[PATCH v3 0/2] capabilities: Ambient capability patchset

[PATCH v3 0/2] capabilities: Ambient capability patchset

[Andy Lutomirski]

This adds ambient capabilities.  See the individual patch changelogs
for details.

Preliminary userspace code is here:

https://git.kernel.org/cgit/linux/kernel/git/luto/util-linux-playground.git/commit/?h=cap_ambient&id=7f5afbd175d2

James or akpm, I think this is ready for 4.2.  Can one of you queue it up?

Thanks,
Andy

Changes from v2:
 - Improve the patch 1 changelog a bit.
 - Add acks.
 - Add comment clarifying the pE' rule.
 - s/except/expect

Changes from v1:
 - Lots of cleanups to the ambient cap code.
 - The securebit is new.

Andy Lutomirski (2):
  capabilities: Ambient capabilities
  capabilities: Add a securebit to disable PR_CAP_AMBIENT_RAISE

 fs/proc/array.c                 |  5 ++-
 include/linux/cred.h            |  8 ++++
 include/uapi/linux/prctl.h      |  6 +++
 include/uapi/linux/securebits.h | 11 ++++-
 kernel/user_namespace.c         |  1 +
 security/commoncap.c            | 92 ++++++++++++++++++++++++++++++++++++-----
 security/keys/process_keys.c    |  1 +
 7 files changed, 112 insertions(+), 12 deletions(-)

-- 
2.1.0

[PATCH v3 1/2] capabilities: Ambient capabilities

[Andy Lutomirski]

Credit where credit is due: this idea comes from Christoph Lameter
with a lot of valuable input from Serge Hallyn.  This patch is
heavily based on Christoph's patch.

===== The status quo =====

On Linux, there are a number of capabilities defined by the kernel.
To perform various privileged tasks, processes can wield
capabilities that they hold.

Each task has four capability masks: effective (pE), permitted (pP),
inheritable (pI), and a bounding set (X).  When the kernel checks
for a capability, it checks pE.  The other capability masks serve to
modify what capabilities can be in pE.

Any task can remove capabilities from pE, pP, or pI at any time.  If
a task has a capability in pP, it can add that capability to pE
and/or pI.  If a task has CAP_SETPCAP, then it can add any
capability to pI, and it can remove capabilities from X.

Tasks are not the only things that can have capabilities; files can
also have capabilities.  A file can have no capabilty information at
all [1].  If a file has capability information, then it has a
permitted mask (fP) and an inheritable mask (fI) as well as a single
effective bit (fE) [2].  File capabilities modify the capabilities
of tasks that execve(2) them.

A task that successfully calls execve has its capabilities modified
for the file ultimately being excecuted (i.e. the binary itself if
that binary is ELF or for the interpreter if the binary is a
script.) [3] In the capability evolution rules, for each mask Z, pZ
represents the old value and pZ' represents the new value.  The
rules are:

  pP' = (X & fP) | (pI & fI)
  pI' = pI
  pE' = (fE ? pP' : 0)
  X is unchanged

For setuid binaries, fP, fI, and fE are modified by a moderately
complicated set of rules that emulate POSIX behavior.  Similarly, if
euid == 0 or ruid == 0, then fP, fI, and fE are modified differently
(primary, fP and fI usually end up being the full set).  For nonroot
users executing binaries with neither setuid nor file caps, fI and
fP are empty and fE is false.

As an extra complication, if you execute a process as nonroot and fE
is set, then the "secure exec" rules are in effect: AT_SECURE gets
set, LD_PRELOAD doesn't work, etc.

This is rather messy.  We've learned that making any changes is
dangerous, though: if a new kernel version allows an unprivileged
program to change its security state in a way that persists cross
execution of a setuid program or a program with file caps, this
persistent state is surprisingly likely to allow setuid or
file-capped programs to be exploited for privilege escalation.

===== The problem =====

Capability inheritance is basically useless.

If you aren't root and you execute an ordinary binary, fI is zero,
so your capabilities have no effect whatsoever on pP'.  This means
that you can't usefully execute a helper process or a shell command
with elevated capabilities if you aren't root.

On current kernels, you can sort of work around this by setting fI
to the full set for most or all non-setuid executable files.  This
causes pP' = pI for nonroot, and inheritance works.  No one does
this because it's a PITA and it isn't even supported on most
filesystems.

If you try this, you'll discover that every nonroot program ends up
with secure exec rules, breaking many things.

This is a problem that has bitten many people who have tried to use
capabilities for anything useful.

===== The proposed change =====

This patch adds a fifth capability mask called the ambient mask
(pA).  pA does what most people expect pI to do.

pA obeys the invariant that no bit can ever be set in pA if it is
not set in both pP and pI.  Dropping a bit from pP or pI drops that
bit from pA.  This ensures that existing programs that try to drop
capabilities still do so, with a complication.  Because capability
inheritance is so broken, setting KEEPCAPS, using setresuid to
switch to nonroot uids, and then calling execve effectively drops
capabilities.  Therefore, setresuid from root to nonroot
conditionally clears pA unless SECBIT_NO_SETUID_FIXUP is set.
Processes that don't like this can re-add bits to pA afterwards.

The capability evolution rules are changed:

  pA' = (file caps or setuid or setgid ? 0 : pA)
  pP' = (X & fP) | (pI & fI) | pA'
  pI' = pI
  pE' = (fE ? pP' : pA')
  X is unchanged

If you are nonroot but you have a capability, you can add it to pA.
If you do so, your children get that capability in pA, pP, and pE.
For example, you can set pA = CAP_NET_BIND_SERVICE, and your
children can automatically bind low-numbered ports.  Hallelujah!

Unprivileged users can create user namespaces, map themselves to a
nonzero uid, and create both privileged (relative to their
namespace) and unprivileged process trees.  This is currently more
or less impossible.  Hallelujah!

You cannot use pA to try to subvert a setuid, setgid, or file-capped
program: if you execute any such program, pA gets cleared and the
resulting evolution rules are unchanged by this patch.

Users with nonzero pA are unlikely to unintentionally leak that
capability.  If they run programs that try to drop privileges,
dropping privileges will still work.

It's worth noting that the degree of paranoia in this patch could
possibly be reduced without causing serious problems.  Specifically,
if we allowed pA to persist across executing non-pA-aware setuid
binaries and across setresuid, then, naively, the only capabilities
that could leak as a result would be the capabilities in pA, and any
attacker *already* has those capabilities.  This would make me
nervous, though -- setuid binaries that tried to privilege-separate
might fail to do so, and putting CAP_DAC_READ_SEARCH or
CAP_DAC_OVERRIDE into pA could have unexpected side effects.
(Whether these unexpected side effects would be exploitable is an
open question.)  I've therefore taken the more paranoid route.  We
can revisit this later.

An alternative would be to require PR_SET_NO_NEW_PRIVS before
setting ambient capabilities.  I think that this would be annoying
and would make granting otherwise unprivileged users minor ambient
capabilities (CAP_NET_BIND_SERVICE or CAP_NET_RAW for example) much
less useful than it is with this patch.

