> default: usage(argv[0]);
> }
> }
>
> /* -M or -G without -U is nonsensical */
>
> if (((uid_map != NULL || gid_map != NULL || map_zero) &&
> !(flags & CLONE_NEWUSER)) ||
> (map_zero && (uid_map != NULL || gid_map != NULL)))
> usage(argv[0]);
>
> args.argv = &argv[optind];
>
> /* We use a pipe to synchronize the parent and child, in order to
> ensure that the parent sets the UID and GID maps before the child
> calls execve(). This ensures that the child maintains its
> capabilities during the execve() in the common case where we
> want to map the child's effective user ID to 0 in the new user
> namespace. Without this synchronization, the child would lose
> its capabilities if it performed an execve() with nonzero
> user IDs (see the capabilities(7) man page for details of the
> transformation of a process's capabilities during execve()). */
>
> if (pipe(args.pipe_fd) == -1)
> errExit("pipe");
>
> /* Create the child in new namespace(s) */
>
> child_pid = clone(childFunc, child_stack + STACK_SIZE,
> flags | SIGCHLD, &args);
> if (child_pid == -1)
> errExit("clone");
>
> /* Parent falls through to here */
>
> if (verbose)
> printf("%s: PID of child created by clone() is %ld\n",
> argv[0], (long) child_pid);
>
> /* Update the UID and GID maps in the child */
>
> if (uid_map != NULL || map_zero) {
> snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
> (long) child_pid);
> if (map_zero) {
> snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid());
> uid_map = map_buf;
> }
> update_map(uid_map, map_path);
> }
> if (gid_map != NULL || map_zero) {
> snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
> (long) child_pid);
> if (map_zero) {
> snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid());
> gid_map = map_buf;
> }
> update_map(gid_map, map_path);
> }
>
> /* Close the write end of the pipe, to signal to the child that we
> have updated the UID and GID maps */
>
> close(args.pipe_fd[1]);
>
> if (waitpid(child_pid, NULL, 0) == -1) /* Wait for child */
> errExit("waitpid");
>
> if (verbose)
> printf("%s: terminating\n", argv[0]);
>
> exit(EXIT_SUCCESS);
> }
>
> SEE ALSO
> newgidmap(1), newuidmap(1), clone(2), setns(2), unshare(2),
> proc(5), subgid(5), subuid(5), credentials(7), capabilities(7),
> namespaces(7), pid_namespaces(7)
>
> The kernel source file Documentation/namespaces/resource-con‐
> trol.txt.
Eric
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-08-22 21:12 ` Serge E. Hallyn
@ 2014-09-01 16:58 ` Michael Kerrisk (man-pages)
0 siblings, 0 replies; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-01 16:58 UTC (permalink / raw)
To: Serge E. Hallyn
Cc: mtk.manpages, Eric W. Biederman, lkml, linux-man, containers,
Andy Lutomirski, richard.weinberger
On 08/22/2014 11:12 PM, Serge E. Hallyn wrote:
> Quoting Michael Kerrisk (man-pages) (mtk.manpages@gmail.com):
>> Hello Eric et al.,
>>
>> For various reasons, my work on the namespaces man pages
>> fell off the table a while back. Nevertheless, the pages have
>> been close to completion for a while now, and I recently restarted,
>> in an effort to finish them. As you also noted to me f2f, there have
>> been recently been some small namespace changes that you may affect
>> the content of the pages. Therefore, I'll take the opportunity to
>> send the namespace-related pages out for further (final?) review.
>>
>> So, here, I start with the user_namespaces(7) page, which is shown
>> in rendered form below, with source attached to this mail. I'll
>> send various other pages in follow-on mails.
>>
>> Review comments/suggestions for improvements / bug fixes welcome.
>>
>> Cheers,
>>
>> Michael
>>
>> ==
>>
>> NAME
>> user_namespaces - overview of Linux user_namespaces
>>
>> DESCRIPTION
>> For an overview of namespaces, see namespaces(7).
>>
>> User namespaces isolate security-related identifiers and
>> attributes, in particular, user IDs and group IDs (see creden‐
>> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
>> ties (see capabilities(7)). A process's user and group IDs can
>> be different inside and outside a user namespace. In particular,
>> a process can have a normal unprivileged user ID outside a user
>> namespace while at the same time having a user ID of 0 inside the
>> namespace; in other words, the process has full privileges for
>> operations inside the user namespace, but is unprivileged for
>> operations outside the namespace.
>>
>> Nested namespaces, namespace membership
>> User namespaces can be nested; that is, each user namespace—
>> except the initial ("root") namespace—has a parent user names‐
>> pace, and can have zero or more child user namespaces. The par‐
>> ent user namespace is the user namespace of the process that cre‐
>> ates the user namespace via a call to unshare(2) or clone(2) with
>> the CLONE_NEWUSER flag.
>>
>> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
>> els of user namespaces. Calls to unshare(2) or clone(2) that
>> would cause this limit to be exceeded fail with the error EUSERS.
>>
>> Each process is a member of exactly one user namespace. A
>> process created via fork(2) or clone(2) without the CLONE_NEWUSER
>> flag is a member of the same user namespace as its parent. A
>> process can join another user namespace with setns(2) if it has
>> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
>> full set of capabilities in that namespace.
>>
>> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
>> makes the new child process (for clone(2)) or the caller (for
>> unshare(2)) a member of the new user namespace created by the
>> call.
>>
>> Capabilities
>> The child process created by clone(2) with the CLONE_NEWUSER flag
>> starts out with a complete set of capabilities in the new user
>> namespace. Likewise, a process that creates a new user namespace
>> using unshare(2) or joins an existing user namespace using
>> setns(2) gains a full set of capabilities in that namespace. On
>> the other hand, that process has no capabilities in the parent
>> (in the case of clone(2)) or previous (in the case of unshare(2)
>> and setns(2)) user namespace, even if the new namespace is cre‐
>> ated or joined by the root user (i.e., a process with user ID 0
>> in the root namespace).
>>
>> Note that a call to execve(2) will cause a process to lose any
>> capabilities that it has, unless it has a user ID of 0 within the
>> namespace. See the discussion of user and group ID mappings,
>> below.
>
> The above is an approximation, but a bit misleading. On exec, the task
> capability set is recalculated according to the usual rules. So if the
> file being executed has file capabilities, the result task may end up
> with capabilities even if it is not root (even if it is uid -1).
>
> Perhaps it should be phrased as:
>
> Note that a call to execve(2) will cause a process' capabilities
> to be recalculated (see capabilities(7)), so that usually, unless
> it has a user ID of 0 within the namespace, it will lose all
> capabilities. See the discussion of user and group ID mappings,
> below.
Thanks, Serge. Changed as you suggest.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-08-30 21:53 ` Eric W. Biederman
@ 2014-09-01 17:31 ` Michael Kerrisk (man-pages)
2014-09-02 1:05 ` Eric W. Biederman
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
2 siblings, 1 reply; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-01 17:31 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
On 08/30/2014 11:53 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>
>> Hello Eric et al.,
>>
>> For various reasons, my work on the namespaces man pages
>> fell off the table a while back. Nevertheless, the pages have
>> been close to completion for a while now, and I recently restarted,
>> in an effort to finish them. As you also noted to me f2f, there have
>> been recently been some small namespace changes that you may affect
>> the content of the pages. Therefore, I'll take the opportunity to
>> send the namespace-related pages out for further (final?) review.
>>
>> So, here, I start with the user_namespaces(7) page, which is shown
>> in rendered form below, with source attached to this mail. I'll
>> send various other pages in follow-on mails.
>>
>> Review comments/suggestions for improvements / bug fixes welcome.
>>
>> Cheers,
>>
>> Michael
>>
>> ==
>>
>> NAME
>> user_namespaces - overview of Linux user_namespaces
>>
>> DESCRIPTION
>> For an overview of namespaces, see namespaces(7).
>>
>> User namespaces isolate security-related identifiers and
>> attributes, in particular, user IDs and group IDs (see creden‐
>> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
>> ties (see capabilities(7)). A process's user and group IDs can
>> be different inside and outside a user namespace. In particular,
>> a process can have a normal unprivileged user ID outside a user
>> namespace while at the same time having a user ID of 0 inside the
>> namespace; in other words, the process has full privileges for
>> operations inside the user namespace, but is unprivileged for
>> operations outside the namespace.
>>
>> Nested namespaces, namespace membership
>> User namespaces can be nested; that is, each user namespace—
>> except the initial ("root") namespace—has a parent user names‐
>> pace, and can have zero or more child user namespaces. The par‐
>> ent user namespace is the user namespace of the process that cre‐
>> ates the user namespace via a call to unshare(2) or clone(2) with
>> the CLONE_NEWUSER flag.
>>
>> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
>> els of user namespaces. Calls to unshare(2) or clone(2) that
>> would cause this limit to be exceeded fail with the error EUSERS.
>>
>> Each process is a member of exactly one user namespace. A
>> process created via fork(2) or clone(2) without the CLONE_NEWUSER
>> flag is a member of the same user namespace as its parent.
>> A
> ^ single-threaded
>
> Because of chroot and other things multi-threaded processes are not
> allowed to join a user namespace. For the documentation just saying
> single-threaded sounds like enough here.
Thanks. Fixed.
>> process can join another user namespace with setns(2) if it has
>> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
>> full set of capabilities in that namespace.
>>
>> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
>> makes the new child process (for clone(2)) or the caller (for
>> unshare(2)) a member of the new user namespace created by the
>> call.
>>
>> Capabilities
>> The child process created by clone(2) with the CLONE_NEWUSER flag
>> starts out with a complete set of capabilities in the new user
>> namespace. Likewise, a process that creates a new user namespace
>> using unshare(2) or joins an existing user namespace using
>> setns(2) gains a full set of capabilities in that namespace. On
>> the other hand, that process has no capabilities in the parent
>> (in the case of clone(2)) or previous (in the case of unshare(2)
>> and setns(2)) user namespace, even if the new namespace is cre‐
>> ated or joined by the root user (i.e., a process with user ID 0
>> in the root namespace).
>>
>> Note that a call to execve(2) will cause a process to lose any
>> capabilities that it has, unless it has a user ID of 0 within the
>> namespace. See the discussion of user and group ID mappings,
>> below.
>>
>> A call to clone(2), unshare(2), or setns(2) using the
>> CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐
>> ties(7)) to their default values (all flags disabled) in the
>> child (for clone(2)) or caller (for unshare(2), or setns(2)).
>> Note that because the caller no longer has capabilities in its
>> original user namespace after a call to setns(2), it is not pos‐
>> sible for a process to reset its "securebits" flags while retain‐
>> ing its user namespace membership by using a pair of setns(2)
>> calls to move to another user namespace and then return to its
>> original user namespace.
>>
>> Having a capability inside a user namespace permits a process to
>> perform operations (that require privilege) only on resources
>> governed by that namespace. The rules for determining whether or
>> not a process has a capability in a particular user namespace are
>> as follows:
>>
>> 1. A process has a capability inside a user namespace if it is a
>> member of that namespace and it has the capability in its
>> effective capability set. A process can gain capabilities in
>> its effective capability set in various ways. For example, it
>> may execute a set-user-ID program or an executable with asso‐
>> ciated file capabilities. In addition, a process may gain
>> capabilities via the effect of clone(2), unshare(2), or
>> setns(2), as already described.
>>
>> 2. If a process has a capability in a user namespace, then it has
>> that capability in all child (and further removed descendant)
>> namespaces as well.
>>
>> 3. When a user namespace is created, the kernel records the
>> effective user ID of the creating process as being the "owner"
>> of the namespace. A process that resides in the parent of the
>> user namespace and whose effective user ID matches the owner
>> of the namespace has all capabilities in the namespace. By
>> virtue of the previous rule, this means that the process has
>> all capabilities in all further removed descendant user names‐
>> paces as well.
