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Date: Sat, 29 Jan 2022 20:07:27 +0100
From: Mathias Krause <>
To: "" <>
Subject: Linux kernel: use-after-free of user namespace on shm and mqueue


A use-after-free vulnerability was found in the way certain rlimit
conversions to 'ucounts' were done, affecting kernels containing merge
commit c54b245d0118 ("Merge branch 'for-linus' of
which is Linux v5.14 and newer.

The underlying issue was already noticed last year in a KASAN report[1]
in the mqueue code but could only be recently root-caused with the help
of our report and reproducer.

The fix was merged yesterday into Linux mainline:

However, in our opinion neither the commit itself nor its merge commit
( clearly expresses the
impact of the vulnerability.

See below for some background information about 'ucounts' and our
analysis of the issue that we previously shared in a similar form with on January 21st:

The 'ucounts' scheme "bubbles up" limit changes to the uppermost user
namespace by attaching and traversing a user namespace to the 'ucounts'
object. However, that user namespace pointer isn't reference-counted. As
the lifetime of a 'ucounts' object isn't strictly tied to that of the
user namespace it was created for, it can outlive the latter, making its
'ns' member pointing to free'd memory. Such usages may happen in the shm
and mqueue code by making use of current_ucounts() and getting a
reference to it via get_ucounts().

We noticed the issue during testing and root-caused it to a
use-after-free of a user namespace object on shm destruction as follows:

1/ A process creates a new shm segment.

2/ It then forks a child that enters a new user namespace, so it gets
   its own 'ucounts' (alloc_ucounts() will create a new one via
   inc_user_namespaces(), as the namespaces differ) that gets attached
   to the new user namespace.

3/ The child process attaches its 'ucounts' to the shm object by a call
   to semctl(SHM_LOCK), see ipc/shm.c:shmctl_do_lock(), lines 1198 and
   1203 in particular:

   1197     if (cmd == SHM_LOCK) {
   1198         struct ucounts *ucounts = current_ucounts();
   1200         err = shmem_lock(shm_file, 1, ucounts);
   1201         if (!err && !(shp->shm_perm.mode & SHM_LOCKED)) {
   1202             shp->shm_perm.mode |= SHM_LOCKED;
   1203             shp->mlock_ucounts = ucounts;
   1204         }
   1205         goto out_unlock0;
   1206     }

   shmem_lock() in line 1200 calls user_shm_unlock() which calls
   get_ucounts() to get a reference to the 'ucounts' object, which
   allows the ucounts object to outlive its user namespace.

4/ The child process terminates, which leads to the destruction of its
   task_struct, the various cred objects and, in turn, the user
   namespace, as there's no reference (but pointers!) to it any more.
   The 'ucounts' object, however, survives, as it still has a live
   reference from the shmem_lock() done before. But it now has a
   dangling 'ns' pointer, as the user namespace was destroyed already.

5/ The parent process now destroys the shm segment which leads to
   shm_destroy() calling shmem_lock() with the (still valid) 'ucounts'
   of the already dead child, leading to ... -> user_shm_unlock() ->
   dec_rlimit_ucounts() dereferencing a dangling 'ns' pointer when
   trying to advance 'iter' in line 285:

   285   for (iter = ucounts; iter; iter = iter->ns->ucounts) {
   286       long dec = atomic_long_sub_return(v, &iter->ucount[type]);
   287       WARN_ON_ONCE(dec < 0);
   288       if (iter == ucounts)
   289           new = dec;
   290   }

We shared a reproducer for the bug including exploitation notes with the
report to, but we don't intend to share it any
further, as the above bug description should allow easy recreation
thereof anyway.

Exploiting this issue for privilege escalation requires the availability
of unprivileged user namespaces. With that granted, a possible way of
exploitation is by reallocating the memory of the released user
namespace object of step 4 and by introducing a type confusion bug
(ensure the user namespace release in step 4 empties the complete slab
page, get it reallocated, e.g. by some kmalloc slab cache and introduce
a fake 'user_namespace' object, e.g. via 'msg_msg' object spraying)
which will allow a decrement operation at an attacker controlled kernel
address (the '->ucounts' pointer of the crafted 'user_namespace'
object). The decrement value is under attacker control as well (the size
of the shm segment, up to RLIMIT_MEMLOCK).

Beside from patching, a possible mitigation is to disable unprivileged
user namespaces:

# sysctl -w kernel.unprivileged_userns_clone=0

To our knowledge, no CVE has been assigned to this issue so far.



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