===== Footnotes =====

[1] Files that are missing the "security.capability" xattr or that
have unrecognized values for that xattr end up with has_cap set to
false.  The code that does that appears to be complicated for no
good reason.

[2] The libcap capability mask parsers and formatters are
dangerously misleading and the documentation is flat-out wrong.  fE
is *not* a mask; it's a single bit.  This has probably confused
every single person who has tried to use file capabilities.

[3] Linux very confusingly processes both the script and the
interpreter if applicable, for reasons that elude me.  The results
from thinking about a script's file capabilities and/or setuid bits
are mostly discarded.

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Aaron Jones <aaronmdjones@gmail.com>
CC: Ted Ts'o <tytso@mit.edu>
Cc: linux-security-module@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: akpm@linuxfoundation.org
Cc: Andrew G. Morgan <morgan@kernel.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: Markku Savela <msa@moth.iki.fi>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Christoph Lameter <cl@linux.com> # Original author
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 fs/proc/array.c              |  5 ++-
 include/linux/cred.h         |  8 ++++
 include/uapi/linux/prctl.h   |  6 +++
 kernel/user_namespace.c      |  1 +
 security/commoncap.c         | 91 +++++++++++++++++++++++++++++++++++++++-----
 security/keys/process_keys.c |  1 +
 6 files changed, 101 insertions(+), 11 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 1295a00ca316..bc15356d6551 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -282,7 +282,8 @@ static void render_cap_t(struct seq_file *m, const char *header,
 static inline void task_cap(struct seq_file *m, struct task_struct *p)
 {
 	const struct cred *cred;
-	kernel_cap_t cap_inheritable, cap_permitted, cap_effective, cap_bset;
+	kernel_cap_t cap_inheritable, cap_permitted, cap_effective,
+			cap_bset, cap_ambient;
 
 	rcu_read_lock();
 	cred = __task_cred(p);
@@ -290,12 +291,14 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
 	cap_permitted	= cred->cap_permitted;
 	cap_effective	= cred->cap_effective;
 	cap_bset	= cred->cap_bset;
+	cap_ambient	= cred->cap_ambient;
 	rcu_read_unlock();
 
 	render_cap_t(m, "CapInh:\t", &cap_inheritable);
 	render_cap_t(m, "CapPrm:\t", &cap_permitted);
 	render_cap_t(m, "CapEff:\t", &cap_effective);
 	render_cap_t(m, "CapBnd:\t", &cap_bset);
+	render_cap_t(m, "CapAmb:\t", &cap_ambient);
 }
 
 static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 2fb2ca2127ed..05178874e771 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -122,6 +122,7 @@ struct cred {
 	kernel_cap_t	cap_permitted;	/* caps we're permitted */
 	kernel_cap_t	cap_effective;	/* caps we can actually use */
 	kernel_cap_t	cap_bset;	/* capability bounding set */
+	kernel_cap_t	cap_ambient;	/* Ambient capability set */
 #ifdef CONFIG_KEYS
 	unsigned char	jit_keyring;	/* default keyring to attach requested
 					 * keys to */
@@ -197,6 +198,13 @@ static inline void validate_process_creds(void)
 }
 #endif
 
+static inline bool cap_ambient_invariant_ok(const struct cred *cred)
+{
+	return cap_issubset(cred->cap_ambient,
+			    cap_intersect(cred->cap_permitted,
+					  cred->cap_inheritable));
+}
+
 /**
  * get_new_cred - Get a reference on a new set of credentials
  * @cred: The new credentials to reference
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 31891d9535e2..65407f867e82 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -190,4 +190,10 @@ struct prctl_mm_map {
 # define PR_FP_MODE_FR		(1 << 0)	/* 64b FP registers */
 # define PR_FP_MODE_FRE		(1 << 1)	/* 32b compatibility */
 
+/* Control the ambient capability set */
+#define PR_CAP_AMBIENT		47
+# define PR_CAP_AMBIENT_GET	1
+# define PR_CAP_AMBIENT_RAISE	2
+# define PR_CAP_AMBIENT_LOWER	3
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 4109f8320684..dab0f808235a 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -39,6 +39,7 @@ static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
 	cred->cap_inheritable = CAP_EMPTY_SET;
 	cred->cap_permitted = CAP_FULL_SET;
 	cred->cap_effective = CAP_FULL_SET;
+	cred->cap_ambient = CAP_EMPTY_SET;
 	cred->cap_bset = CAP_FULL_SET;
 #ifdef CONFIG_KEYS
 	key_put(cred->request_key_auth);
diff --git a/security/commoncap.c b/security/commoncap.c
index f66713bd7450..835a7584f7ea 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -272,6 +272,16 @@ int cap_capset(struct cred *new,
 	new->cap_effective   = *effective;
 	new->cap_inheritable = *inheritable;
 	new->cap_permitted   = *permitted;
+
+	/*
+	 * Mask off ambient bits that are no longer both permitted and
+	 * inheritable.
+	 */
+	new->cap_ambient = cap_intersect(new->cap_ambient,
+					 cap_intersect(*permitted,
+						       *inheritable));
+	if (WARN_ON(!cap_ambient_invariant_ok(new)))
+		return -EINVAL;
 	return 0;
 }
 
@@ -352,6 +362,7 @@ static inline int bprm_caps_from_vfs_caps(struct cpu_vfs_cap_data *caps,
 
 		/*
 		 * pP' = (X & fP) | (pI & fI)
+		 * The addition of pA' is handled later.
 		 */
 		new->cap_permitted.cap[i] =
 			(new->cap_bset.cap[i] & permitted) |
@@ -479,10 +490,13 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 {
 	const struct cred *old = current_cred();
 	struct cred *new = bprm->cred;
-	bool effective, has_cap = false;
+	bool effective, has_cap = false, is_setid;
 	int ret;
 	kuid_t root_uid;
 
+	if (WARN_ON(!cap_ambient_invariant_ok(old)))
+		return -EPERM;
+
 	effective = false;
 	ret = get_file_caps(bprm, &effective, &has_cap);
 	if (ret < 0)
@@ -527,8 +541,9 @@ skip:
 	 *
 	 * In addition, if NO_NEW_PRIVS, then ensure we get no new privs.
 	 */
-	if ((!uid_eq(new->euid, old->uid) ||
-	     !gid_eq(new->egid, old->gid) ||
+	is_setid = !uid_eq(new->euid, old->uid) || !gid_eq(new->egid, old->gid);
+
+	if ((is_setid ||
 	     !cap_issubset(new->cap_permitted, old->cap_permitted)) &&
 	    bprm->unsafe & ~LSM_UNSAFE_PTRACE_CAP) {
 		/* downgrade; they get no more than they had, and maybe less */
@@ -544,10 +559,28 @@ skip:
 	new->suid = new->fsuid = new->euid;
 	new->sgid = new->fsgid = new->egid;
 