>>
>> Interaction of user namespaces and other types of namespaces
>> Starting in Linux 3.8, unprivileged processes can create user
>> namespaces, and mount, PID, IPC, network, and UTS namespaces can
>> be created with just the CAP_SYS_ADMIN capability in the caller's
>> user namespace.
>>
>> If CLONE_NEWUSER is specified along with other CLONE_NEW* flags
>> in a single clone(2) or unshare(2) call, the user namespace is
>> guaranteed to be created first, giving the child (clone(2)) or
>> caller (unshare(2)) privileges over the remaining namespaces cre‐
>> ated by the call. Thus, it is possible for an unprivileged call‐
>> er to specify this combination of flags.
>>
>> When a new IPC, mount, network, PID, or UTS namespace is created
>> via clone(2) or unshare(2), the kernel records the user namespace
>> of the creating process against the new namespace. (This associ‐
>> ation can't be changed.) When a process in the new namespace
>> subsequently performs privileged operations that operate on
>> global resources isolated by the namespace, the permission checks
>> are performed according to the process's capabilities in the user
>> namespace that the kernel associated with the new namespace.
>
> Restrictions on mount namespaces.
>
> - A mount namespace has a owner user namespace. A mount namespace whose
> owner user namespace is different than the owerner user namespace of
> it's parent mount namespace is considered a less privileged mount
> namespace.
>
> - When creating a less privileged mount namespace shared mounts are
> reduced to slave mounts. This ensures that mappings performed in less
> privileged mount namespaces will not propogate to more privielged
> mount namespaces.
>
> - Mounts that come as a single unit from more privileged mount are
> locked together and may not be separated in a less privielged mount
> namespace.
>
> - The mount flags readonly, nodev, nosuid, noexec, and the mount atime
> settings when propogated from a more privielged to a less privileged
> mount namespace become locked, and may not be changed in the less
> privielged mount namespace.
>
> - (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or
> directory that is a mountpoint in one namespace that is not a mount
> point in another namespace, may be renamed, unlinked, or rmdired in
> the mount namespace in which it is not a mount namespace if the
> ordinary permission checks pass.
>
> Previously attemping to rmdir, unlink or rename a file or directory
> that was a mount point in another mount namespace would result in
> -EBUSY. This behavior had technical problems of enforcement (nfs)
> and resulted in a nice denial of servial attack against more
> privileged users. (Aka preventing individual files from being updated
> by bind mounting on top of them).
I need some help here. What is your intention for the above text.
Do you mean I should add it pretty much as is under a subheading
"Restrictions on mount namespaces"?
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-08-20 23:36 For review: user_namespace(7) man page Michael Kerrisk (man-pages)
2014-08-22 21:12 ` Serge E. Hallyn
2014-08-30 21:53 ` Eric W. Biederman
@ 2014-09-01 20:57 ` Andy Lutomirski
2014-09-09 14:00 ` Michael Kerrisk (man-pages)
2 siblings, 1 reply; 26+ messages in thread
From: Andy Lutomirski @ 2014-09-01 20:57 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: Eric W. Biederman, lkml, linux-man, Linux Containers,
richard -rw- weinberger, Serge E. Hallyn
On Wed, Aug 20, 2014 at 4:36 PM, Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
> Hello Eric et al.,
>
> For various reasons, my work on the namespaces man pages
> fell off the table a while back. Nevertheless, the pages have
> been close to completion for a while now, and I recently restarted,
> in an effort to finish them. As you also noted to me f2f, there have
> been recently been some small namespace changes that you may affect
> the content of the pages. Therefore, I'll take the opportunity to
> send the namespace-related pages out for further (final?) review.
>
> So, here, I start with the user_namespaces(7) page, which is shown
> in rendered form below, with source attached to this mail. I'll
> send various other pages in follow-on mails.
>
> Review comments/suggestions for improvements / bug fixes welcome.
>
> Cheers,
>
> Michael
>
> ==
>
> NAME
> user_namespaces - overview of Linux user_namespaces
>
> DESCRIPTION
> For an overview of namespaces, see namespaces(7).
>
> User namespaces isolate security-related identifiers and
> attributes, in particular, user IDs and group IDs (see creden‐
> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
Putting "root directory" here is odd -- that's really part of a
different namespace. But user namespaces sort of isolate the other
namespaces from each other.
Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?
> ties (see capabilities(7)). A process's user and group IDs can
> be different inside and outside a user namespace. In particular,
> a process can have a normal unprivileged user ID outside a user
> namespace while at the same time having a user ID of 0 inside the
> namespace; in other words, the process has full privileges for
> operations inside the user namespace, but is unprivileged for
> operations outside the namespace.
>
> Nested namespaces, namespace membership
> User namespaces can be nested; that is, each user namespace—
> except the initial ("root") namespace—has a parent user names‐
> pace, and can have zero or more child user namespaces. The par‐
> ent user namespace is the user namespace of the process that cre‐
> ates the user namespace via a call to unshare(2) or clone(2) with
> the CLONE_NEWUSER flag.
>
> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
> els of user namespaces. Calls to unshare(2) or clone(2) that
> would cause this limit to be exceeded fail with the error EUSERS.
>
> Each process is a member of exactly one user namespace. A
> process created via fork(2) or clone(2) without the CLONE_NEWUSER
> flag is a member of the same user namespace as its parent. A
> process can join another user namespace with setns(2) if it has
> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
> full set of capabilities in that namespace.
>
> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
> makes the new child process (for clone(2)) or the caller (for
> unshare(2)) a member of the new user namespace created by the
> call.
>
> Capabilities
> The child process created by clone(2) with the CLONE_NEWUSER flag
> starts out with a complete set of capabilities in the new user
> namespace. Likewise, a process that creates a new user namespace
> using unshare(2) or joins an existing user namespace using
> setns(2) gains a full set of capabilities in that namespace. On
> the other hand, that process has no capabilities in the parent
> (in the case of clone(2)) or previous (in the case of unshare(2)
> and setns(2)) user namespace, even if the new namespace is cre‐
> ated or joined by the root user (i.e., a process with user ID 0
> in the root namespace).
>
> Note that a call to execve(2) will cause a process to lose any
> capabilities that it has, unless it has a user ID of 0 within the
> namespace.
Or unless file capabilities have a non-empty inheritable mask.
It may be worth mentioning that execve in a user namespace works
exactly like execve outside a userns.
> $ cat /proc/$$/uid_map
> 0 0 4294967295
>
> This mapping tells us that the range starting at user ID 0 in
> this namespace maps to a range starting at 0 in the (nonexistent)
> parent namespace, and the length of the range is the largest
> 32-bit unsigned integer.
>
> Defining user and group ID mappings: writing to uid_map and gid_map
> After the creation of a new user namespace, the uid_map file of
> one of the processes in the namespace may be written to once to
> define the mapping of user IDs in the new user namespace. An
> attempt to write more than once to a uid_map file in a user
> namespace fails with the error EPERM. Similar rules apply for
> gid_map files.
>
> The lines written to uid_map (gid_map) must conform to the fol‐
> lowing rules:
>
> * The three fields must be valid numbers, and the last field
> must be greater than 0.
>
> * Lines are terminated by newline characters.
>
> * There is an (arbitrary) limit on the number of lines in the
> file. As at Linux 3.8, the limit is five lines. In addition,
> the number of bytes written to the file must be less than the
> system page size, and the write must be performed at the start
> of the file (i.e., lseek(2) and pwrite(2) can't be used to
> write to nonzero offsets in the file).
>
> * The range of user IDs (group IDs) specified in each line can‐
> not overlap with the ranges in any other lines. In the ini‐
> tial implementation (Linux 3.8), this requirement was satis‐
> fied by a simplistic implementation that imposed the further
> requirement that the values in both field 1 and field 2 of
> successive lines must be in ascending numerical order, which
> prevented some otherwise valid maps from being created. Linux
> 3.9 and later fix this limitation, allowing any valid set of
> nonoverlapping maps.
>
> * At least one line must be written to the file.
>
> Writes that violate the above rules fail with the error EINVAL.
>
> In order for a process to write to the /proc/[pid]/uid_map
> (/proc/[pid]/gid_map) file, all of the following requirements
> must be met:
>
> 1. The writing process must have the CAP_SETUID (CAP_SETGID)
> capability in the user namespace of the process pid.
This checked for the opening process (and I don't actually remember
whether it's checked for the writing process).
>
> 2. The writing process must be in either the user namespace of
> the process pid or inside the parent user namespace of the
> process pid.
>
> 3. The mapped user IDs (group IDs) must in turn have a mapping in
> the parent user namespace.
>
> 4. One of the following is true:
>
> * The data written to uid_map (gid_map) consists of a single
> line that maps the writing process's filesystem user ID
> (group ID) in the parent user namespace to a user ID (group
> ID) in the user namespace. The usual case here is that
> this single line provides a mapping for user ID of the
> process that created the namespace.
>
> * The process has the CAP_SETUID (CAP_SETGID) capability in
> the parent user namespace. Thus, a privileged process can
> make mappings to arbitrary user IDs (group IDs) in the par‐
> ent user namespace.
The opening process.
One other thing that could be worth mentioning it: any non-user
namespace that's created is owned by the user namespace of the process
that created it at the time of creation. Actions on those namespaces
require capabilities in the corresponding user namespace.
Thanks for doing this!
--Andy
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-01 17:31 ` Michael Kerrisk (man-pages)
@ 2014-09-02 1:05 ` Eric W. Biederman
2014-09-09 14:00 ` Michael Kerrisk (man-pages)
0 siblings, 1 reply; 26+ messages in thread
From: Eric W. Biederman @ 2014-09-02 1:05 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: lkml, linux-man, containers, Andy Lutomirski, richard.weinberger,
Serge E. Hallyn
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> On 08/30/2014 11:53 PM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>> For various reasons, my work on the namespaces man pages
>>> fell off the table a while back. Nevertheless, the pages have
>>> been close to completion for a while now, and I recently restarted,
>>> in an effort to finish them. As you also noted to me f2f, there have
>>> been recently been some small namespace changes that you may affect
>>> the content of the pages. Therefore, I'll take the opportunity to
>>> send the namespace-related pages out for further (final?) review.
>>>
>>> So, here, I start with the user_namespaces(7) page, which is shown
>>> in rendered form below, with source attached to this mail. I'll
>>> send various other pages in follow-on mails.
>>>
>>> Review comments/suggestions for improvements / bug fixes welcome.
>>>
>>> Cheers,
>>>
>>> Michael
>>>
>>> ==
>>>
>>> NAME
>>> user_namespaces - overview of Linux user_namespaces
>>>
>>> DESCRIPTION
>>> For an overview of namespaces, see namespaces(7).
>>>
>>> User namespaces isolate security-related identifiers and
>>> attributes, in particular, user IDs and group IDs (see creden‐
>>> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
>>> ties (see capabilities(7)). A process's user and group IDs can
>>> be different inside and outside a user namespace. In particular,
>>> a process can have a normal unprivileged user ID outside a user
>>> namespace while at the same time having a user ID of 0 inside the
>>> namespace; in other words, the process has full privileges for
>>> operations inside the user namespace, but is unprivileged for
>>> operations outside the namespace.
>>>
>>> Nested namespaces, namespace membership
>>> User namespaces can be nested; that is, each user namespace—
>>> except the initial ("root") namespace—has a parent user names‐
>>> pace, and can have zero or more child user namespaces. The par‐
>>> ent user namespace is the user namespace of the process that cre‐
>>> ates the user namespace via a call to unshare(2) or clone(2) with
>>> the CLONE_NEWUSER flag.
