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Date: Sun, 30 Jan 2022 08:45:41 +0100
From: Salvatore Bonaccorso <carnil@...ian.org>
To: oss-security@...ts.openwall.com
Subject: Re: Linux kernel: use-after-free of user namespace on
shm and mqueue destruction
Hi,
On Sat, Jan 29, 2022 at 08:07:27PM +0100, Mathias Krause wrote:
> Hi!
>
> 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
> git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace")
> 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:
> https://git.kernel.org/linus/f9d87929d451d3e649699d0f1d74f71f77ad38f5
>
> However, in our opinion neither the commit itself nor its merge commit
> (https://git.kernel.org/linus/76fcbc9c7c57a5d4) 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
> security@...nel.org 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();
> 1199
> 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 security@...nel.org, 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.
This issue has been assigned CVE-2022-24122 by MITRE via
https://cveform.mitre.org/ .
Regards,
Salvatore
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