+	/* File caps or setid cancels ambient. */
+	if (has_cap || is_setid)
+		cap_clear(new->cap_ambient);
+
+	/*
+	 * Now that we've computed pA', update pP' to give:
+	 *   pP' = (X & fP) | (pI & fI) | pA'
+	 */
+	new->cap_permitted = cap_combine(new->cap_permitted, new->cap_ambient);
+
+	/*
+	 * Set pE' = (fE ? pP' : pA').  Because pA' is zero if fE is set,
+	 * this is the same as pE' = (fE ? pP' : 0) | pA'.
+	 */
 	if (effective)
 		new->cap_effective = new->cap_permitted;
 	else
-		cap_clear(new->cap_effective);
+		new->cap_effective = new->cap_ambient;
+
+	if (WARN_ON(!cap_ambient_invariant_ok(new)))
+		return -EPERM;
+
 	bprm->cap_effective = effective;
 
 	/*
@@ -562,7 +595,7 @@ skip:
 	 * Number 1 above might fail if you don't have a full bset, but I think
 	 * that is interesting information to audit.
 	 */
-	if (!cap_isclear(new->cap_effective)) {
+	if (!cap_issubset(new->cap_effective, new->cap_ambient)) {
 		if (!cap_issubset(CAP_FULL_SET, new->cap_effective) ||
 		    !uid_eq(new->euid, root_uid) || !uid_eq(new->uid, root_uid) ||
 		    issecure(SECURE_NOROOT)) {
@@ -573,6 +606,10 @@ skip:
 	}
 
 	new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
+
+	if (WARN_ON(!cap_ambient_invariant_ok(new)))
+		return -EPERM;
+
 	return 0;
 }
 
@@ -594,7 +631,7 @@ int cap_bprm_secureexec(struct linux_binprm *bprm)
 	if (!uid_eq(cred->uid, root_uid)) {
 		if (bprm->cap_effective)
 			return 1;
-		if (!cap_isclear(cred->cap_permitted))
+		if (!cap_issubset(cred->cap_permitted, cred->cap_ambient))
 			return 1;
 	}
 
@@ -696,10 +733,18 @@ static inline void cap_emulate_setxuid(struct cred *new, const struct cred *old)
 	     uid_eq(old->suid, root_uid)) &&
 	    (!uid_eq(new->uid, root_uid) &&
 	     !uid_eq(new->euid, root_uid) &&
-	     !uid_eq(new->suid, root_uid)) &&
-	    !issecure(SECURE_KEEP_CAPS)) {
-		cap_clear(new->cap_permitted);
-		cap_clear(new->cap_effective);
+	     !uid_eq(new->suid, root_uid))) {
+		if (!issecure(SECURE_KEEP_CAPS)) {
+			cap_clear(new->cap_permitted);
+			cap_clear(new->cap_effective);
+		}
+
+		/*
+		 * Pre-ambient programs expect setresuid to nonroot followed
+		 * by exec to drop capabilities.  We should make sure that
+		 * this remains the case.
+		 */
+		cap_clear(new->cap_ambient);
 	}
 	if (uid_eq(old->euid, root_uid) && !uid_eq(new->euid, root_uid))
 		cap_clear(new->cap_effective);
@@ -929,6 +974,32 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
 		return commit_creds(new);
 
+	case PR_CAP_AMBIENT:
+		if (((!cap_valid(arg3)) | arg4 | arg5))
+			return -EINVAL;
+
+		if (arg2 == PR_CAP_AMBIENT_GET) {
+			return !!cap_raised(current_cred()->cap_ambient, arg3);
+		} else if (arg2 != PR_CAP_AMBIENT_RAISE &&
+			   arg2 != PR_CAP_AMBIENT_LOWER) {
+			return -EINVAL;
+		} else {
+			if (arg2 == PR_CAP_AMBIENT_RAISE &&
+			    (!cap_raised(current_cred()->cap_permitted, arg3) ||
+			     !cap_raised(current_cred()->cap_inheritable,
+					 arg3)))
+				return -EPERM;
+
+			new = prepare_creds();
+			if (!new)
+				return -ENOMEM;
+			if (arg2 == PR_CAP_AMBIENT_RAISE)
+				cap_raise(new->cap_ambient, arg3);
+			else
+				cap_lower(new->cap_ambient, arg3);
+			return commit_creds(new);
+		}
+
 	default:
 		/* No functionality available - continue with default */
 		return -ENOSYS;
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index bd536cb221e2..43b4cddbf2b3 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -848,6 +848,7 @@ void key_change_session_keyring(struct callback_head *twork)
 	new->cap_inheritable	= old->cap_inheritable;
 	new->cap_permitted	= old->cap_permitted;
 	new->cap_effective	= old->cap_effective;
+	new->cap_ambient	= old->cap_ambient;
 	new->cap_bset		= old->cap_bset;
 
 	new->jit_keyring	= old->jit_keyring;
-- 
2.1.0

[PATCH v3 2/2] capabilities: Add a securebit to disable PR_CAP_AMBIENT_RAISE

[Andy Lutomirski]

Per Andrew Morgan's request, add a securebit to allow admins to
disable PR_CAP_AMBIENT_RAISE.  This securebit will prevent processes
from adding capabilities to their ambient set.

For simplicity, this disables PR_CAP_AMBIENT_RAISE entirely rather
than just disabling setting previously cleared bits.

Acked-By: Andrew G. Morgan <morgan@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Aaron Jones <aaronmdjones@gmail.com>
CC: Ted Ts'o <tytso@mit.edu>
Cc: linux-security-module@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: akpm@linuxfoundation.org
Cc: Andrew G. Morgan <morgan@kernel.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: Markku Savela <msa@moth.iki.fi>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 include/uapi/linux/securebits.h | 11 ++++++++++-
 security/commoncap.c            |  3 ++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/securebits.h b/include/uapi/linux/securebits.h
index 985aac9e6bf8..35ac35cef217 100644
--- a/include/uapi/linux/securebits.h
+++ b/include/uapi/linux/securebits.h
@@ -43,9 +43,18 @@
 #define SECBIT_KEEP_CAPS	(issecure_mask(SECURE_KEEP_CAPS))
 #define SECBIT_KEEP_CAPS_LOCKED (issecure_mask(SECURE_KEEP_CAPS_LOCKED))
 
+/* When set, a process cannot add new capabilities to its ambient set. */
+#define SECURE_NO_CAP_AMBIENT_RAISE		6
+#define SECURE_NO_CAP_AMBIENT_RAISE_LOCKED	7  /* make bit-6 immutable */
+
+#define SECBIT_NO_CAP_AMBIENT_RAISE (issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE))
+#define SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED \
+			(issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE_LOCKED))
+
 #define SECURE_ALL_BITS		(issecure_mask(SECURE_NOROOT) | \
 				 issecure_mask(SECURE_NO_SETUID_FIXUP) | \
-				 issecure_mask(SECURE_KEEP_CAPS))
+				 issecure_mask(SECURE_KEEP_CAPS) | \
+				 issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE))
 #define SECURE_ALL_LOCKS	(SECURE_ALL_BITS << 1)
 