>>>
>>> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
>>> els of user namespaces. Calls to unshare(2) or clone(2) that
>>> would cause this limit to be exceeded fail with the error EUSERS.
>>>
>>> Each process is a member of exactly one user namespace. A
>>> process created via fork(2) or clone(2) without the CLONE_NEWUSER
>>> flag is a member of the same user namespace as its parent.
>>> A
>> ^ single-threaded
>>
>> Because of chroot and other things multi-threaded processes are not
>> allowed to join a user namespace. For the documentation just saying
>> single-threaded sounds like enough here.
>
> Thanks. Fixed.
>
>>> process can join another user namespace with setns(2) if it has
>>> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
>>> full set of capabilities in that namespace.
>>>
>>> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
>>> makes the new child process (for clone(2)) or the caller (for
>>> unshare(2)) a member of the new user namespace created by the
>>> call.
>>>
>>> Capabilities
>>> The child process created by clone(2) with the CLONE_NEWUSER flag
>>> starts out with a complete set of capabilities in the new user
>>> namespace. Likewise, a process that creates a new user namespace
>>> using unshare(2) or joins an existing user namespace using
>>> setns(2) gains a full set of capabilities in that namespace. On
>>> the other hand, that process has no capabilities in the parent
>>> (in the case of clone(2)) or previous (in the case of unshare(2)
>>> and setns(2)) user namespace, even if the new namespace is cre‐
>>> ated or joined by the root user (i.e., a process with user ID 0
>>> in the root namespace).
>>>
>>> Note that a call to execve(2) will cause a process to lose any
>>> capabilities that it has, unless it has a user ID of 0 within the
>>> namespace. See the discussion of user and group ID mappings,
>>> below.
>>>
>>> A call to clone(2), unshare(2), or setns(2) using the
>>> CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐
>>> ties(7)) to their default values (all flags disabled) in the
>>> child (for clone(2)) or caller (for unshare(2), or setns(2)).
>>> Note that because the caller no longer has capabilities in its
>>> original user namespace after a call to setns(2), it is not pos‐
>>> sible for a process to reset its "securebits" flags while retain‐
>>> ing its user namespace membership by using a pair of setns(2)
>>> calls to move to another user namespace and then return to its
>>> original user namespace.
>>>
>>> Having a capability inside a user namespace permits a process to
>>> perform operations (that require privilege) only on resources
>>> governed by that namespace. The rules for determining whether or
>>> not a process has a capability in a particular user namespace are
>>> as follows:
>>>
>>> 1. A process has a capability inside a user namespace if it is a
>>> member of that namespace and it has the capability in its
>>> effective capability set. A process can gain capabilities in
>>> its effective capability set in various ways. For example, it
>>> may execute a set-user-ID program or an executable with asso‐
>>> ciated file capabilities. In addition, a process may gain
>>> capabilities via the effect of clone(2), unshare(2), or
>>> setns(2), as already described.
>>>
>>> 2. If a process has a capability in a user namespace, then it has
>>> that capability in all child (and further removed descendant)
>>> namespaces as well.
>>>
>>> 3. When a user namespace is created, the kernel records the
>>> effective user ID of the creating process as being the "owner"
>>> of the namespace. A process that resides in the parent of the
>>> user namespace and whose effective user ID matches the owner
>>> of the namespace has all capabilities in the namespace. By
>>> virtue of the previous rule, this means that the process has
>>> all capabilities in all further removed descendant user names‐
>>> paces as well.
>>>
>>> Interaction of user namespaces and other types of namespaces
>>> Starting in Linux 3.8, unprivileged processes can create user
>>> namespaces, and mount, PID, IPC, network, and UTS namespaces can
>>> be created with just the CAP_SYS_ADMIN capability in the caller's
>>> user namespace.
>>>
>>> If CLONE_NEWUSER is specified along with other CLONE_NEW* flags
>>> in a single clone(2) or unshare(2) call, the user namespace is
>>> guaranteed to be created first, giving the child (clone(2)) or
>>> caller (unshare(2)) privileges over the remaining namespaces cre‐
>>> ated by the call. Thus, it is possible for an unprivileged call‐
>>> er to specify this combination of flags.
>>>
>>> When a new IPC, mount, network, PID, or UTS namespace is created
>>> via clone(2) or unshare(2), the kernel records the user namespace
>>> of the creating process against the new namespace. (This associ‐
>>> ation can't be changed.) When a process in the new namespace
>>> subsequently performs privileged operations that operate on
>>> global resources isolated by the namespace, the permission checks
>>> are performed according to the process's capabilities in the user
>>> namespace that the kernel associated with the new namespace.
>>
>> Restrictions on mount namespaces.
>>
>> - A mount namespace has a owner user namespace. A mount namespace whose
>> owner user namespace is different than the owerner user namespace of
>> it's parent mount namespace is considered a less privileged mount
>> namespace.
>>
>> - When creating a less privileged mount namespace shared mounts are
>> reduced to slave mounts. This ensures that mappings performed in less
>> privileged mount namespaces will not propogate to more privielged
>> mount namespaces.
>>
>> - Mounts that come as a single unit from more privileged mount are
>> locked together and may not be separated in a less privielged mount
>> namespace.
>>
>> - The mount flags readonly, nodev, nosuid, noexec, and the mount atime
>> settings when propogated from a more privielged to a less privileged
>> mount namespace become locked, and may not be changed in the less
>> privielged mount namespace.
>>
>> - (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or
>> directory that is a mountpoint in one namespace that is not a mount
>> point in another namespace, may be renamed, unlinked, or rmdired in
>> the mount namespace in which it is not a mount namespace if the
>> ordinary permission checks pass.
>>
>> Previously attemping to rmdir, unlink or rename a file or directory
>> that was a mount point in another mount namespace would result in
>> -EBUSY. This behavior had technical problems of enforcement (nfs)
>> and resulted in a nice denial of servial attack against more
>> privileged users. (Aka preventing individual files from being updated
>> by bind mounting on top of them).
>
> I need some help here. What is your intention for the above text.
> Do you mean I should add it pretty much as is under a subheading
> "Restrictions on mount namespaces"?
You have the heading "Interactions of user namespaces and other types of
namespaces" and looking through the man pages you have posted for review
somewhere under that heading is the best place I could find for this
content. (Better suggestions are welcome).
My experience with working with you previously is that you tended to
reword what I had written to make the content more readable. Perhaps
straying from exactly accurate to the practical.
So rather than try and write the perfect deathless end-user prose I
figured I would write down what the weird restrictions are on mount
namespaces when you create them in user namespaces accurately.
The test will do fine as a subsection entitled "Restrictions on mount
namespaces" as I have written it. Or it is fine as a seed for something
better. But those restrictions are important to document so that people
know what to expect from mount namespaces.
On a related note. One thing that has come up recently (in 3 separate
implementations is that mount(MS_REMOUNT|...,...) must include all of
the mount flags that need to be preserved. People creating read-only
bind mounts tend to miss that and the locked flags in mount namespaces.
That issue was flushed out now that the kernel is now not allowing most
mount flags to be cleared in mount namespaces.
Eric
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-08-30 21:53 ` Eric W. Biederman
2014-09-01 17:31 ` Michael Kerrisk (man-pages)
@ 2014-09-09 13:59 ` Michael Kerrisk (man-pages)
2014-09-09 15:49 ` Eric W. Biederman
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
2 siblings, 1 reply; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-09 13:59 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
Hi Eric,
On 08/30/2014 02:53 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>
>> Hello Eric et al.,
>>
>> For various reasons, my work on the namespaces man pages
>> fell off the table a while back. Nevertheless, the pages have
>> been close to completion for a while now, and I recently restarted,
>> in an effort to finish them. As you also noted to me f2f, there have
>> been recently been some small namespace changes that you may affect
>> the content of the pages. Therefore, I'll take the opportunity to
>> send the namespace-related pages out for further (final?) review.
>>
>> So, here, I start with the user_namespaces(7) page, which is shown
>> in rendered form below, with source attached to this mail. I'll
>> send various other pages in follow-on mails.
>>
>> Review comments/suggestions for improvements / bug fixes welcome.
>>
>> Cheers,
>>
>> Michael
>>
>> ==
>>
>> NAME
>> user_namespaces - overview of Linux user_namespaces
>>
[...]
>> When a new IPC, mount, network, PID, or UTS namespace is created
>> via clone(2) or unshare(2), the kernel records the user namespace
>> of the creating process against the new namespace. (This associ‐
>> ation can't be changed.) When a process in the new namespace
>> subsequently performs privileged operations that operate on
>> global resources isolated by the namespace, the permission checks
>> are performed according to the process's capabilities in the user
>> namespace that the kernel associated with the new namespace.
>
> Restrictions on mount namespaces.
>
> - A mount namespace has a owner user namespace. A mount namespace whose
> owner user namespace is different than the owerner user namespace of
> it's parent mount namespace is considered a less privileged mount
> namespace.
>
> - When creating a less privileged mount namespace shared mounts are
> reduced to slave mounts. This ensures that mappings performed in less
> privileged mount namespaces will not propogate to more privielged
> mount namespaces.
>
> - Mounts that come as a single unit from more privileged mount are
> locked together and may not be separated in a less privielged mount
> namespace.
Could you clarify what you mean by "Mounts that come as a single unit"?
> - The mount flags readonly, nodev, nosuid, noexec, and the mount atime
> settings when propogated from a more privielged to a less privileged
> mount namespace become locked, and may not be changed in the less
> privielged mount namespace.
>
> - (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or
> directory that is a mountpoint in one namespace that is not a mount
> point in another namespace, may be renamed, unlinked, or rmdired in
> the mount namespace in which it is not a mount namespace if the
> ordinary permission checks pass.
>
> Previously attemping to rmdir, unlink or rename a file or directory
> that was a mount point in another mount namespace would result in
> -EBUSY. This behavior had technical problems of enforcement (nfs)
> and resulted in a nice denial of servial attack against more
> privileged users. (Aka preventing individual files from being updated
> by bind mounting on top of them).
I have reworked the text above a little so that now we have the following.
Aside from question above, does it look okay?
Restrictions on mount namespaces
Note the following points with respect to mount namespaces:
* A mount namespace has na owner user namespace. A mount
namespace whose owner user namespace is different from the
owner user namespace of its parent mount namespace is con‐
sidered a less privileged mount namespace.
* When creating a less privileged mount namespace, shared
mounts are reduced to slave mounts. This ensures that map‐
pings performed in less privileged mount namespaces will not
propagate to more privileged mount namespaces.
* Mounts that come as a single unit from more privileged mount
are locked together and may not be separated in a less priv‐
ileged mount namespace.
* The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the
"atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME) set‐
tings become locked when propagated from a more privileged
to a less privileged mount namespace, and may not be changed
in the less privileged mount namespace.
* A file or directory that is a mount point in one namespace
that is not a mount point in another namespace, may be
renamed, unlinked, or removed (rmdir(2)) in the mount names‐
pace in which it is not a mount point (subject to the usual
permission checks).
Previously, attempting to unlink, rename, or remove a file
or directory that was a mount point in another mount names‐
pace would result in the error EBUSY. That behavior had
technical problems of enforcement (e.g., for NFS) and per‐
mitted denial-of-service attacks against more privileged
users. (i.e., preventing individual files from being
updated by bind mounting on top of them).
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-08-30 21:53 ` Eric W. Biederman
2014-09-01 17:31 ` Michael Kerrisk (man-pages)
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
@ 2014-09-09 13:59 ` Michael Kerrisk (man-pages)
2014-09-09 15:51 ` Eric W. Biederman
2 siblings, 1 reply; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-09 13:59 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
On 08/30/2014 02:53 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
[...]
