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Message-ID: <CAGXu5jKfvswdUMcrGbr6+wn1ME_JkySMvdNd0wvvHpbQ1C7HUg@mail.gmail.com>
Date: Thu, 12 Mar 2015 14:49:14 -0700
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...nel.org>
Cc: Christoph Lameter <cl@...ux.com>,
Serge Hallyn <serge.hallyn@...onical.com>,
Andy Lutomirski <luto@...capital.net>,
Jonathan Corbet <corbet@....net>,
Aaron Jones <aaronmdjones@...il.com>,
"Ted Ts'o" <tytso@....edu>,
linux-security-module <linux-security-module@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
akpm@...uxfoundation.org, "Andrew G. Morgan" <morgan@...nel.org>,
Mimi Zohar <zohar@...ux.vnet.ibm.com>,
Austin S Hemmelgarn <ahferroin7@...il.com>,
Markku Savela <msa@...h.iki.fi>,
Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC] capabilities: Ambient capabilities
On Thu, Mar 12, 2015 at 11:08 AM, Andy Lutomirski <luto@...nel.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 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@...omium.org>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Serge Hallyn <serge.hallyn@...onical.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Aaron Jones <aaronmdjones@...il.com>
> CC: Ted Ts'o <tytso@....edu>
> Cc: linux-security-module@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-api@...r.kernel.org
> Cc: akpm@...uxfoundation.org
> Cc: Andrew G. Morgan <morgan@...nel.org>
> Cc: Mimi Zohar <zohar@...ux.vnet.ibm.com>
> Cc: Austin S Hemmelgarn <ahferroin7@...il.com>
> Cc: Markku Savela <msa@...h.iki.fi>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> Cc: Michael Kerrisk <mtk.manpages@...il.com>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
This would be quite welcome for things we're doing in Chrome OS.
Presently, we're able to use fscaps to keep non-root caps across exec
and haven't encountered issues with AT_SECURE (yet), but using pA
would be much nicer and exactly matches how we want to use it: a
launcher is creating a tree of processes that are non-root but need
some capabilities. Right now the tree is very small and we're able to
sprinkle our fscaps lightly. :) This would be better.
-Kees
> ---
>
> 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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--
Kees Cook
Chrome OS Security
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