 #endif /* _UAPI_LINUX_SECUREBITS_H */
diff --git a/security/commoncap.c b/security/commoncap.c
index 835a7584f7ea..22b7b91c5eae 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -987,7 +987,8 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			if (arg2 == PR_CAP_AMBIENT_RAISE &&
 			    (!cap_raised(current_cred()->cap_permitted, arg3) ||
 			     !cap_raised(current_cred()->cap_inheritable,
-					 arg3)))
+					 arg3) ||
+			     issecure(SECURE_NO_CAP_AMBIENT_RAISE)))
 				return -EPERM;
 
 			new = prepare_creds();
-- 
2.1.0

Re: [PATCH v3 0/2] capabilities: Ambient capability patchset

[Andy Lutomirski]

On Wed, May 27, 2015 at 4:47 PM, Andy Lutomirski <luto@kernel.org> wrote:
> This adds ambient capabilities.  See the individual patch changelogs
> for details.
>
> Preliminary userspace code is here:
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/util-linux-playground.git/commit/?h=cap_ambient&id=7f5afbd175d2
>
> James or akpm, I think this is ready for 4.2.  Can one of you queue it up?

akpm / James: Ping?  Do I need more acks?

--Andy

>
> Thanks,
> Andy
>
> Changes from v2:
>  - Improve the patch 1 changelog a bit.
>  - Add acks.
>  - Add comment clarifying the pE' rule.
>  - s/except/expect
>
> Changes from v1:
>  - Lots of cleanups to the ambient cap code.
>  - The securebit is new.
>
> Andy Lutomirski (2):
>   capabilities: Ambient capabilities
>   capabilities: Add a securebit to disable PR_CAP_AMBIENT_RAISE
>
>  fs/proc/array.c                 |  5 ++-
>  include/linux/cred.h            |  8 ++++
>  include/uapi/linux/prctl.h      |  6 +++
>  include/uapi/linux/securebits.h | 11 ++++-
>  kernel/user_namespace.c         |  1 +
>  security/commoncap.c            | 92 ++++++++++++++++++++++++++++++++++++-----
>  security/keys/process_keys.c    |  1 +
>  7 files changed, 112 insertions(+), 12 deletions(-)
>
> --
> 2.1.0
>



-- 
Andy Lutomirski
AMA Capital Management, LLC

Re: [PATCH v3 2/2] capabilities: Add a securebit to disable PR_CAP_AMBIENT_RAISE

[Serge E. Hallyn]

On Wed, May 27, 2015 at 04:47:59PM -0700, Andy Lutomirski wrote:
> Per Andrew Morgan's request, add a securebit to allow admins to
> disable PR_CAP_AMBIENT_RAISE.  This securebit will prevent processes
> from adding capabilities to their ambient set.
> 
> For simplicity, this disables PR_CAP_AMBIENT_RAISE entirely rather
> than just disabling setting previously cleared bits.
> 
> Acked-By: Andrew G. Morgan <morgan@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Serge Hallyn <serge.hallyn@canonical.com>

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Aaron Jones <aaronmdjones@gmail.com>
> CC: Ted Ts'o <tytso@mit.edu>
> Cc: linux-security-module@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-api@vger.kernel.org
> Cc: akpm@linuxfoundation.org
> Cc: Andrew G. Morgan <morgan@kernel.org>
> Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
> Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
> Cc: Markku Savela <msa@moth.iki.fi>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  include/uapi/linux/securebits.h | 11 ++++++++++-
>  security/commoncap.c            |  3 ++-
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/securebits.h b/include/uapi/linux/securebits.h
> index 985aac9e6bf8..35ac35cef217 100644
> --- a/include/uapi/linux/securebits.h
> +++ b/include/uapi/linux/securebits.h
> @@ -43,9 +43,18 @@
>  #define SECBIT_KEEP_CAPS	(issecure_mask(SECURE_KEEP_CAPS))
>  #define SECBIT_KEEP_CAPS_LOCKED (issecure_mask(SECURE_KEEP_CAPS_LOCKED))
>  
> +/* When set, a process cannot add new capabilities to its ambient set. */
> +#define SECURE_NO_CAP_AMBIENT_RAISE		6
> +#define SECURE_NO_CAP_AMBIENT_RAISE_LOCKED	7  /* make bit-6 immutable */
> +
> +#define SECBIT_NO_CAP_AMBIENT_RAISE (issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE))
> +#define SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED \
> +			(issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE_LOCKED))
> +
>  #define SECURE_ALL_BITS		(issecure_mask(SECURE_NOROOT) | \
>  				 issecure_mask(SECURE_NO_SETUID_FIXUP) | \
> -				 issecure_mask(SECURE_KEEP_CAPS))
> +				 issecure_mask(SECURE_KEEP_CAPS) | \
> +				 issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE))
>  #define SECURE_ALL_LOCKS	(SECURE_ALL_BITS << 1)
>  
>  #endif /* _UAPI_LINUX_SECUREBITS_H */
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 835a7584f7ea..22b7b91c5eae 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -987,7 +987,8 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  			if (arg2 == PR_CAP_AMBIENT_RAISE &&
>  			    (!cap_raised(current_cred()->cap_permitted, arg3) ||
>  			     !cap_raised(current_cred()->cap_inheritable,
> -					 arg3)))
> +					 arg3) ||
> +			     issecure(SECURE_NO_CAP_AMBIENT_RAISE)))
>  				return -EPERM;
>  
>  			new = prepare_creds();
> -- 
> 2.1.0
> 
> --
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Re: [PATCH v3 1/2] capabilities: Ambient capabilities

[Kees Cook]