>> The initial user namespace has no parent namespace, but, for con‐
>> sistency, the kernel provides dummy user and group ID mapping
>> files for this namespace. Looking at the uid_map file (gid_map
>> is the same) from a shell in the initial namespace shows:
>>
>> $ cat /proc/$$/uid_map
>> 0 0 4294967295
>>
>> This mapping tells us that the range starting at user ID 0 in
>> this namespace maps to a range starting at 0 in the (nonexistent)
>> parent namespace, and the length of the range is the largest
>> 32-bit unsigned integer.
>
> Which deliberately leaves 4294967295 32bit (-1) unmapped. (uid_t)-1 is
> used in several interfaces (like setreuid) as a way to specify no uid
> leaving it unmapped and unusuable guarantees that there will be no
> confusion when using those kernel methods.
So, I worked that piece into the text to give:
This mapping tells us that the range starting at user ID 0 in
this namespace maps to a range starting at 0 in the (nonexis‐
tent) parent namespace, and the length of the range is the
largest 32-bit unsigned integer. (This deliberately leaves
4294967295 (the 32-bit signed -1 value) unmapped. This is
deliberate: (uid_t) -1 is used in several interfaces (e.g.,
setreuid(2)) as a way to specify "no user ID". Leaving
setreuid(2)) unmapped and unusuable guarantees that there will
be no confusion when using these interfaces.
Okay?
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-02 1:05 ` Eric W. Biederman
@ 2014-09-09 14:00 ` Michael Kerrisk (man-pages)
2014-09-09 16:16 ` Eric W. Biederman
0 siblings, 1 reply; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-09 14:00 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
Hi Eric,
> On a related note. One thing that has come up recently (in 3 separate
> implementations is that mount(MS_REMOUNT|...,...) must include all of
> the mount flags that need to be preserved. People creating read-only
> bind mounts tend to miss that and the locked flags in mount namespaces.
> That issue was flushed out now that the kernel is now not allowing most
> mount flags to be cleared in mount namespaces.
So, are you meaning that something needs to be added to the page
regarding this point?
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-01 20:57 ` Andy Lutomirski
@ 2014-09-09 14:00 ` Michael Kerrisk (man-pages)
2014-09-09 16:05 ` Eric W. Biederman
0 siblings, 1 reply; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-09 14:00 UTC (permalink / raw)
To: Andy Lutomirski
Cc: mtk.manpages, Eric W. Biederman, lkml, linux-man,
Linux Containers, richard -rw- weinberger, Serge E. Hallyn
Hi Andy, and Eric,
On 09/01/2014 01:57 PM, Andy Lutomirski wrote:
> On Wed, Aug 20, 2014 at 4:36 PM, Michael Kerrisk (man-pages)
> <mtk.manpages@gmail.com> wrote:
>> Hello Eric et al.,
>>
>> For various reasons, my work on the namespaces man pages
>> fell off the table a while back. Nevertheless, the pages have
>> been close to completion for a while now, and I recently restarted,
>> in an effort to finish them. As you also noted to me f2f, there have
>> been recently been some small namespace changes that you may affect
>> the content of the pages. Therefore, I'll take the opportunity to
>> send the namespace-related pages out for further (final?) review.
>>
>> So, here, I start with the user_namespaces(7) page, which is shown
>> in rendered form below, with source attached to this mail. I'll
>> send various other pages in follow-on mails.
>>
>> Review comments/suggestions for improvements / bug fixes welcome.
>>
>> Cheers,
>>
>> Michael
>>
>> ==
>>
>> NAME
>> user_namespaces - overview of Linux user_namespaces
>>
>> DESCRIPTION
>> For an overview of namespaces, see namespaces(7).
>>
>> User namespaces isolate security-related identifiers and
>> attributes, in particular, user IDs and group IDs (see creden‐
>> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
>
> Putting "root directory" here is odd -- that's really part of a
> different namespace. But user namespaces sort of isolate the other
> namespaces from each other.
I'm trying to remember the details here. I think this piece originally
came after a discussion with Eric, but I am not sure. Eric?
> Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?
>
>> ties (see capabilities(7)). A process's user and group IDs can
>> be different inside and outside a user namespace. In particular,
>> a process can have a normal unprivileged user ID outside a user
>> namespace while at the same time having a user ID of 0 inside the
>> namespace; in other words, the process has full privileges for
>> operations inside the user namespace, but is unprivileged for
>> operations outside the namespace.
>>
>> Nested namespaces, namespace membership
>> User namespaces can be nested; that is, each user namespace—
>> except the initial ("root") namespace—has a parent user names‐
>> pace, and can have zero or more child user namespaces. The par‐
>> ent user namespace is the user namespace of the process that cre‐
>> ates the user namespace via a call to unshare(2) or clone(2) with
>> the CLONE_NEWUSER flag.
>>
>> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
>> els of user namespaces. Calls to unshare(2) or clone(2) that
>> would cause this limit to be exceeded fail with the error EUSERS.
>>
>> Each process is a member of exactly one user namespace. A
>> process created via fork(2) or clone(2) without the CLONE_NEWUSER
>> flag is a member of the same user namespace as its parent. A
>> process can join another user namespace with setns(2) if it has
>> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
>> full set of capabilities in that namespace.
>>
>> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
>> makes the new child process (for clone(2)) or the caller (for
>> unshare(2)) a member of the new user namespace created by the
>> call.
>>
>> Capabilities
>> The child process created by clone(2) with the CLONE_NEWUSER flag
>> starts out with a complete set of capabilities in the new user
>> namespace. Likewise, a process that creates a new user namespace
>> using unshare(2) or joins an existing user namespace using
>> setns(2) gains a full set of capabilities in that namespace. On
>> the other hand, that process has no capabilities in the parent
>> (in the case of clone(2)) or previous (in the case of unshare(2)
>> and setns(2)) user namespace, even if the new namespace is cre‐
>> ated or joined by the root user (i.e., a process with user ID 0
>> in the root namespace).
>>
>> Note that a call to execve(2) will cause a process to lose any
>> capabilities that it has, unless it has a user ID of 0 within the
>> namespace.
>
> Or unless file capabilities have a non-empty inheritable mask.
>
> It may be worth mentioning that execve in a user namespace works
> exactly like execve outside a userns.
I';ve reworded that para to say:
Note that a call to execve(2) will cause a process's capabili‐
ties to be recalculated in the usual way (see capabilities(7)),
so that usually, unless it has a user ID of 0 within the names‐
pace or the executable file has a nonempty inheritable capabil‐
ities mask, it will lose all capabilities. See the discussion
of user and group ID mappings, below.
Okay?
>
>> $ cat /proc/$$/uid_map
>> 0 0 4294967295
>>
>> This mapping tells us that the range starting at user ID 0 in
>> this namespace maps to a range starting at 0 in the (nonexistent)
>> parent namespace, and the length of the range is the largest
>> 32-bit unsigned integer.
>>
>> Defining user and group ID mappings: writing to uid_map and gid_map
>> After the creation of a new user namespace, the uid_map file of
>> one of the processes in the namespace may be written to once to
>> define the mapping of user IDs in the new user namespace. An
>> attempt to write more than once to a uid_map file in a user
>> namespace fails with the error EPERM. Similar rules apply for
>> gid_map files.
>>
>> The lines written to uid_map (gid_map) must conform to the fol‐
>> lowing rules:
>>
>> * The three fields must be valid numbers, and the last field
>> must be greater than 0.
>>
>> * Lines are terminated by newline characters.
>>
>> * There is an (arbitrary) limit on the number of lines in the
>> file. As at Linux 3.8, the limit is five lines. In addition,
>> the number of bytes written to the file must be less than the
>> system page size, and the write must be performed at the start
>> of the file (i.e., lseek(2) and pwrite(2) can't be used to
>> write to nonzero offsets in the file).
>>
>> * The range of user IDs (group IDs) specified in each line can‐
>> not overlap with the ranges in any other lines. In the ini‐
>> tial implementation (Linux 3.8), this requirement was satis‐
>> fied by a simplistic implementation that imposed the further
>> requirement that the values in both field 1 and field 2 of
>> successive lines must be in ascending numerical order, which
>> prevented some otherwise valid maps from being created. Linux
>> 3.9 and later fix this limitation, allowing any valid set of
>> nonoverlapping maps.
>>
>> * At least one line must be written to the file.
>>
>> Writes that violate the above rules fail with the error EINVAL.
>>
>> In order for a process to write to the /proc/[pid]/uid_map
>> (/proc/[pid]/gid_map) file, all of the following requirements
>> must be met:
>>
>> 1. The writing process must have the CAP_SETUID (CAP_SETGID)
>> capability in the user namespace of the process pid.
>
> This checked for the opening process (and I don't actually remember
> whether it's checked for the writing process).
Eric, can you comment?
>>
>> 2. The writing process must be in either the user namespace of
>> the process pid or inside the parent user namespace of the
>> process pid.
>>
>> 3. The mapped user IDs (group IDs) must in turn have a mapping in
>> the parent user namespace.
>>
>> 4. One of the following is true:
>>
>> * The data written to uid_map (gid_map) consists of a single
>> line that maps the writing process's filesystem user ID
>> (group ID) in the parent user namespace to a user ID (group
>> ID) in the user namespace. The usual case here is that
>> this single line provides a mapping for user ID of the
>> process that created the namespace.
>>
>> * The process has the CAP_SETUID (CAP_SETGID) capability in
>> the parent user namespace. Thus, a privileged process can
>> make mappings to arbitrary user IDs (group IDs) in the par‐
>> ent user namespace.
>
> The opening process.
Fixed.
> One other thing that could be worth mentioning it: any non-user
> namespace that's created is owned by the user namespace of the process
> that created it at the time of creation. Actions on those namespaces
> require capabilities in the corresponding user namespace.
I added:
[[
When a non-user-namespace is created,
it is owned by the user namespace in which the creating process
was a member at the time of the creation of the namespace.
Actions on the non-user-namespace
require capabilities in the corresponding user namespace.
]]
> Thanks for doing this!
You're welcome. Thanks for the review!
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
@ 2014-09-09 15:49 ` Eric W. Biederman
2014-09-11 14:40 ` Michael Kerrisk (man-pages)
0 siblings, 1 reply; 26+ messages in thread
From: Eric W. Biederman @ 2014-09-09 15:49 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: lkml, linux-man, containers, Andy Lutomirski, richard.weinberger,
Serge E. Hallyn
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> Hi Eric,
>
> On 08/30/2014 02:53 PM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>
>>> Hello Eric et al.,
>>>
>>> For various reasons, my work on the namespaces man pages
>>> fell off the table a while back. Nevertheless, the pages have
>>> been close to completion for a while now, and I recently restarted,
>>> in an effort to finish them. As you also noted to me f2f, there have
>>> been recently been some small namespace changes that you may affect
>>> the content of the pages. Therefore, I'll take the opportunity to
>>> send the namespace-related pages out for further (final?) review.
>>>
>>> So, here, I start with the user_namespaces(7) page, which is shown
>>> in rendered form below, with source attached to this mail. I'll
>>> send various other pages in follow-on mails.
>>>
>>> Review comments/suggestions for improvements / bug fixes welcome.
>>>
>>> Cheers,
>>>
>>> Michael
>>>
>>> ==
>>>
>>> NAME
>>> user_namespaces - overview of Linux user_namespaces
>>>
> [...]
>
>>> When a new IPC, mount, network, PID, or UTS namespace is created
>>> via clone(2) or unshare(2), the kernel records the user namespace
>>> of the creating process against the new namespace. (This associ‐
>>> ation can't be changed.) When a process in the new namespace
>>> subsequently performs privileged operations that operate on
>>> global resources isolated by the namespace, the permission checks
>>> are performed according to the process's capabilities in the user
>>> namespace that the kernel associated with the new namespace.
