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From: Casey Schaufler <casey@schaufler-ca.com>
To: Micah Morton <mortonm@chromium.org>, serge@hallyn.com
Cc: jmorris@namei.org, Kees Cook <keescook@chromium.org>,
	linux-security-module@vger.kernel.org
Subject: Re: [PATCH] LSM: add SafeSetID module that gates setid calls
Date: Thu, 1 Nov 2018 10:08:03 -0700
Message-ID: <41daea64-03c0-0970-c405-d1a5ae134181@schaufler-ca.com> (raw)
In-Reply-To: <CAJ-EccMaAu+g55Nuh1nZqt3Ld-YEgDiT_ewdQzGtFGmzRgXckw@mail.gmail.com>

On 11/1/2018 9:11 AM, Micah Morton wrote:
> On Wed, Oct 31, 2018 at 11:07 PM Serge E. Hallyn <serge@hallyn.com> wrote:
>> On Wed, Oct 31, 2018 at 09:02:45PM +0000, Serge E. Hallyn wrote:
>>> Quoting mortonm@chromium.org (mortonm@chromium.org):
>>>> From: Micah Morton <mortonm@chromium.org>
>>>>
>>>> SafeSetID gates the setid family of syscalls to restrict UID/GID
>>>> transitions from a given UID/GID to only those approved by a
>>>> system-wide whitelist. These restrictions also prohibit the given
>>>> UIDs/GIDs from obtaining auxiliary privileges associated with
>>>> CAP_SET{U/G}ID, such as allowing a user to set up user namespace UID
>>>> mappings. For now, only gating the set*uid family of syscalls is
>>>> supported, with support for set*gid coming in a future patch set.
>>>>
>>>> Signed-off-by: Micah Morton <mortonm@chromium.org>
>>>> ---
>>>>
>>>> NOTE: See the TODO above setuid_syscall() in lsm.c for an aspect of this
>>>> code that likely needs improvement before being an acceptable approach.
>>>> I'm specifically interested to see if there are better ideas for how
>>>> this could be done.
>>>>
>>>>  Documentation/admin-guide/LSM/SafeSetID.rst |  94 ++++++
>>>>  Documentation/admin-guide/LSM/index.rst     |   1 +
>>>>  arch/Kconfig                                |   5 +
>>>>  arch/arm/Kconfig                            |   1 +
>>>>  arch/arm64/Kconfig                          |   1 +
>>>>  arch/x86/Kconfig                            |   1 +
>>>>  security/Kconfig                            |   1 +
>>>>  security/Makefile                           |   2 +
>>>>  security/safesetid/Kconfig                  |  13 +
>>>>  security/safesetid/Makefile                 |   7 +
>>>>  security/safesetid/lsm.c                    | 334 ++++++++++++++++++++
>>>>  security/safesetid/lsm.h                    |  30 ++
>>>>  security/safesetid/securityfs.c             | 189 +++++++++++
>>>>  13 files changed, 679 insertions(+)
>>>>  create mode 100644 Documentation/admin-guide/LSM/SafeSetID.rst
>>>>  create mode 100644 security/safesetid/Kconfig
>>>>  create mode 100644 security/safesetid/Makefile
>>>>  create mode 100644 security/safesetid/lsm.c
>>>>  create mode 100644 security/safesetid/lsm.h
>>>>  create mode 100644 security/safesetid/securityfs.c
>>>>
>>>> diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentation/admin-guide/LSM/SafeSetID.rst
>>>> new file mode 100644
>>>> index 000000000000..e7d072124424
>>>> --- /dev/null
>>>> +++ b/Documentation/admin-guide/LSM/SafeSetID.rst
>>>> @@ -0,0 +1,94 @@
>>>> +=========
>>>> +SafeSetID
>>>> +=========
>>>> +SafeSetID is an LSM module that gates the setid family of syscalls to restrict
>>>> +UID/GID transitions from a given UID/GID to only those approved by a
>>>> +system-wide whitelist. These restrictions also prohibit the given UIDs/GIDs
>>>> +from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, such as
>>>> +allowing a user to set up user namespace UID mappings.
>>>> +
>>>> +
>>>> +Background
>>>> +==========
>>>> +In absence of file capabilities, processes spawned on a Linux system that need
>>>> +to switch to a different user must be spawned with CAP_SETUID privileges.
>>>> +CAP_SETUID is granted to programs running as root or those running as a non-root
>>>> +user that have been explicitly given the CAP_SETUID runtime capability. It is
>>>> +often preferable to use Linux runtime capabilities rather than file
>>>> +capabilities, since using file capabilities to run a program with elevated
>>>> +privileges opens up possible security holes since any user with access to the
>>>> +file can exec() that program to gain the elevated privileges.
>>> Not true, see inheritable capabilities.  You also might look at ambient
>>> capabilities.
>> So for example with pam_cap.so you could have your N uids each be given
>> the desired pI, and assign the corrsponding fIs to the files they should
>> be able to exec with privilege.  No other uids will run those files with
>> privilege.  *1
> Sorry, what are "pl" and "fls" here? "Privilege level" and "files"?
>
>> Can you give some more details about exactly how you see SafeSetID being
>> used?
> Sure. The main use case for this LSM is to allow a non-root program to
> transition to other untrusted uids without full blown CAP_SETUID
> capabilities. The non-root program would still need CAP_SETUID to do
> any kind of transition, but the additional restrictions imposed by
> this LSM would mean it is a "safer" version of CAP_SETUID since the
> non-root program cannot take advantage of CAP_SETUID to do any
> unapproved actions (i.e. setuid to uid 0 or create/enter new user
> namespace). The higher level goal is to allow for uid-based sandboxing
> of system services without having to give out CAP_SETUID all over the
> place just so that non-root programs can drop to
> even-further-non-privileged uids. This is especially relevant when one
> non-root daemon on the system should be allowed to spawn other
> processes as different uids, but its undesirable to give the daemon a
> basically-root-equivalent CAP_SETUID.