On Wed, May 27, 2015 at 4:47 PM, Andy Lutomirski <luto@kernel.org> wrote:
> Credit where credit is due: this idea comes from Christoph Lameter
> with a lot of valuable input from Serge Hallyn.  This patch is
> heavily based on Christoph's patch.
>
> ===== The status quo =====
>
> On Linux, there are a number of capabilities defined by the kernel.
> To perform various privileged tasks, processes can wield
> capabilities that they hold.
>
> Each task has four capability masks: effective (pE), permitted (pP),
> inheritable (pI), and a bounding set (X).  When the kernel checks
> for a capability, it checks pE.  The other capability masks serve to
> modify what capabilities can be in pE.
>
> Any task can remove capabilities from pE, pP, or pI at any time.  If
> a task has a capability in pP, it can add that capability to pE
> and/or pI.  If a task has CAP_SETPCAP, then it can add any
> capability to pI, and it can remove capabilities from X.
>
> Tasks are not the only things that can have capabilities; files can
> also have capabilities.  A file can have no capabilty information at
> all [1].  If a file has capability information, then it has a
> permitted mask (fP) and an inheritable mask (fI) as well as a single
> effective bit (fE) [2].  File capabilities modify the capabilities
> of tasks that execve(2) them.
>
> A task that successfully calls execve has its capabilities modified
> for the file ultimately being excecuted (i.e. the binary itself if
> that binary is ELF or for the interpreter if the binary is a
> script.) [3] In the capability evolution rules, for each mask Z, pZ
> represents the old value and pZ' represents the new value.  The
> rules are:
>
>   pP' = (X & fP) | (pI & fI)
>   pI' = pI
>   pE' = (fE ? pP' : 0)
>   X is unchanged
>
> For setuid binaries, fP, fI, and fE are modified by a moderately
> complicated set of rules that emulate POSIX behavior.  Similarly, if
> euid == 0 or ruid == 0, then fP, fI, and fE are modified differently
> (primary, fP and fI usually end up being the full set).  For nonroot
> users executing binaries with neither setuid nor file caps, fI and
> fP are empty and fE is false.
>
> As an extra complication, if you execute a process as nonroot and fE
> is set, then the "secure exec" rules are in effect: AT_SECURE gets
> set, LD_PRELOAD doesn't work, etc.
>
> This is rather messy.  We've learned that making any changes is
> dangerous, though: if a new kernel version allows an unprivileged
> program to change its security state in a way that persists cross
> execution of a setuid program or a program with file caps, this
> persistent state is surprisingly likely to allow setuid or
> file-capped programs to be exploited for privilege escalation.
>
> ===== The problem =====
>
> Capability inheritance is basically useless.
>
> If you aren't root and you execute an ordinary binary, fI is zero,
> so your capabilities have no effect whatsoever on pP'.  This means
> that you can't usefully execute a helper process or a shell command
> with elevated capabilities if you aren't root.
>
> On current kernels, you can sort of work around this by setting fI
> to the full set for most or all non-setuid executable files.  This
> causes pP' = pI for nonroot, and inheritance works.  No one does
> this because it's a PITA and it isn't even supported on most
> filesystems.
>
> If you try this, you'll discover that every nonroot program ends up
> with secure exec rules, breaking many things.
>
> This is a problem that has bitten many people who have tried to use
> capabilities for anything useful.
>
> ===== The proposed change =====
>
> This patch adds a fifth capability mask called the ambient mask
> (pA).  pA does what most people expect pI to do.
>
> pA obeys the invariant that no bit can ever be set in pA if it is
> not set in both pP and pI.  Dropping a bit from pP or pI drops that
> bit from pA.  This ensures that existing programs that try to drop
> capabilities still do so, with a complication.  Because capability
> inheritance is so broken, setting KEEPCAPS, using setresuid to
> switch to nonroot uids, and then calling execve effectively drops
> capabilities.  Therefore, setresuid from root to nonroot
> conditionally clears pA unless SECBIT_NO_SETUID_FIXUP is set.
> Processes that don't like this can re-add bits to pA afterwards.
>
> The capability evolution rules are changed:
>
>   pA' = (file caps or setuid or setgid ? 0 : pA)
>   pP' = (X & fP) | (pI & fI) | pA'
>   pI' = pI
>   pE' = (fE ? pP' : pA')
>   X is unchanged
>
> If you are nonroot but you have a capability, you can add it to pA.
> If you do so, your children get that capability in pA, pP, and pE.
> For example, you can set pA = CAP_NET_BIND_SERVICE, and your
> children can automatically bind low-numbered ports.  Hallelujah!

Chrome OS could use this right now. :)

> Unprivileged users can create user namespaces, map themselves to a
> nonzero uid, and create both privileged (relative to their
> namespace) and unprivileged process trees.  This is currently more
> or less impossible.  Hallelujah!
>
> You cannot use pA to try to subvert a setuid, setgid, or file-capped
> program: if you execute any such program, pA gets cleared and the
> resulting evolution rules are unchanged by this patch.
>
> Users with nonzero pA are unlikely to unintentionally leak that
> capability.  If they run programs that try to drop privileges,
> dropping privileges will still work.
>
> It's worth noting that the degree of paranoia in this patch could
> possibly be reduced without causing serious problems.  Specifically,
> if we allowed pA to persist across executing non-pA-aware setuid
> binaries and across setresuid, then, naively, the only capabilities
> that could leak as a result would be the capabilities in pA, and any
> attacker *already* has those capabilities.  This would make me
> nervous, though -- setuid binaries that tried to privilege-separate
> might fail to do so, and putting CAP_DAC_READ_SEARCH or
> CAP_DAC_OVERRIDE into pA could have unexpected side effects.
> (Whether these unexpected side effects would be exploitable is an
> open question.)  I've therefore taken the more paranoid route.  We
> can revisit this later.

I think this is correct. Stuff using file caps, or set*id bits are
fundamentally using a different privilege management model. Keeping pA
separate makes a lot of sense to me.

> An alternative would be to require PR_SET_NO_NEW_PRIVS before
> setting ambient capabilities.  I think that this would be annoying
> and would make granting otherwise unprivileged users minor ambient
> capabilities (CAP_NET_BIND_SERVICE or CAP_NET_RAW for example) much
> less useful than it is with this patch.

Agreed: we should keep nnp out of this.

> ===== Footnotes =====
>
> [1] Files that are missing the "security.capability" xattr or that
> have unrecognized values for that xattr end up with has_cap set to
> false.  The code that does that appears to be complicated for no
> good reason.

Would it make more sense to have has_cap true, but have it lack any actual caps?

> [2] The libcap capability mask parsers and formatters are
> dangerously misleading and the documentation is flat-out wrong.  fE
> is *not* a mask; it's a single bit.  This has probably confused
> every single person who has tried to use file capabilities.

Sounds like it would be a valuable documentation patch.

> [3] Linux very confusingly processes both the script and the
> interpreter if applicable, for reasons that elude me.  The results
> from thinking about a script's file capabilities and/or setuid bits
> are mostly discarded.

I wonder if this is important enough to fix?