>>
>> Restrictions on mount namespaces.
>>
>> - A mount namespace has a owner user namespace. A mount namespace whose
>> owner user namespace is different than the owerner user namespace of
>> it's parent mount namespace is considered a less privileged mount
>> namespace.
>>
>> - When creating a less privileged mount namespace shared mounts are
>> reduced to slave mounts. This ensures that mappings performed in less
>> privileged mount namespaces will not propogate to more privielged
>> mount namespaces.
>>
>> - Mounts that come as a single unit from more privileged mount are
>> locked together and may not be separated in a less privielged mount
>> namespace.
>
> Could you clarify what you mean by "Mounts that come as a single
> unit"?
unshare(CLONE_NEWNS) brings across all of the mounts from the original
mount namespace as a single unit.
recursive mounts that propogate between mount namespaces propogate as a
single unit.
The importance of this is allow the global root to mount over things
and not have to worry that someone from a user namespace root can
peek underneath.
>> - The mount flags readonly, nodev, nosuid, noexec, and the mount atime
>> settings when propogated from a more privielged to a less privileged
>> mount namespace become locked, and may not be changed in the less
>> privielged mount namespace.
>>
>> - (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or
>> directory that is a mountpoint in one namespace that is not a mount
>> point in another namespace, may be renamed, unlinked, or rmdired in
>> the mount namespace in which it is not a mount namespace if the
>> ordinary permission checks pass.
>>
>> Previously attemping to rmdir, unlink or rename a file or directory
>> that was a mount point in another mount namespace would result in
>> -EBUSY. This behavior had technical problems of enforcement (nfs)
>> and resulted in a nice denial of servial attack against more
>> privileged users. (Aka preventing individual files from being updated
>> by bind mounting on top of them).
>
> I have reworked the text above a little so that now we have the following.
> Aside from question above, does it look okay?
>
> Restrictions on mount namespaces
> Note the following points with respect to mount namespaces:
>
> * A mount namespace has na owner user namespace. A mount
^s/na/an/
> namespace whose owner user namespace is different from the
> owner user namespace of its parent mount namespace is con‐
> sidered a less privileged mount namespace.
>
> * When creating a less privileged mount namespace, shared
> mounts are reduced to slave mounts. This ensures that map‐
> pings performed in less privileged mount namespaces will not
> propagate to more privileged mount namespaces.
>
> * Mounts that come as a single unit from more privileged mount
^ namespaces
> are locked together and may not be separated in a less priv‐
> ileged mount namespace.
>
> * The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the
> "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME) set‐
> tings become locked when propagated from a more privileged
> to a less privileged mount namespace, and may not be changed
> in the less privileged mount namespace.
>
> * A file or directory that is a mount point in one namespace
> that is not a mount point in another namespace, may be
> renamed, unlinked, or removed (rmdir(2)) in the mount names‐
> pace in which it is not a mount point (subject to the usual
> permission checks).
>
> Previously, attempting to unlink, rename, or remove a file
> or directory that was a mount point in another mount names‐
> pace would result in the error EBUSY. That behavior had
> technical problems of enforcement (e.g., for NFS) and per‐
> mitted denial-of-service attacks against more privileged
> users. (i.e., preventing individual files from being
> updated by bind mounting on top of them).
Subject to tiny typo corrections that looks fine.
Eric
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
@ 2014-09-09 15:51 ` Eric W. Biederman
2014-09-11 14:40 ` Michael Kerrisk (man-pages)
0 siblings, 1 reply; 26+ messages in thread
From: Eric W. Biederman @ 2014-09-09 15:51 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: lkml, linux-man, containers, Andy Lutomirski, richard.weinberger,
Serge E. Hallyn
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> On 08/30/2014 02:53 PM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> [...]
>
>
>>> The initial user namespace has no parent namespace, but, for con‐
>>> sistency, the kernel provides dummy user and group ID mapping
>>> files for this namespace. Looking at the uid_map file (gid_map
>>> is the same) from a shell in the initial namespace shows:
>>>
>>> $ cat /proc/$$/uid_map
>>> 0 0 4294967295
>>>
>>> This mapping tells us that the range starting at user ID 0 in
>>> this namespace maps to a range starting at 0 in the (nonexistent)
>>> parent namespace, and the length of the range is the largest
>>> 32-bit unsigned integer.
>>
>> Which deliberately leaves 4294967295 32bit (-1) unmapped. (uid_t)-1 is
>> used in several interfaces (like setreuid) as a way to specify no uid
>> leaving it unmapped and unusuable guarantees that there will be no
>> confusion when using those kernel methods.
>
> So, I worked that piece into the text to give:
>
> This mapping tells us that the range starting at user ID 0 in
> this namespace maps to a range starting at 0 in the (nonexis‐
> tent) parent namespace, and the length of the range is the
> largest 32-bit unsigned integer. (This deliberately leaves
> 4294967295 (the 32-bit signed -1 value) unmapped. This is
> deliberate: (uid_t) -1 is used in several interfaces (e.g.,
> setreuid(2)) as a way to specify "no user ID". Leaving
> setreuid(2)) unmapped and unusuable guarantees that there will
^^^^ (uid_t) -1 (not setreuid(2)
> be no confusion when using these interfaces.
>
> Okay?
Other than the typo fix above this looks good.
Eric
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 14:00 ` Michael Kerrisk (man-pages)
@ 2014-09-09 16:05 ` Eric W. Biederman
2014-09-09 19:26 ` Andy Lutomirski
2014-09-11 14:46 ` Michael Kerrisk (man-pages)
0 siblings, 2 replies; 26+ messages in thread
From: Eric W. Biederman @ 2014-09-09 16:05 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: Andy Lutomirski, lkml, linux-man, Linux Containers,
richard -rw- weinberger, Serge E. Hallyn
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> Hi Andy, and Eric,
>
> On 09/01/2014 01:57 PM, Andy Lutomirski wrote:
>> On Wed, Aug 20, 2014 at 4:36 PM, Michael Kerrisk (man-pages)
>> <mtk.manpages@gmail.com> wrote:
>>> Hello Eric et al.,
>>>
>>> For various reasons, my work on the namespaces man pages
>>> fell off the table a while back. Nevertheless, the pages have
>>> been close to completion for a while now, and I recently restarted,
>>> in an effort to finish them. As you also noted to me f2f, there have
>>> been recently been some small namespace changes that you may affect
>>> the content of the pages. Therefore, I'll take the opportunity to
>>> send the namespace-related pages out for further (final?) review.
>>>
>>> So, here, I start with the user_namespaces(7) page, which is shown
>>> in rendered form below, with source attached to this mail. I'll
>>> send various other pages in follow-on mails.
>>>
>>> Review comments/suggestions for improvements / bug fixes welcome.
>>>
>>> Cheers,
>>>
>>> Michael
>>>
>>> ==
>>>
>>> NAME
>>> user_namespaces - overview of Linux user_namespaces
>>>
>>> DESCRIPTION
>>> For an overview of namespaces, see namespaces(7).
>>>
>>> User namespaces isolate security-related identifiers and
>>> attributes, in particular, user IDs and group IDs (see creden‐
>>> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
>>
>> Putting "root directory" here is odd -- that's really part of a
>> different namespace. But user namespaces sort of isolate the other
>> namespaces from each other.
>
> I'm trying to remember the details here. I think this piece originally
> came after a discussion with Eric, but I am not sure. Eric?
Probably.
I am not certain what the best way to say it but we do need to document
that an unprivileged user that creates a user namespace can now call
chroot.
We may also want to discuss the specific restrictions on chroot.
The text about chroot at least gives people a strong hint that the
chroot rules are affected by user namespaces.
The restrictions that we have settled on to avoid chroot being a problem
are the creator of a user namespace must not be chrooted in their
current mount namespace, and the creator of the user namespace must not
be threaded.
Andy can you check me on this it looks like unshare is currently buggy
in that it will allow a threaded application to create a user namespace.
>> Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?
>>
>>> ties (see capabilities(7)). A process's user and group IDs can
>>> be different inside and outside a user namespace. In particular,
>>> a process can have a normal unprivileged user ID outside a user
>>> namespace while at the same time having a user ID of 0 inside the
>>> namespace; in other words, the process has full privileges for
>>> operations inside the user namespace, but is unprivileged for
>>> operations outside the namespace.
>>>
>>> Nested namespaces, namespace membership
>>> User namespaces can be nested; that is, each user namespace—
>>> except the initial ("root") namespace—has a parent user names‐
>>> pace, and can have zero or more child user namespaces. The par‐
>>> ent user namespace is the user namespace of the process that cre‐
>>> ates the user namespace via a call to unshare(2) or clone(2) with
>>> the CLONE_NEWUSER flag.
>>>
>>> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
>>> els of user namespaces. Calls to unshare(2) or clone(2) that
>>> would cause this limit to be exceeded fail with the error EUSERS.
>>>
>>> Each process is a member of exactly one user namespace. A
>>> process created via fork(2) or clone(2) without the CLONE_NEWUSER
>>> flag is a member of the same user namespace as its parent. A
>>> process can join another user namespace with setns(2) if it has
>>> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
>>> full set of capabilities in that namespace.
>>>
>>> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
>>> makes the new child process (for clone(2)) or the caller (for
>>> unshare(2)) a member of the new user namespace created by the
>>> call.
>>>
>>> Capabilities
>>> The child process created by clone(2) with the CLONE_NEWUSER flag
>>> starts out with a complete set of capabilities in the new user
>>> namespace. Likewise, a process that creates a new user namespace
>>> using unshare(2) or joins an existing user namespace using
>>> setns(2) gains a full set of capabilities in that namespace. On
>>> the other hand, that process has no capabilities in the parent
>>> (in the case of clone(2)) or previous (in the case of unshare(2)
>>> and setns(2)) user namespace, even if the new namespace is cre‐
>>> ated or joined by the root user (i.e., a process with user ID 0
>>> in the root namespace).
>>>
>>> Note that a call to execve(2) will cause a process to lose any
>>> capabilities that it has, unless it has a user ID of 0 within the
>>> namespace.
>>
>> Or unless file capabilities have a non-empty inheritable mask.
>>
>> It may be worth mentioning that execve in a user namespace works
>> exactly like execve outside a userns.
>
>
> I';ve reworded that para to say:
>
> Note that a call to execve(2) will cause a process's capabili‐
> ties to be recalculated in the usual way (see capabilities(7)),
> so that usually, unless it has a user ID of 0 within the names‐
> pace or the executable file has a nonempty inheritable capabil‐
> ities mask, it will lose all capabilities. See the discussion
> of user and group ID mappings, below.
>
> Okay?
That seems reasonable to me.
>>> $ cat /proc/$$/uid_map
>>> 0 0 4294967295
>>>
>>> This mapping tells us that the range starting at user ID 0 in
>>> this namespace maps to a range starting at 0 in the (nonexistent)
>>> parent namespace, and the length of the range is the largest
>>> 32-bit unsigned integer.
>>>
>>> Defining user and group ID mappings: writing to uid_map and gid_map
>>> After the creation of a new user namespace, the uid_map file of
>>> one of the processes in the namespace may be written to once to
>>> define the mapping of user IDs in the new user namespace. An
>>> attempt to write more than once to a uid_map file in a user
>>> namespace fails with the error EPERM. Similar rules apply for
>>> gid_map files.
>>>
>>> The lines written to uid_map (gid_map) must conform to the fol‐
>>> lowing rules:
>>>
>>> * The three fields must be valid numbers, and the last field
>>> must be greater than 0.