I don't want to sound stupid(er than usual), but it sounds like
you could do all this using setuid bits prudently. Based on this
description, I don't see that anything new is needed.

>> I'm still not quite clear on whether you want N completely unprivileged
>> uids to be used by some user (i.e. uid 1000), or whether one or more of
>> those should also have some privileged, or whether one of the uids might
>> or might not b uid 0.  Years ago I used to use N separate uids to
>> somewhat segragate workloads on my laptop, and I'd like my browser to
>> do something like that.  Is that the kind of uid switching you have
>> in mind?
> "N completely unprivileged uids to be used by some user (i.e. uid
> 1000)" is the closest description of what this LSM has in mind. For
> example, uid 123 is some system service that needs runtime
> capabilities X, Y and Z and a bunch of DBus permissions associated
> with uid 123, but also wants to spawn another program without any of
> these capabilities/permissions. In this case we would like to avoid
> giving the system service CAP_SETUID.
>
>> -serge
>>
>> *1 And maybe with one of the p9auth/factotem proposals out there you
>> could have a userspace daemon hand out the tokens for setuid, but that's
>> getting "out there" and probably derailing this conversation :)


  parent reply index

Thread overview: 88+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-31 15:28 mortonm
2018-10-31 21:02 ` Serge E. Hallyn
2018-10-31 21:57   ` Kees Cook
2018-10-31 22:37     ` Casey Schaufler
2018-11-01  1:12       ` Micah Morton
2018-11-01  6:13         ` Serge E. Hallyn
2018-11-01 15:39           ` Casey Schaufler
2018-11-01 15:56             ` Serge E. Hallyn
2018-11-01 16:18             ` Micah Morton
2018-11-01  6:07   ` Serge E. Hallyn
2018-11-01 16:11     ` Micah Morton
2018-11-01 16:22       ` Micah Morton
2018-11-01 16:41       ` Micah Morton
2018-11-01 17:08       ` Casey Schaufler [this message]
2018-11-01 19:52         ` Micah Morton
2018-11-02 16:05           ` Casey Schaufler
2018-11-02 17:12             ` Micah Morton
2018-11-02 18:19               ` Casey Schaufler
2018-11-02 18:30                 ` Serge E. Hallyn
2018-11-02 19:02                   ` Casey Schaufler
2018-11-02 19:22                     ` Serge E. Hallyn
2018-11-08 20:53                       ` Micah Morton
2018-11-08 21:34                         ` Casey Schaufler
2018-11-09  0:30                           ` Micah Morton
2018-11-09 23:21                             ` [PATCH] LSM: generalize flag passing to security_capable mortonm
2018-11-21 16:54                             ` [PATCH] LSM: add SafeSetID module that gates setid calls mortonm
2018-12-06  0:08                               ` Kees Cook
2018-12-06 17:51                                 ` Micah Morton
2019-01-11 17:13                                 ` [PATCH v2] " mortonm
2019-01-15  0:38                                   ` Kees Cook
2019-01-15 18:04                                     ` [PATCH v3 1/2] LSM: mark all set*uid call sites in kernel/sys.c mortonm
2019-01-15 19:34                                       ` Kees Cook
2019-01-15 18:04                                     ` [PATCH v3 2/2] LSM: add SafeSetID module that gates setid calls mortonm
2019-01-15 19:44                                       ` Kees Cook
2019-01-15 21:50                                         ` [PATCH v4 " mortonm
2019-01-15 22:32                                           ` Kees Cook
2019-01-16 15:46                                             ` [PATCH v5 " mortonm
2019-01-16 16:10                                               ` Casey Schaufler