>
> Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Aaron Jones <aaronmdjones@gmail.com>
> CC: Ted Ts'o <tytso@mit.edu>
> Cc: linux-security-module@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-api@vger.kernel.org
> Cc: akpm@linuxfoundation.org
> Cc: Andrew G. Morgan <morgan@kernel.org>
> Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
> Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
> Cc: Markku Savela <msa@moth.iki.fi>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> Signed-off-by: Christoph Lameter <cl@linux.com> # Original author
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  fs/proc/array.c              |  5 ++-
>  include/linux/cred.h         |  8 ++++
>  include/uapi/linux/prctl.h   |  6 +++
>  kernel/user_namespace.c      |  1 +
>  security/commoncap.c         | 91 +++++++++++++++++++++++++++++++++++++++-----
>  security/keys/process_keys.c |  1 +
>  6 files changed, 101 insertions(+), 11 deletions(-)
>
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 1295a00ca316..bc15356d6551 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -282,7 +282,8 @@ static void render_cap_t(struct seq_file *m, const char *header,
>  static inline void task_cap(struct seq_file *m, struct task_struct *p)
>  {
>         const struct cred *cred;
> -       kernel_cap_t cap_inheritable, cap_permitted, cap_effective, cap_bset;
> +       kernel_cap_t cap_inheritable, cap_permitted, cap_effective,
> +                       cap_bset, cap_ambient;
>
>         rcu_read_lock();
>         cred = __task_cred(p);
> @@ -290,12 +291,14 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
>         cap_permitted   = cred->cap_permitted;
>         cap_effective   = cred->cap_effective;
>         cap_bset        = cred->cap_bset;
> +       cap_ambient     = cred->cap_ambient;
>         rcu_read_unlock();
>
>         render_cap_t(m, "CapInh:\t", &cap_inheritable);
>         render_cap_t(m, "CapPrm:\t", &cap_permitted);
>         render_cap_t(m, "CapEff:\t", &cap_effective);
>         render_cap_t(m, "CapBnd:\t", &cap_bset);
> +       render_cap_t(m, "CapAmb:\t", &cap_ambient);
>  }
>
>  static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
> diff --git a/include/linux/cred.h b/include/linux/cred.h
> index 2fb2ca2127ed..05178874e771 100644
> --- a/include/linux/cred.h
> +++ b/include/linux/cred.h
> @@ -122,6 +122,7 @@ struct cred {
>         kernel_cap_t    cap_permitted;  /* caps we're permitted */
>         kernel_cap_t    cap_effective;  /* caps we can actually use */
>         kernel_cap_t    cap_bset;       /* capability bounding set */
> +       kernel_cap_t    cap_ambient;    /* Ambient capability set */
>  #ifdef CONFIG_KEYS
>         unsigned char   jit_keyring;    /* default keyring to attach requested
>                                          * keys to */
> @@ -197,6 +198,13 @@ static inline void validate_process_creds(void)
>  }
>  #endif
>
> +static inline bool cap_ambient_invariant_ok(const struct cred *cred)
> +{
> +       return cap_issubset(cred->cap_ambient,
> +                           cap_intersect(cred->cap_permitted,
> +                                         cred->cap_inheritable));
> +}
> +
>  /**
>   * get_new_cred - Get a reference on a new set of credentials
>   * @cred: The new credentials to reference
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 31891d9535e2..65407f867e82 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -190,4 +190,10 @@ struct prctl_mm_map {
>  # define PR_FP_MODE_FR         (1 << 0)        /* 64b FP registers */
>  # define PR_FP_MODE_FRE                (1 << 1)        /* 32b compatibility */
>
> +/* Control the ambient capability set */
> +#define PR_CAP_AMBIENT         47
> +# define PR_CAP_AMBIENT_GET    1
> +# define PR_CAP_AMBIENT_RAISE  2
> +# define PR_CAP_AMBIENT_LOWER  3

I assume this is to avoid bumping the structures in capset(2)?
Bikeshed: Instead of GET/RAISE/LOWER, why not READ/DROP/ADD, which
would follow the existing names used for the bounding set (though we
add "ADD", which is not available to bset)?

> +
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 4109f8320684..dab0f808235a 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -39,6 +39,7 @@ static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
>         cred->cap_inheritable = CAP_EMPTY_SET;
>         cred->cap_permitted = CAP_FULL_SET;
>         cred->cap_effective = CAP_FULL_SET;
> +       cred->cap_ambient = CAP_EMPTY_SET;
>         cred->cap_bset = CAP_FULL_SET;
>  #ifdef CONFIG_KEYS
>         key_put(cred->request_key_auth);
> diff --git a/security/commoncap.c b/security/commoncap.c
> index f66713bd7450..835a7584f7ea 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -272,6 +272,16 @@ int cap_capset(struct cred *new,
>         new->cap_effective   = *effective;
>         new->cap_inheritable = *inheritable;
>         new->cap_permitted   = *permitted;
> +
> +       /*
> +        * Mask off ambient bits that are no longer both permitted and
> +        * inheritable.
> +        */
> +       new->cap_ambient = cap_intersect(new->cap_ambient,
> +                                        cap_intersect(*permitted,
> +                                                      *inheritable));
> +       if (WARN_ON(!cap_ambient_invariant_ok(new)))
> +               return -EINVAL;
>         return 0;
>  }
>
> @@ -352,6 +362,7 @@ static inline int bprm_caps_from_vfs_caps(struct cpu_vfs_cap_data *caps,
>
>                 /*
>                  * pP' = (X & fP) | (pI & fI)
> +                * The addition of pA' is handled later.
>                  */
>                 new->cap_permitted.cap[i] =
>                         (new->cap_bset.cap[i] & permitted) |
> @@ -479,10 +490,13 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
>  {
>         const struct cred *old = current_cred();
>         struct cred *new = bprm->cred;
> -       bool effective, has_cap = false;
> +       bool effective, has_cap = false, is_setid;
>         int ret;
>         kuid_t root_uid;
>
> +       if (WARN_ON(!cap_ambient_invariant_ok(old)))