>>>
>>> * Lines are terminated by newline characters.
>>>
>>> * There is an (arbitrary) limit on the number of lines in the
>>> file. As at Linux 3.8, the limit is five lines. In addition,
>>> the number of bytes written to the file must be less than the
>>> system page size, and the write must be performed at the start
>>> of the file (i.e., lseek(2) and pwrite(2) can't be used to
>>> write to nonzero offsets in the file).
>>>
>>> * The range of user IDs (group IDs) specified in each line can‐
>>> not overlap with the ranges in any other lines. In the ini‐
>>> tial implementation (Linux 3.8), this requirement was satis‐
>>> fied by a simplistic implementation that imposed the further
>>> requirement that the values in both field 1 and field 2 of
>>> successive lines must be in ascending numerical order, which
>>> prevented some otherwise valid maps from being created. Linux
>>> 3.9 and later fix this limitation, allowing any valid set of
>>> nonoverlapping maps.
>>>
>>> * At least one line must be written to the file.
>>>
>>> Writes that violate the above rules fail with the error EINVAL.
>>>
>>> In order for a process to write to the /proc/[pid]/uid_map
>>> (/proc/[pid]/gid_map) file, all of the following requirements
>>> must be met:
>>>
>>> 1. The writing process must have the CAP_SETUID (CAP_SETGID)
>>> capability in the user namespace of the process pid.
>>
>> This checked for the opening process (and I don't actually remember
>> whether it's checked for the writing process).
>
> Eric, can you comment?
We have to check for the opening processes and that changes was made
after I implemented my interface. Pieces of the code appear to also
examine the writing process and verify everything applies to it as well.
I goofed when I designed the interface originall and had not realized
what a classic design error it can be to not restrict by the opening
process.
>>> 2. The writing process must be in either the user namespace of
>>> the process pid or inside the parent user namespace of the
>>> process pid.
>>>
>>> 3. The mapped user IDs (group IDs) must in turn have a mapping in
>>> the parent user namespace.
>>>
>>> 4. One of the following is true:
>>>
>>> * The data written to uid_map (gid_map) consists of a single
>>> line that maps the writing process's filesystem user ID
>>> (group ID) in the parent user namespace to a user ID (group
>>> ID) in the user namespace. The usual case here is that
>>> this single line provides a mapping for user ID of the
>>> process that created the namespace.
>>>
>>> * The process has the CAP_SETUID (CAP_SETGID) capability in
>>> the parent user namespace. Thus, a privileged process can
>>> make mappings to arbitrary user IDs (group IDs) in the par‐
>>> ent user namespace.
>>
>> The opening process.
>
> Fixed.
>
>> One other thing that could be worth mentioning it: any non-user
>> namespace that's created is owned by the user namespace of the process
>> that created it at the time of creation. Actions on those namespaces
>> require capabilities in the corresponding user namespace.
>
> I added:
>
> [[
> When a non-user-namespace is created,
> it is owned by the user namespace in which the creating process
> was a member at the time of the creation of the namespace.
> Actions on the non-user-namespace
> require capabilities in the corresponding user namespace.
> ]]
>
>> Thanks for doing this!
>
> You're welcome. Thanks for the review!
Eric
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 14:00 ` Michael Kerrisk (man-pages)
@ 2014-09-09 16:16 ` Eric W. Biederman
2014-09-11 14:40 ` Michael Kerrisk (man-pages)
0 siblings, 1 reply; 26+ messages in thread
From: Eric W. Biederman @ 2014-09-09 16:16 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: lkml, linux-man, containers, Andy Lutomirski, richard.weinberger,
Serge E. Hallyn
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> Hi Eric,
>
>> On a related note. One thing that has come up recently (in 3 separate
>> implementations is that mount(MS_REMOUNT|...,...) must include all of
>> the mount flags that need to be preserved. People creating read-only
>> bind mounts tend to miss that and the locked flags in mount namespaces.
>> That issue was flushed out now that the kernel is now not allowing most
>> mount flags to be cleared in mount namespaces.
>
> So, are you meaning that something needs to be added to the page
> regarding this point?
Yes. The interface is non-intuitive and we should at least document
the weirdness.
I recommend updating the mount(2) man page to say something like:
MS_REMOUNT
Remount an existing mount. This allows you to change the
mountflags and data of an existing mount without having
to unmount and remount the file system. target should be
the same value specified in the initial mount() call;
source and filesystemtype are ignored.
^^^^^^^^^^^^^
Mountflags and data should match the original mount system
call except those parameters that are being deliberately
changed.
The following mountflags can be changed: MS_RDONLY,
MS_SYNCHRONOUS, MS_MANDLOCK; before kernel 2.6.16, the
following could also be changed: MS_NOATIME and
MS_NODIRATIME; and, additionally, before kernel 2.4.10,
the following could also be changed: MS_NOSUID, MS_NODEV,
MS_NOEXEC.
Eric
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 16:05 ` Eric W. Biederman
@ 2014-09-09 19:26 ` Andy Lutomirski
2014-09-09 19:39 ` Andy Lutomirski
2014-09-11 14:47 ` Michael Kerrisk (man-pages)
2014-09-11 14:46 ` Michael Kerrisk (man-pages)
1 sibling, 2 replies; 26+ messages in thread
From: Andy Lutomirski @ 2014-09-09 19:26 UTC (permalink / raw)
To: Eric W. Biederman
Cc: Michael Kerrisk (man-pages),
lkml, linux-man, Linux Containers, richard -rw- weinberger,
Serge E. Hallyn
On Tue, Sep 9, 2014 at 9:05 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> We may also want to discuss the specific restrictions on chroot.
>
> The text about chroot at least gives people a strong hint that the
> chroot rules are affected by user namespaces.
>
> The restrictions that we have settled on to avoid chroot being a problem
> are the creator of a user namespace must not be chrooted in their
> current mount namespace, and the creator of the user namespace must not
> be threaded.
>
> Andy can you check me on this it looks like unshare is currently buggy
> in that it will allow a threaded application to create a user namespace.
I think it's this code in unshare:
/*
* If unsharing a user namespace must also unshare the thread.
*/
if (unshare_flags & CLONE_NEWUSER)
unshare_flags |= CLONE_THREAD | CLONE_FS;
I suppose that this should be documented.
CLONE_FS prevents the chroot from leaking out of the namespace. (But
see the other thread that I'm about to start...)
--Andy
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 19:26 ` Andy Lutomirski
@ 2014-09-09 19:39 ` Andy Lutomirski
2014-09-11 14:47 ` Michael Kerrisk (man-pages)
1 sibling, 0 replies; 26+ messages in thread
From: Andy Lutomirski @ 2014-09-09 19:39 UTC (permalink / raw)
To: Eric W. Biederman
Cc: Michael Kerrisk (man-pages),
lkml, linux-man, Linux Containers, richard -rw- weinberger,
Serge E. Hallyn
On Tue, Sep 9, 2014 at 12:26 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Tue, Sep 9, 2014 at 9:05 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>> We may also want to discuss the specific restrictions on chroot.
>>
>> The text about chroot at least gives people a strong hint that the
>> chroot rules are affected by user namespaces.
>>
>> The restrictions that we have settled on to avoid chroot being a problem
>> are the creator of a user namespace must not be chrooted in their
>> current mount namespace, and the creator of the user namespace must not
>> be threaded.
>>
>> Andy can you check me on this it looks like unshare is currently buggy
>> in that it will allow a threaded application to create a user namespace.
>
> I think it's this code in unshare:
>
> /*
> * If unsharing a user namespace must also unshare the thread.
> */
> if (unshare_flags & CLONE_NEWUSER)
> unshare_flags |= CLONE_THREAD | CLONE_FS;
>
> I suppose that this should be documented.
>
> CLONE_FS prevents the chroot from leaking out of the namespace. (But
> see the other thread that I'm about to start...)
Never mind. I thought I saw a bug, but I was wrong.
--Andy
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 16:16 ` Eric W. Biederman
@ 2014-09-11 14:40 ` Michael Kerrisk (man-pages)
0 siblings, 0 replies; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-11 14:40 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
On 09/09/2014 09:16 AM, Eric W. Biederman wrote:
>>> On a related note. One thing that has come up recently (in 3 separate
>>> >> implementations is that mount(MS_REMOUNT|...,...) must include all of
>>> >> the mount flags that need to be preserved. People creating read-only
>>> >> bind mounts tend to miss that and the locked flags in mount namespaces.
>>> >> That issue was flushed out now that the kernel is now not allowing most
>>> >> mount flags to be cleared in mount namespaces.
>> >
>> > So, are you meaning that something needs to be added to the page
>> > regarding this point?
> Yes. The interface is non-intuitive and we should at least document
> the weirdness.
Okay -- I have added that piece to mount(2).
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 15:49 ` Eric W. Biederman
@ 2014-09-11 14:40 ` Michael Kerrisk (man-pages)
0 siblings, 0 replies; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-11 14:40 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
On 09/09/2014 08:49 AM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>
>> Hi Eric,
>>
>> On 08/30/2014 02:53 PM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>
>>>> Hello Eric et al.,
>>>>
>>>> For various reasons, my work on the namespaces man pages
>>>> fell off the table a while back. Nevertheless, the pages have
>>>> been close to completion for a while now, and I recently restarted,
>>>> in an effort to finish them. As you also noted to me f2f, there have
>>>> been recently been some small namespace changes that you may affect
>>>> the content of the pages. Therefore, I'll take the opportunity to
>>>> send the namespace-related pages out for further (final?) review.
>>>>
>>>> So, here, I start with the user_namespaces(7) page, which is shown
>>>> in rendered form below, with source attached to this mail. I'll
>>>> send various other pages in follow-on mails.
>>>>
>>>> Review comments/suggestions for improvements / bug fixes welcome.
>>>>
>>>> Cheers,
>>>>
>>>> Michael
>>>>
>>>> ==
>>>>
>>>> NAME
>>>> user_namespaces - overview of Linux user_namespaces
>>>>
>> [...]
>>
>>>> When a new IPC, mount, network, PID, or UTS namespace is created
>>>> via clone(2) or unshare(2), the kernel records the user namespace
>>>> of the creating process against the new namespace. (This associ‐
>>>> ation can't be changed.) When a process in the new namespace
>>>> subsequently performs privileged operations that operate on
>>>> global resources isolated by the namespace, the permission checks
>>>> are performed according to the process's capabilities in the user
>>>> namespace that the kernel associated with the new namespace.
>>>
>>> Restrictions on mount namespaces.
>>>
>>> - A mount namespace has a owner user namespace. A mount namespace whose
>>> owner user namespace is different than the owerner user namespace of
>>> it's parent mount namespace is considered a less privileged mount
>>> namespace.
>>>
>>> - When creating a less privileged mount namespace shared mounts are
>>> reduced to slave mounts. This ensures that mappings performed in less
>>> privileged mount namespaces will not propogate to more privielged
>>> mount namespaces.
>>>
>>> - Mounts that come as a single unit from more privileged mount are
>>> locked together and may not be separated in a less privielged mount
>>> namespace.
>>
>> Could you clarify what you mean by "Mounts that come as a single
>> unit"?
>
> unshare(CLONE_NEWNS) brings across all of the mounts from the original
> mount namespace as a single unit.
>
> recursive mounts that propogate between mount namespaces propogate as a
> single unit.
Thanks, I've added those details to the page.
> The importance of this is allow the global root to mount over things
> and not have to worry that someone from a user namespace root can
> peek underneath.
>
>>> - The mount flags readonly, nodev, nosuid, noexec, and the mount atime
>>> settings when propogated from a more privielged to a less privileged
>>> mount namespace become locked, and may not be changed in the less
>>> privielged mount namespace.
