On Sat, Jan 18, 2020 at 02:19:08AM +0100, Christian Brauner wrote:

Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat") introduced the ability to opt out of audit messages for accesses to various proc files since they are not violations of policy. While doing so it somehow switched the check from ns_capable() to has_ns_capability{_noaudit}(). That means it switched from checking the subjective credentials of the task to using the objective credentials. This is wrong since. ptrace_has_cap() is currently only used in ptrace_may_access() And is used to check whether the calling task (subject) has the CAP_SYS_PTRACE capability in the provided user namespace to operate on the target task (object). According to the cred.h comments this would mean the subjective credentials of the calling task need to be used. This switches ptrace_has_cap() to use security_capable(). Because we only call ptrace_has_cap() in ptrace_may_access() and in there we already have a stable reference to the calling task's creds under rcu_read_lock() there's no need to go through another series of dereferences and rcu locking done in ns_capable{_noaudit}().

As one example where this might be particularly problematic, Jann pointed out that in combination with the upcoming IORING_OP_OPENAT feature, this bug might allow unprivileged users to bypass the capability checks while asynchronously opening files like /proc/*/mem, because the capability checks for this would be performed against kernel credentials.

To illustrate on the former point about this being exploitable: When io_uring creates a new context it records the subjective credentials of the caller. Later on, when it starts to do work it creates a kernel thread and registers a callback. The callback runs with kernel creds for ktask->real_cred and ktask->cred. To prevent this from becoming a full-blown 0-day io_uring will call override_cred() and override ktask->cred with the subjective credentials of the creator of the io_uring instance. With ptrace_has_cap() currently looking at ktask->real_cred this override will be ineffective and the caller will be able to open arbitray proc files as mentioned above. Luckily, this is currently not exploitable but will turn into a 0-day once IORING_OP_OPENAT{2} land in v5.6. Fix it now!

Cc: Oleg Nesterov oleg@redhat.com Cc: Eric Paris eparis@redhat.com Cc: stable@vger.kernel.org Reviewed-by: Kees Cook keescook@chromium.org

Looks good! Just to close the loop, yes, my Reviewed-by can stand. :)

-Kees

Reviewed-by: Serge Hallyn serge@hallyn.com Reviewed-by: Jann Horn jannh@google.com Fixes: 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat") Signed-off-by: Christian Brauner christian.brauner@ubuntu.com

---

/* v1 */ Link: https://lore.kernel.org/r/20200115171736.16994-1-christian.brauner@ubuntu.co...

/* v2 */ Link: https://lore.kernel.org/r/20200116224518.30598-1-christian.brauner@ubuntu.co...

• Christian Brauner christian.brauner@ubuntu.com:
  • fix incorrect CAP_OPT_NOAUDIT, CAPT_OPT_NONE order

/* v3 */ Link: https://lore.kernel.org/r/20200117105717.29803-1-christian.brauner@ubuntu.co...

• Kees Cook keescook@chromium.org:
  • remove misleading reference to cread guard mutex from commit message
  • replace if-branches with ternary ?: operator

/* v4 */

• Kees Cook keescook@chromium.org:
  • use security_capable() == 0 on return
• Christian Brauner christian.brauner@ubuntu.com:
  • replace ?: operator with if-branches since we need to check against 0. This makes it more legible.

---

kernel/ptrace.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c index cb9ddcc08119..43d6179508d6 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -264,12 +264,17 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state) return ret; } -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode) +static bool ptrace_has_cap(const struct cred *cred, struct user_namespace *ns,

• 	   unsigned int mode)
  

{

• int ret;
• if (mode & PTRACE_MODE_NOAUDIT)

• return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
  

• ret = security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NOAUDIT);
  

  else

• return has_ns_capability(current, ns, CAP_SYS_PTRACE);
  

• ret = security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NONE);
  

• return ret == 0;

} /* Returns 0 on success, -errno on denial. */ @@ -321,7 +326,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode) gid_eq(caller_gid, tcred->sgid) && gid_eq(caller_gid, tcred->gid)) goto ok;

• if (ptrace_has_cap(tcred->user_ns, mode))

• if (ptrace_has_cap(cred, tcred->user_ns, mode)) goto ok; rcu_read_unlock(); return -EPERM;

@@ -340,7 +345,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode) mm = task->mm; if (mm && ((get_dumpable(mm) != SUID_DUMP_USER) &&

•     !ptrace_has_cap(mm->user_ns, mode)))
  

•     !ptrace_has_cap(cred, mm->user_ns, mode)))
   return -EPERM;
  

return security_ptrace_access_check(task, mode);

base-commit: b3a987b0264d3ddbb24293ebff10eddfc472f653

2.25.0