> +               return -EPERM;
> +
>         effective = false;
>         ret = get_file_caps(bprm, &effective, &has_cap);
>         if (ret < 0)
> @@ -527,8 +541,9 @@ skip:
>          *
>          * In addition, if NO_NEW_PRIVS, then ensure we get no new privs.
>          */
> -       if ((!uid_eq(new->euid, old->uid) ||
> -            !gid_eq(new->egid, old->gid) ||
> +       is_setid = !uid_eq(new->euid, old->uid) || !gid_eq(new->egid, old->gid);
> +
> +       if ((is_setid ||
>              !cap_issubset(new->cap_permitted, old->cap_permitted)) &&
>             bprm->unsafe & ~LSM_UNSAFE_PTRACE_CAP) {
>                 /* downgrade; they get no more than they had, and maybe less */
> @@ -544,10 +559,28 @@ skip:
>         new->suid = new->fsuid = new->euid;
>         new->sgid = new->fsgid = new->egid;
>
> +       /* File caps or setid cancels ambient. */
> +       if (has_cap || is_setid)
> +               cap_clear(new->cap_ambient);
> +
> +       /*
> +        * Now that we've computed pA', update pP' to give:
> +        *   pP' = (X & fP) | (pI & fI) | pA'
> +        */
> +       new->cap_permitted = cap_combine(new->cap_permitted, new->cap_ambient);
> +
> +       /*
> +        * Set pE' = (fE ? pP' : pA').  Because pA' is zero if fE is set,
> +        * this is the same as pE' = (fE ? pP' : 0) | pA'.
> +        */
>         if (effective)
>                 new->cap_effective = new->cap_permitted;
>         else
> -               cap_clear(new->cap_effective);
> +               new->cap_effective = new->cap_ambient;
> +
> +       if (WARN_ON(!cap_ambient_invariant_ok(new)))
> +               return -EPERM;
> +
>         bprm->cap_effective = effective;
>
>         /*
> @@ -562,7 +595,7 @@ skip:
>          * Number 1 above might fail if you don't have a full bset, but I think
>          * that is interesting information to audit.
>          */
> -       if (!cap_isclear(new->cap_effective)) {
> +       if (!cap_issubset(new->cap_effective, new->cap_ambient)) {
>                 if (!cap_issubset(CAP_FULL_SET, new->cap_effective) ||
>                     !uid_eq(new->euid, root_uid) || !uid_eq(new->uid, root_uid) ||
>                     issecure(SECURE_NOROOT)) {
> @@ -573,6 +606,10 @@ skip:
>         }
>
>         new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
> +
> +       if (WARN_ON(!cap_ambient_invariant_ok(new)))
> +               return -EPERM;
> +
>         return 0;
>  }
>
> @@ -594,7 +631,7 @@ int cap_bprm_secureexec(struct linux_binprm *bprm)
>         if (!uid_eq(cred->uid, root_uid)) {
>                 if (bprm->cap_effective)
>                         return 1;
> -               if (!cap_isclear(cred->cap_permitted))
> +               if (!cap_issubset(cred->cap_permitted, cred->cap_ambient))
>                         return 1;
>         }
>
> @@ -696,10 +733,18 @@ static inline void cap_emulate_setxuid(struct cred *new, const struct cred *old)
>              uid_eq(old->suid, root_uid)) &&
>             (!uid_eq(new->uid, root_uid) &&
>              !uid_eq(new->euid, root_uid) &&
> -            !uid_eq(new->suid, root_uid)) &&
> -           !issecure(SECURE_KEEP_CAPS)) {
> -               cap_clear(new->cap_permitted);
> -               cap_clear(new->cap_effective);
> +            !uid_eq(new->suid, root_uid))) {
> +               if (!issecure(SECURE_KEEP_CAPS)) {
> +                       cap_clear(new->cap_permitted);
> +                       cap_clear(new->cap_effective);
> +               }
> +
> +               /*
> +                * Pre-ambient programs expect setresuid to nonroot followed
> +                * by exec to drop capabilities.  We should make sure that
> +                * this remains the case.
> +                */
> +               cap_clear(new->cap_ambient);
>         }
>         if (uid_eq(old->euid, root_uid) && !uid_eq(new->euid, root_uid))
>                 cap_clear(new->cap_effective);
> @@ -929,6 +974,32 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>                         new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
>                 return commit_creds(new);
>
> +       case PR_CAP_AMBIENT:
> +               if (((!cap_valid(arg3)) | arg4 | arg5))
> +                       return -EINVAL;
> +
> +               if (arg2 == PR_CAP_AMBIENT_GET) {
> +                       return !!cap_raised(current_cred()->cap_ambient, arg3);
> +               } else if (arg2 != PR_CAP_AMBIENT_RAISE &&
> +                          arg2 != PR_CAP_AMBIENT_LOWER) {
> +                       return -EINVAL;
> +               } else {
> +                       if (arg2 == PR_CAP_AMBIENT_RAISE &&
> +                           (!cap_raised(current_cred()->cap_permitted, arg3) ||
> +                            !cap_raised(current_cred()->cap_inheritable,
> +                                        arg3)))
> +                               return -EPERM;
> +
> +                       new = prepare_creds();
> +                       if (!new)
> +                               return -ENOMEM;
> +                       if (arg2 == PR_CAP_AMBIENT_RAISE)
> +                               cap_raise(new->cap_ambient, arg3);
> +                       else
> +                               cap_lower(new->cap_ambient, arg3);
> +                       return commit_creds(new);
> +               }
> +
>         default:
>                 /* No functionality available - continue with default */
>                 return -ENOSYS;
> diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
> index bd536cb221e2..43b4cddbf2b3 100644
> --- a/security/keys/process_keys.c
> +++ b/security/keys/process_keys.c
> @@ -848,6 +848,7 @@ void key_change_session_keyring(struct callback_head *twork)
>         new->cap_inheritable    = old->cap_inheritable;
>         new->cap_permitted      = old->cap_permitted;
>         new->cap_effective      = old->cap_effective;
> +       new->cap_ambient        = old->cap_ambient;
>         new->cap_bset           = old->cap_bset;
>
>         new->jit_keyring        = old->jit_keyring;
> --
> 2.1.0
>