>>>
>>> - (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or
>>> directory that is a mountpoint in one namespace that is not a mount
>>> point in another namespace, may be renamed, unlinked, or rmdired in
>>> the mount namespace in which it is not a mount namespace if the
>>> ordinary permission checks pass.
>>>
>>> Previously attemping to rmdir, unlink or rename a file or directory
>>> that was a mount point in another mount namespace would result in
>>> -EBUSY. This behavior had technical problems of enforcement (nfs)
>>> and resulted in a nice denial of servial attack against more
>>> privileged users. (Aka preventing individual files from being updated
>>> by bind mounting on top of them).
>>
>> I have reworked the text above a little so that now we have the following.
>> Aside from question above, does it look okay?
>>
>> Restrictions on mount namespaces
>> Note the following points with respect to mount namespaces:
>>
>> * A mount namespace has na owner user namespace. A mount
> ^s/na/an/
>> namespace whose owner user namespace is different from the
>> owner user namespace of its parent mount namespace is con‐
>> sidered a less privileged mount namespace.
>>
>> * When creating a less privileged mount namespace, shared
>> mounts are reduced to slave mounts. This ensures that map‐
>> pings performed in less privileged mount namespaces will not
>> propagate to more privileged mount namespaces.
>>
>> * Mounts that come as a single unit from more privileged mount
> ^ namespaces
>> are locked together and may not be separated in a less priv‐
>> ileged mount namespace.
>>
>> * The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the
>> "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME) set‐
>> tings become locked when propagated from a more privileged
>> to a less privileged mount namespace, and may not be changed
>> in the less privileged mount namespace.
>>
>> * A file or directory that is a mount point in one namespace
>> that is not a mount point in another namespace, may be
>> renamed, unlinked, or removed (rmdir(2)) in the mount names‐
>> pace in which it is not a mount point (subject to the usual
>> permission checks).
>>
>> Previously, attempting to unlink, rename, or remove a file
>> or directory that was a mount point in another mount names‐
>> pace would result in the error EBUSY. That behavior had
>> technical problems of enforcement (e.g., for NFS) and per‐
>> mitted denial-of-service attacks against more privileged
>> users. (i.e., preventing individual files from being
>> updated by bind mounting on top of them).
>
> Subject to tiny typo corrections that looks fine.
Yup, I already found and fixed ;-).
Thanks, Eric.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 15:51 ` Eric W. Biederman
@ 2014-09-11 14:40 ` Michael Kerrisk (man-pages)
0 siblings, 0 replies; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-11 14:40 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, containers, Andy Lutomirski,
richard.weinberger, Serge E. Hallyn
On 09/09/2014 08:51 AM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>
>> On 08/30/2014 02:53 PM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>> [...]
>>
>>
>>>> The initial user namespace has no parent namespace, but, for con‐
>>>> sistency, the kernel provides dummy user and group ID mapping
>>>> files for this namespace. Looking at the uid_map file (gid_map
>>>> is the same) from a shell in the initial namespace shows:
>>>>
>>>> $ cat /proc/$$/uid_map
>>>> 0 0 4294967295
>>>>
>>>> This mapping tells us that the range starting at user ID 0 in
>>>> this namespace maps to a range starting at 0 in the (nonexistent)
>>>> parent namespace, and the length of the range is the largest
>>>> 32-bit unsigned integer.
>>>
>>> Which deliberately leaves 4294967295 32bit (-1) unmapped. (uid_t)-1 is
>>> used in several interfaces (like setreuid) as a way to specify no uid
>>> leaving it unmapped and unusuable guarantees that there will be no
>>> confusion when using those kernel methods.
>>
>> So, I worked that piece into the text to give:
>>
>> This mapping tells us that the range starting at user ID 0 in
>> this namespace maps to a range starting at 0 in the (nonexis‐
>> tent) parent namespace, and the length of the range is the
>> largest 32-bit unsigned integer. (This deliberately leaves
>> 4294967295 (the 32-bit signed -1 value) unmapped. This is
>> deliberate: (uid_t) -1 is used in several interfaces (e.g.,
>> setreuid(2)) as a way to specify "no user ID". Leaving
>> setreuid(2)) unmapped and unusuable guarantees that there will
> ^^^^ (uid_t) -1 (not setreuid(2)
>> be no confusion when using these interfaces.
>>
>> Okay?
>
> Other than the typo fix above this looks good.
Ahhh -- thanks for catching that, Eric. Fixed now.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 16:05 ` Eric W. Biederman
2014-09-09 19:26 ` Andy Lutomirski
@ 2014-09-11 14:46 ` Michael Kerrisk (man-pages)
2014-09-11 15:14 ` Andy Lutomirski
1 sibling, 1 reply; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-11 14:46 UTC (permalink / raw)
To: Eric W. Biederman
Cc: mtk.manpages, Andy Lutomirski, lkml, linux-man, Linux Containers,
richard -rw- weinberger, Serge E. Hallyn
Hi Eric,
On 09/09/2014 09:05 AM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>
>> Hi Andy, and Eric,
>>
>> On 09/01/2014 01:57 PM, Andy Lutomirski wrote:
>>> On Wed, Aug 20, 2014 at 4:36 PM, Michael Kerrisk (man-pages)
>>> <mtk.manpages@gmail.com> wrote:
>>>> Hello Eric et al.,
>>>>
>>>> For various reasons, my work on the namespaces man pages
>>>> fell off the table a while back. Nevertheless, the pages have
>>>> been close to completion for a while now, and I recently restarted,
>>>> in an effort to finish them. As you also noted to me f2f, there have
>>>> been recently been some small namespace changes that you may affect
>>>> the content of the pages. Therefore, I'll take the opportunity to
>>>> send the namespace-related pages out for further (final?) review.
>>>>
>>>> So, here, I start with the user_namespaces(7) page, which is shown
>>>> in rendered form below, with source attached to this mail. I'll
>>>> send various other pages in follow-on mails.
>>>>
>>>> Review comments/suggestions for improvements / bug fixes welcome.
>>>>
>>>> Cheers,
>>>>
>>>> Michael
>>>>
>>>> ==
>>>>
>>>> NAME
>>>> user_namespaces - overview of Linux user_namespaces
>>>>
>>>> DESCRIPTION
>>>> For an overview of namespaces, see namespaces(7).
>>>>
>>>> User namespaces isolate security-related identifiers and
>>>> attributes, in particular, user IDs and group IDs (see creden‐
>>>> tials(7), the root directory, keys (see keyctl(2)), and capabili‐
>>>
>>> Putting "root directory" here is odd -- that's really part of a
>>> different namespace. But user namespaces sort of isolate the other
>>> namespaces from each other.
>>
>> I'm trying to remember the details here. I think this piece originally
>> came after a discussion with Eric, but I am not sure. Eric?
>
> Probably.
>
> I am not certain what the best way to say it but we do need to document
> that an unprivileged user that creates a user namespace can now call
> chroot.
>
> We may also want to discuss the specific restrictions on chroot.
>
> The text about chroot at least gives people a strong hint that the
> chroot rules are affected by user namespaces.
>
> The restrictions that we have settled on to avoid chroot being a problem
> are the creator of a user namespace must not be chrooted in their
> current mount namespace, and the creator of the user namespace must not
> be threaded.
>
> Andy can you check me on this it looks like unshare is currently buggy
> in that it will allow a threaded application to create a user namespace.
So, somewhere we should have some text such as:
[[
An unprivileged user who creates a namespace can call chroot(2)
within that namesapce, subject to the restriction that the
creator of a user namespace must not be chrooted in their
current mount namespace, and the creator of the user namespace must not
be threaded.
]]
Right?
>>> Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?
>>>
>>>> ties (see capabilities(7)). A process's user and group IDs can
>>>> be different inside and outside a user namespace. In particular,
>>>> a process can have a normal unprivileged user ID outside a user
>>>> namespace while at the same time having a user ID of 0 inside the
>>>> namespace; in other words, the process has full privileges for
>>>> operations inside the user namespace, but is unprivileged for
>>>> operations outside the namespace.
>>>>
>>>> Nested namespaces, namespace membership
>>>> User namespaces can be nested; that is, each user namespace—
>>>> except the initial ("root") namespace—has a parent user names‐
>>>> pace, and can have zero or more child user namespaces. The par‐
>>>> ent user namespace is the user namespace of the process that cre‐
>>>> ates the user namespace via a call to unshare(2) or clone(2) with
>>>> the CLONE_NEWUSER flag.
>>>>
>>>> The kernel imposes (since version 3.11) a limit of 32 nested lev‐
>>>> els of user namespaces. Calls to unshare(2) or clone(2) that
>>>> would cause this limit to be exceeded fail with the error EUSERS.
>>>>
>>>> Each process is a member of exactly one user namespace. A
>>>> process created via fork(2) or clone(2) without the CLONE_NEWUSER
>>>> flag is a member of the same user namespace as its parent. A
>>>> process can join another user namespace with setns(2) if it has
>>>> the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
>>>> full set of capabilities in that namespace.
>>>>
>>>> A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
>>>> makes the new child process (for clone(2)) or the caller (for
>>>> unshare(2)) a member of the new user namespace created by the
>>>> call.
>>>>
>>>> Capabilities
>>>> The child process created by clone(2) with the CLONE_NEWUSER flag
>>>> starts out with a complete set of capabilities in the new user
>>>> namespace. Likewise, a process that creates a new user namespace
>>>> using unshare(2) or joins an existing user namespace using
>>>> setns(2) gains a full set of capabilities in that namespace. On
>>>> the other hand, that process has no capabilities in the parent
>>>> (in the case of clone(2)) or previous (in the case of unshare(2)
>>>> and setns(2)) user namespace, even if the new namespace is cre‐
>>>> ated or joined by the root user (i.e., a process with user ID 0
>>>> in the root namespace).
>>>>
>>>> Note that a call to execve(2) will cause a process to lose any
>>>> capabilities that it has, unless it has a user ID of 0 within the
>>>> namespace.
>>>
>>> Or unless file capabilities have a non-empty inheritable mask.
>>>
>>> It may be worth mentioning that execve in a user namespace works
>>> exactly like execve outside a userns.
>>
>>
>> I';ve reworded that para to say:
>>
>> Note that a call to execve(2) will cause a process's capabili‐
>> ties to be recalculated in the usual way (see capabilities(7)),
>> so that usually, unless it has a user ID of 0 within the names‐
>> pace or the executable file has a nonempty inheritable capabil‐
>> ities mask, it will lose all capabilities. See the discussion
>> of user and group ID mappings, below.
>>
>> Okay?
>
> That seems reasonable to me.
>
>>>> $ cat /proc/$$/uid_map
>>>> 0 0 4294967295
>>>>
>>>> This mapping tells us that the range starting at user ID 0 in
>>>> this namespace maps to a range starting at 0 in the (nonexistent)
>>>> parent namespace, and the length of the range is the largest
>>>> 32-bit unsigned integer.
>>>>
>>>> Defining user and group ID mappings: writing to uid_map and gid_map
>>>> After the creation of a new user namespace, the uid_map file of
>>>> one of the processes in the namespace may be written to once to
>>>> define the mapping of user IDs in the new user namespace. An
>>>> attempt to write more than once to a uid_map file in a user
>>>> namespace fails with the error EPERM. Similar rules apply for
>>>> gid_map files.
>>>>
>>>> The lines written to uid_map (gid_map) must conform to the fol‐
>>>> lowing rules:
>>>>
>>>> * The three fields must be valid numbers, and the last field
>>>> must be greater than 0.
>>>>
>>>> * Lines are terminated by newline characters.