2019-01-22 20:40                                                 ` Micah Morton
2019-01-22 22:28                                                   ` James Morris
2019-01-22 22:40                                                     ` Micah Morton
2019-01-22 22:42                                                       ` [PATCH v3 1/2] " mortonm
2019-01-25 15:51                                                         ` Micah Morton
2019-01-25 20:15                                               ` [PATCH v5 2/2] " James Morris
2019-01-25 21:06                                                 ` Micah Morton
2019-01-28 19:47                                                   ` Micah Morton
2019-01-28 19:56                                                     ` Kees Cook
2019-01-28 20:09                                                       ` James Morris
2019-01-28 20:19                                                       ` Micah Morton
2019-01-28 20:30                                                         ` [PATCH] LSM: Add 'name' field for SafeSetID in DEFINE_LSM mortonm
2019-01-28 22:12                                                           ` James Morris
2019-01-28 22:33                                                         ` [PATCH v5 2/2] LSM: add SafeSetID module that gates setid calls Micah Morton
2019-01-29 17:25                                                           ` James Morris
2019-01-29 21:14                                                             ` Micah Morton
2019-01-30  7:15                                                               ` Kees Cook
2019-02-06 19:03                                                                 ` [PATCH] LSM: SafeSetID: add selftest mortonm
2019-02-06 19:26                                                                   ` Edwin Zimmerman
2019-02-07 21:54                                                                     ` Micah Morton
2019-02-12 19:01                                                                   ` James Morris
2019-01-15 21:58                                         ` [PATCH v3 2/2] LSM: add SafeSetID module that gates setid calls Micah Morton
2019-01-15 19:49                                     ` [PATCH v2] " Micah Morton
2019-01-15 19:53                                       ` Kees Cook
2019-01-15  4:07                                   ` James Morris
2019-01-15 19:42                                     ` Micah Morton
2018-11-02 19:28                 ` [PATCH] " Micah Morton
2018-11-06 19:09                 ` [PATCH v2] " mortonm
2018-11-06 20:59       ` [PATCH] " James Morris
2018-11-06 21:21         ` [PATCH v3] " mortonm
2018-11-02 18:07 ` [PATCH] " Stephen Smalley
2018-11-02 19:13   ` Micah Morton
2018-11-19 18:54   ` [PATCH] [PATCH] LSM: generalize flag passing to security_capable mortonm
2018-12-13 22:29     ` Micah Morton
2018-12-13 23:09       ` Casey Schaufler
2018-12-14  0:05         ` Micah Morton
2018-12-18 22:37         ` [PATCH v2] " mortonm
2019-01-07 17:55           ` Micah Morton
2019-01-07 18:16             ` Casey Schaufler
2019-01-07 18:36               ` Micah Morton
2019-01-07 18:46                 ` Casey Schaufler
2019-01-07 19:02                   ` Micah Morton
2019-01-07 22:57                     ` [PATCH v3] " mortonm
2019-01-07 23:13           ` [PATCH v2] " Kees Cook
2019-01-08  0:10             ` [PATCH v4] " mortonm
2019-01-08  0:20               ` Kees Cook
2019-01-09 18:39                 ` Micah Morton
2019-01-10 22:31               ` James Morris
2019-01-10 23:03                 ` Micah Morton
2019-01-08  0:10             ` [PATCH v2] " Micah Morton

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