Do you have tests for the capability behaviors we could add to the
selftests/ tree?

-Kees

-- 
Kees Cook
Chrome OS Security

Re: [PATCH v3 1/2] capabilities: Ambient capabilities

[Andy Lutomirski]

On Tue, Jun 9, 2015 at 4:09 PM, Kees Cook <keescook@chromium.org> wrote:
> On Wed, May 27, 2015 at 4:47 PM, Andy Lutomirski <luto@kernel.org> wrote:
>> Credit where credit is due: this idea comes from Christoph Lameter
>> with a lot of valuable input from Serge Hallyn.  This patch is
>> heavily based on Christoph's patch.
>>
>> ===== The status quo =====
>>
>> On Linux, there are a number of capabilities defined by the kernel.
>> To perform various privileged tasks, processes can wield
>> capabilities that they hold.
>>
>> Each task has four capability masks: effective (pE), permitted (pP),
>> inheritable (pI), and a bounding set (X).  When the kernel checks
>> for a capability, it checks pE.  The other capability masks serve to
>> modify what capabilities can be in pE.
>>
>> Any task can remove capabilities from pE, pP, or pI at any time.  If
>> a task has a capability in pP, it can add that capability to pE
>> and/or pI.  If a task has CAP_SETPCAP, then it can add any
>> capability to pI, and it can remove capabilities from X.
>>
>> Tasks are not the only things that can have capabilities; files can
>> also have capabilities.  A file can have no capabilty information at
>> all [1].  If a file has capability information, then it has a
>> permitted mask (fP) and an inheritable mask (fI) as well as a single
>> effective bit (fE) [2].  File capabilities modify the capabilities
>> of tasks that execve(2) them.
>>
>> A task that successfully calls execve has its capabilities modified
>> for the file ultimately being excecuted (i.e. the binary itself if
>> that binary is ELF or for the interpreter if the binary is a
>> script.) [3] In the capability evolution rules, for each mask Z, pZ
>> represents the old value and pZ' represents the new value.  The
>> rules are:
>>
>>   pP' = (X & fP) | (pI & fI)
>>   pI' = pI
>>   pE' = (fE ? pP' : 0)
>>   X is unchanged
>>
>> For setuid binaries, fP, fI, and fE are modified by a moderately
>> complicated set of rules that emulate POSIX behavior.  Similarly, if
>> euid == 0 or ruid == 0, then fP, fI, and fE are modified differently
>> (primary, fP and fI usually end up being the full set).  For nonroot
>> users executing binaries with neither setuid nor file caps, fI and
>> fP are empty and fE is false.
>>
>> As an extra complication, if you execute a process as nonroot and fE
>> is set, then the "secure exec" rules are in effect: AT_SECURE gets
>> set, LD_PRELOAD doesn't work, etc.
>>
>> This is rather messy.  We've learned that making any changes is
>> dangerous, though: if a new kernel version allows an unprivileged
>> program to change its security state in a way that persists cross
>> execution of a setuid program or a program with file caps, this
>> persistent state is surprisingly likely to allow setuid or
>> file-capped programs to be exploited for privilege escalation.
>>
>> ===== The problem =====
>>
>> Capability inheritance is basically useless.
>>
>> If you aren't root and you execute an ordinary binary, fI is zero,
>> so your capabilities have no effect whatsoever on pP'.  This means
>> that you can't usefully execute a helper process or a shell command
>> with elevated capabilities if you aren't root.
>>
>> On current kernels, you can sort of work around this by setting fI
>> to the full set for most or all non-setuid executable files.  This
>> causes pP' = pI for nonroot, and inheritance works.  No one does
>> this because it's a PITA and it isn't even supported on most
>> filesystems.
>>
>> If you try this, you'll discover that every nonroot program ends up
>> with secure exec rules, breaking many things.
>>
>> This is a problem that has bitten many people who have tried to use
>> capabilities for anything useful.
>>
>> ===== The proposed change =====
>>
>> This patch adds a fifth capability mask called the ambient mask
>> (pA).  pA does what most people expect pI to do.
>>
>> pA obeys the invariant that no bit can ever be set in pA if it is
>> not set in both pP and pI.  Dropping a bit from pP or pI drops that
>> bit from pA.  This ensures that existing programs that try to drop
>> capabilities still do so, with a complication.  Because capability
>> inheritance is so broken, setting KEEPCAPS, using setresuid to
>> switch to nonroot uids, and then calling execve effectively drops
>> capabilities.  Therefore, setresuid from root to nonroot
>> conditionally clears pA unless SECBIT_NO_SETUID_FIXUP is set.
>> Processes that don't like this can re-add bits to pA afterwards.
>>
>> The capability evolution rules are changed:
>>
>>   pA' = (file caps or setuid or setgid ? 0 : pA)
>>   pP' = (X & fP) | (pI & fI) | pA'
>>   pI' = pI
>>   pE' = (fE ? pP' : pA')
>>   X is unchanged
>>
>> If you are nonroot but you have a capability, you can add it to pA.
>> If you do so, your children get that capability in pA, pP, and pE.
>> For example, you can set pA = CAP_NET_BIND_SERVICE, and your
>> children can automatically bind low-numbered ports.  Hallelujah!
>
> Chrome OS could use this right now. :)
>
>> Unprivileged users can create user namespaces, map themselves to a
>> nonzero uid, and create both privileged (relative to their
>> namespace) and unprivileged process trees.  This is currently more
>> or less impossible.  Hallelujah!
>>
>> You cannot use pA to try to subvert a setuid, setgid, or file-capped
>> program: if you execute any such program, pA gets cleared and the
>> resulting evolution rules are unchanged by this patch.
>>
>> Users with nonzero pA are unlikely to unintentionally leak that
>> capability.  If they run programs that try to drop privileges,
>> dropping privileges will still work.
>>
>> It's worth noting that the degree of paranoia in this patch could
>> possibly be reduced without causing serious problems.  Specifically,
>> if we allowed pA to persist across executing non-pA-aware setuid
>> binaries and across setresuid, then, naively, the only capabilities
>> that could leak as a result would be the capabilities in pA, and any
>> attacker *already* has those capabilities.  This would make me
>> nervous, though -- setuid binaries that tried to privilege-separate
>> might fail to do so, and putting CAP_DAC_READ_SEARCH or
>> CAP_DAC_OVERRIDE into pA could have unexpected side effects.
>> (Whether these unexpected side effects would be exploitable is an
>> open question.)  I've therefore taken the more paranoid route.  We
>> can revisit this later.
>
> I think this is correct. Stuff using file caps, or set*id bits are
> fundamentally using a different privilege management model. Keeping pA
> separate makes a lot of sense to me.
>
>> An alternative would be to require PR_SET_NO_NEW_PRIVS before
>> setting ambient capabilities.  I think that this would be annoying
>> and would make granting otherwise unprivileged users minor ambient
>> capabilities (CAP_NET_BIND_SERVICE or CAP_NET_RAW for example) much
>> less useful than it is with this patch.
>
> Agreed: we should keep nnp out of this.
>
>> ===== Footnotes =====
>>
>> [1] Files that are missing the "security.capability" xattr or that
>> have unrecognized values for that xattr end up with has_cap set to
>> false.  The code that does that appears to be complicated for no
>> good reason.
>
> Would it make more sense to have has_cap true, but have it lack any actual caps?

I assume you're referring to the case where we fail to parse the
xattr.  If so, I don't really know if or when this happens.  Should
that be addressed separately from this patch set?

>
>> [2] The libcap capability mask parsers and formatters are
>> dangerously misleading and the documentation is flat-out wrong.  fE
>> is *not* a mask; it's a single bit.  This has probably confused
>> every single person who has tried to use file capabilities.
>
> Sounds like it would be a valuable documentation patch.

I'll try.  Let's get the current thing done first.

>
>> [3] Linux very confusingly processes both the script and the
>> interpreter if applicable, for reasons that elude me.  The results
>> from thinking about a script's file capabilities and/or setuid bits
>> are mostly discarded.
>
> I wonder if this is important enough to fix?

Not sure.

However, the fact that AFAICT LSM due to a script (as opposed to an
interpreter) is preserved sounds rather dangerous to me.  I'm not sure
whether we can safely fix that at this point.

--Andy

Re: [PATCH v3 1/2] capabilities: Ambient capabilities

[Kees Cook]

On Tue, Jun 9, 2015 at 5:00 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Tue, Jun 9, 2015 at 4:09 PM, Kees Cook <keescook@chromium.org> wrote:
>> On Wed, May 27, 2015 at 4:47 PM, Andy Lutomirski <luto@kernel.org> wrote:
>>> [1] Files that are missing the "security.capability" xattr or that
>>> have unrecognized values for that xattr end up with has_cap set to
>>> false.  The code that does that appears to be complicated for no
>>> good reason.
>>
>> Would it make more sense to have has_cap true, but have it lack any actual caps?
>
> I assume you're referring to the case where we fail to parse the
> xattr.  If so, I don't really know if or when this happens.  Should
> that be addressed separately from this patch set?

No, no. All of these footnotes seem to be separate from the series. I
just wanted to chime in on them, since none of them really sounded
like they should get dropped.

>>> [2] The libcap capability mask parsers and formatters are
>>> dangerously misleading and the documentation is flat-out wrong.  fE
>>> is *not* a mask; it's a single bit.  This has probably confused
>>> every single person who has tried to use file capabilities.
>>
>> Sounds like it would be a valuable documentation patch.
>
> I'll try.  Let's get the current thing done first.

Yup! No worries. I'm happy to help with the docs, too.

>>> [3] Linux very confusingly processes both the script and the
>>> interpreter if applicable, for reasons that elude me.  The results
>>> from thinking about a script's file capabilities and/or setuid bits
>>> are mostly discarded.
>>
>> I wonder if this is important enough to fix?
>
> Not sure.
>
> However, the fact that AFAICT LSM due to a script (as opposed to an
> interpreter) is preserved sounds rather dangerous to me.  I'm not sure
> whether we can safely fix that at this point.

Agreed.

I missed adding:

Acked-by: Kees Cook <keescook@chromium.org>

I would love to use this already. :)

-Kees

-- 
Kees Cook
Chrome OS Security