>>>>
>>>> * There is an (arbitrary) limit on the number of lines in the
>>>> file. As at Linux 3.8, the limit is five lines. In addition,
>>>> the number of bytes written to the file must be less than the
>>>> system page size, and the write must be performed at the start
>>>> of the file (i.e., lseek(2) and pwrite(2) can't be used to
>>>> write to nonzero offsets in the file).
>>>>
>>>> * The range of user IDs (group IDs) specified in each line can‐
>>>> not overlap with the ranges in any other lines. In the ini‐
>>>> tial implementation (Linux 3.8), this requirement was satis‐
>>>> fied by a simplistic implementation that imposed the further
>>>> requirement that the values in both field 1 and field 2 of
>>>> successive lines must be in ascending numerical order, which
>>>> prevented some otherwise valid maps from being created. Linux
>>>> 3.9 and later fix this limitation, allowing any valid set of
>>>> nonoverlapping maps.
>>>>
>>>> * At least one line must be written to the file.
>>>>
>>>> Writes that violate the above rules fail with the error EINVAL.
>>>>
>>>> In order for a process to write to the /proc/[pid]/uid_map
>>>> (/proc/[pid]/gid_map) file, all of the following requirements
>>>> must be met:
>>>>
>>>> 1. The writing process must have the CAP_SETUID (CAP_SETGID)
>>>> capability in the user namespace of the process pid.
>>>
>>> This checked for the opening process (and I don't actually remember
>>> whether it's checked for the writing process).
>>
>> Eric, can you comment?
>
> We have to check for the opening processes and that changes was made
> after I implemented my interface. Pieces of the code appear to also
> examine the writing process and verify everything applies to it as well.
>
> I goofed when I designed the interface originall and had not realized
> what a classic design error it can be to not restrict by the opening
> process.
So, I still need some help here. Should the sentence above just read:
1. The *opening* process must have the CAP_SETUID (CAP_SETGID)
capability in the user namespace of the process pid.
or must something also be said about the writing process? (If so, i'd
appreciate a completely formed sentence or two that I can just drop into
the man page..)
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-09 19:26 ` Andy Lutomirski
2014-09-09 19:39 ` Andy Lutomirski
@ 2014-09-11 14:47 ` Michael Kerrisk (man-pages)
2014-09-11 15:15 ` Andy Lutomirski
1 sibling, 1 reply; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-11 14:47 UTC (permalink / raw)
To: Andy Lutomirski, Eric W. Biederman
Cc: mtk.manpages, lkml, linux-man, Linux Containers,
richard -rw- weinberger, Serge E. Hallyn
Hi Andy,
On 09/09/2014 12:26 PM, Andy Lutomirski wrote:
> On Tue, Sep 9, 2014 at 9:05 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>> We may also want to discuss the specific restrictions on chroot.
>>
>> The text about chroot at least gives people a strong hint that the
>> chroot rules are affected by user namespaces.
>>
>> The restrictions that we have settled on to avoid chroot being a problem
>> are the creator of a user namespace must not be chrooted in their
>> current mount namespace, and the creator of the user namespace must not
>> be threaded.
>>
>> Andy can you check me on this it looks like unshare is currently buggy
>> in that it will allow a threaded application to create a user namespace.
>
> I think it's this code in unshare:
>
> /*
> * If unsharing a user namespace must also unshare the thread.
> */
> if (unshare_flags & CLONE_NEWUSER)
> unshare_flags |= CLONE_THREAD | CLONE_FS;
>
> I suppose that this should be documented.
>
> CLONE_FS prevents the chroot from leaking out of the namespace. (But
> see the other thread that I'm about to start...)
So, in the current draft of the setns(2) page, there is
CLONE_NEWNS
...
Since Linux 3.9, CLONE_NEWUSER also automatically implies
CLONE_FS.
Does that cover your point? Or did you mean that more needs to be said?
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-11 14:46 ` Michael Kerrisk (man-pages)
@ 2014-09-11 15:14 ` Andy Lutomirski
2014-09-14 2:42 ` Michael Kerrisk (man-pages)
0 siblings, 1 reply; 26+ messages in thread
From: Andy Lutomirski @ 2014-09-11 15:14 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: Eric W. Biederman, lkml, linux-man, Linux Containers,
richard -rw- weinberger, Serge E. Hallyn
On Thu, Sep 11, 2014 at 7:46 AM, Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
> Hi Eric,
>
> On 09/09/2014 09:05 AM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>
>>> Hi Andy, and Eric,
>>>>> 1. The writing process must have the CAP_SETUID (CAP_SETGID)
>>>>> capability in the user namespace of the process pid.
>>>>
>>>> This checked for the opening process (and I don't actually remember
>>>> whether it's checked for the writing process).
>>>
>>> Eric, can you comment?
>>
>> We have to check for the opening processes and that changes was made
>> after I implemented my interface. Pieces of the code appear to also
>> examine the writing process and verify everything applies to it as well.
>>
>> I goofed when I designed the interface originall and had not realized
>> what a classic design error it can be to not restrict by the opening
>> process.
>
> So, I still need some help here. Should the sentence above just read:
>
> 1. The *opening* process must have the CAP_SETUID (CAP_SETGID)
> capability in the user namespace of the process pid.
I think this is sufficient.
>
> or must something also be said about the writing process? (If so, i'd
> appreciate a completely formed sentence or two that I can just drop into
> the man page..)
There might be a restriction there, too, but I think it could be
removed, and I also think that it's unlikely that anyone will
encounter it.
--Andy
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-11 14:47 ` Michael Kerrisk (man-pages)
@ 2014-09-11 15:15 ` Andy Lutomirski
2014-09-14 2:58 ` Michael Kerrisk (man-pages)
0 siblings, 1 reply; 26+ messages in thread
From: Andy Lutomirski @ 2014-09-11 15:15 UTC (permalink / raw)
To: Michael Kerrisk (man-pages)
Cc: Eric W. Biederman, lkml, linux-man, Linux Containers,
richard -rw- weinberger, Serge E. Hallyn
On Thu, Sep 11, 2014 at 7:47 AM, Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
>
> So, in the current draft of the setns(2) page, there is
>
> CLONE_NEWNS
> ...
> Since Linux 3.9, CLONE_NEWUSER also automatically implies
> CLONE_FS.
>
> Does that cover your point? Or did you mean that more needs to be said?
Looks good, although you could add CLONE_THREAD and the rest of the
things implied by CLONE_THREAD if you want to be fancier.
--Andy
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-11 15:14 ` Andy Lutomirski
@ 2014-09-14 2:42 ` Michael Kerrisk (man-pages)
0 siblings, 0 replies; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-14 2:42 UTC (permalink / raw)
To: Andy Lutomirski
Cc: mtk.manpages, Eric W. Biederman, lkml, linux-man,
Linux Containers, richard -rw- weinberger, Serge E. Hallyn
On 09/11/2014 08:14 AM, Andy Lutomirski wrote:
> On Thu, Sep 11, 2014 at 7:46 AM, Michael Kerrisk (man-pages)
> <mtk.manpages@gmail.com> wrote:
>> Hi Eric,
>>
>> On 09/09/2014 09:05 AM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>
>>>> Hi Andy, and Eric,
>>>>>> 1. The writing process must have the CAP_SETUID (CAP_SETGID)
>>>>>> capability in the user namespace of the process pid.
>>>>>
>>>>> This checked for the opening process (and I don't actually remember
>>>>> whether it's checked for the writing process).
>>>>
>>>> Eric, can you comment?
>>>
>>> We have to check for the opening processes and that changes was made
>>> after I implemented my interface. Pieces of the code appear to also
>>> examine the writing process and verify everything applies to it as well.
>>>
>>> I goofed when I designed the interface originall and had not realized
>>> what a classic design error it can be to not restrict by the opening
>>> process.
>>
>> So, I still need some help here. Should the sentence above just read:
>>
>> 1. The *opening* process must have the CAP_SETUID (CAP_SETGID)
>> capability in the user namespace of the process pid.
>
> I think this is sufficient.
>
>>
>> or must something also be said about the writing process? (If so, i'd
>> appreciate a completely formed sentence or two that I can just drop into
>> the man page..)
>
> There might be a restriction there, too, but I think it could be
> removed, and I also think that it's unlikely that anyone will
> encounter it.
Okay. Thanks, Andy.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
* Re: For review: user_namespace(7) man page
2014-09-11 15:15 ` Andy Lutomirski
@ 2014-09-14 2:58 ` Michael Kerrisk (man-pages)
0 siblings, 0 replies; 26+ messages in thread
From: Michael Kerrisk (man-pages) @ 2014-09-14 2:58 UTC (permalink / raw)
To: Andy Lutomirski
Cc: mtk.manpages, Eric W. Biederman, lkml, linux-man,
Linux Containers, richard -rw- weinberger, Serge E. Hallyn
On 09/11/2014 08:15 AM, Andy Lutomirski wrote:
> On Thu, Sep 11, 2014 at 7:47 AM, Michael Kerrisk (man-pages)
> <mtk.manpages@gmail.com> wrote:
>>
>> So, in the current draft of the setns(2) page, there is
>>
>> CLONE_NEWNS
>> ...
>> Since Linux 3.9, CLONE_NEWUSER also automatically implies
>> CLONE_FS.
>>
>> Does that cover your point? Or did you mean that more needs to be said?
>
> Looks good, although you could add CLONE_THREAD and the rest of the
> things implied by CLONE_THREAD if you want to be fancier.
Yes, under CLONE_NEWUSER there is also a statement that that flag
implies CLONE_THREAD, and elsewhere in the page there is the
following text:
[[
In addition, CLONE_THREAD, CLONE_SIGHAND, and CLONE_VM can be
specified in flags if the caller is single threaded (i.e., it
is not sharing its address space with another process or
thread). In this case, these flags have no effect. (Note also
that specifying CLONE_THREAD automatically implies CLONE_VM,
and specifying CLONE_VM automatically implies CLONE_SIGHAND.)
If the process is multithreaded, then the use of these flags
results in an error.
]]
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
^ permalink raw reply [flat|nested] 26+ messages in thread
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-- links below jump to the message on this page --
2014-08-20 23:36 For review: user_namespace(7) man page Michael Kerrisk (man-pages)
2014-08-22 21:12 ` Serge E. Hallyn
2014-09-01 16:58 ` Michael Kerrisk (man-pages)
2014-08-30 21:53 ` Eric W. Biederman
2014-09-01 17:31 ` Michael Kerrisk (man-pages)
2014-09-02 1:05 ` Eric W. Biederman
2014-09-09 14:00 ` Michael Kerrisk (man-pages)
2014-09-09 16:16 ` Eric W. Biederman
2014-09-11 14:40 ` Michael Kerrisk (man-pages)
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
2014-09-09 15:49 ` Eric W. Biederman
2014-09-11 14:40 ` Michael Kerrisk (man-pages)
2014-09-09 13:59 ` Michael Kerrisk (man-pages)
2014-09-09 15:51 ` Eric W. Biederman
2014-09-11 14:40 ` Michael Kerrisk (man-pages)
2014-09-01 20:57 ` Andy Lutomirski
2014-09-09 14:00 ` Michael Kerrisk (man-pages)
2014-09-09 16:05 ` Eric W. Biederman
2014-09-09 19:26 ` Andy Lutomirski
2014-09-09 19:39 ` Andy Lutomirski
2014-09-11 14:47 ` Michael Kerrisk (man-pages)
2014-09-11 15:15 ` Andy Lutomirski
2014-09-14 2:58 ` Michael Kerrisk (man-pages)
2014-09-11 14:46 ` Michael Kerrisk (man-pages)
2014-09-11 15:14 ` Andy Lutomirski
2014-09-14 2:42 ` Michael Kerrisk (man-pages)
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