[RFC] capabilities: Ambient capabilities

From: Andy Lutomirski
Date: Thu Mar 12 2015 - 14:08:19 EST

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 pI should have done.

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 calling execve effectively drops
capabilities. Therefore, setresuid from root to nonroot
unconditionally clears pA. 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 relaxed 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.

An alternative would be to either 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 ==
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 the script and the interpreter if
applicable, for reasons that escape me. The results from thinking
about a script's file capabilities and/or setuid bits are mostly discarded.

Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Aaron Jones <aaronmdjones@xxxxxxxxx>
CC: Ted Ts'o <tytso@xxxxxxx>
Cc: linux-security-module@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxx
Cc: Andrew G. Morgan <morgan@xxxxxxxxxx>
Cc: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
Cc: Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>
Cc: Markku Savela <msa@xxxxxxxxxxx>
Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---

Preliminary userspace code is here:

https://git.kernel.org/cgit/linux/kernel/git/luto/util-linux-playground.git/commit/?h=cap_ambient&id=860c73ac1acaaae976bdd3bb83b89b0180f0702a

fs/proc/array.c | 5 ++-
include/linux/cred.h | 15 +++++++++
include/uapi/linux/prctl.h | 6 ++++
kernel/user_namespace.c | 1 +
security/commoncap.c | 75 ++++++++++++++++++++++++++++++++++++++------
security/keys/process_keys.c | 1 +
6 files changed, 92 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..a21bcba6ef84 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 +198,20 @@ static inline void validate_process_creds(void)
}
#endif

+static inline void cap_enforce_ambient_invariants(struct cred *cred)
+{
+ cred->cap_ambient = cap_intersect(cred->cap_ambient,
+ cap_intersect(cred->cap_permitted,
+ cred->cap_inheritable));
+}
+
+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..b3253886ecad 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -272,6 +272,7 @@ int cap_capset(struct cred *new,
new->cap_effective = *effective;
new->cap_inheritable = *inheritable;
new->cap_permitted = *permitted;
+ cap_enforce_ambient_invariants(new);
return 0;
}

@@ -352,6 +353,7 @@ 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 +481,12 @@ 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;

+ BUG_ON(!cap_ambient_invariant_ok(old));
+
effective = false;
ret = get_file_caps(bprm, &effective, &has_cap);
if (ret < 0)
@@ -527,8 +531,9 @@ 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 +549,23 @@ skip:
new->suid = new->fsuid = new->euid;
new->sgid = new->fsgid = new->egid;

+ /* File caps or setid cancel 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);
+
if (effective)
new->cap_effective = new->cap_permitted;
else
- cap_clear(new->cap_effective);
+ new->cap_effective = new->cap_ambient;
+
+ BUG_ON(!cap_ambient_invariant_ok(new));
+
bprm->cap_effective = effective;

/*
@@ -562,7 +580,7 @@ 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 +591,9 @@ skip:
}

new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
+
+ BUG_ON(!cap_ambient_invariant_ok(new));
+
return 0;
}

@@ -594,7 +615,7 @@ 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 +717,18 @@ 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 except 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 +958,32 @@ 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;
--
2.3.0

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