Message-ID: <2025022629-CVE-2022-49700-2b3e@gregkh>
Date: Wed, 26 Feb 2025 03:24:23 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2022-49700: mm/slub: add missing TID updates on slab deactivation

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

mm/slub: add missing TID updates on slab deactivation

The fastpath in slab_alloc_node() assumes that c->slab is stable as long as
the TID stays the same. However, two places in __slab_alloc() currently
don't update the TID when deactivating the CPU slab.

If multiple operations race the right way, this could lead to an object
getting lost; or, in an even more unlikely situation, it could even lead to
an object being freed onto the wrong slab's freelist, messing up the
`inuse` counter and eventually causing a page to be freed to the page
allocator while it still contains slab objects.

(I haven't actually tested these cases though, this is just based on
looking at the code. Writing testcases for this stuff seems like it'd be
a pain...)

The race leading to state inconsistency is (all operations on the same CPU
and kmem_cache):

 - task A: begin do_slab_free():
    - read TID
    - read pcpu freelist (==NULL)
    - check `slab == c->slab` (true)
 - [PREEMPT A->B]
 - task B: begin slab_alloc_node():
    - fastpath fails (`c->freelist` is NULL)
    - enter __slab_alloc()
    - slub_get_cpu_ptr() (disables preemption)
    - enter ___slab_alloc()
    - take local_lock_irqsave()
    - read c->freelist as NULL
    - get_freelist() returns NULL
    - write `c->slab = NULL`
    - drop local_unlock_irqrestore()
    - goto new_slab
    - slub_percpu_partial() is NULL
    - get_partial() returns NULL
    - slub_put_cpu_ptr() (enables preemption)
 - [PREEMPT B->A]
 - task A: finish do_slab_free():
    - this_cpu_cmpxchg_double() succeeds()
    - [CORRUPT STATE: c->slab==NULL, c->freelist!=NULL]

>>From there, the object on c->freelist will get lost if task B is allowed to
continue from here: It will proceed to the retry_load_slab label,
set c->slab, then jump to load_freelist, which clobbers c->freelist.

But if we instead continue as follows, we get worse corruption:

 - task A: run __slab_free() on object from other struct slab:
    - CPU_PARTIAL_FREE case (slab was on no list, is now on pcpu partial)
 - task A: run slab_alloc_node() with NUMA node constraint:
    - fastpath fails (c->slab is NULL)
    - call __slab_alloc()
    - slub_get_cpu_ptr() (disables preemption)
    - enter ___slab_alloc()
    - c->slab is NULL: goto new_slab
    - slub_percpu_partial() is non-NULL
    - set c->slab to slub_percpu_partial(c)
    - [CORRUPT STATE: c->slab points to slab-1, c->freelist has objects
      from slab-2]
    - goto redo
    - node_match() fails
    - goto deactivate_slab
    - existing c->freelist is passed into deactivate_slab()
    - inuse count of slab-1 is decremented to account for object from
      slab-2

At this point, the inuse count of slab-1 is 1 lower than it should be.
This means that if we free all allocated objects in slab-1 except for one,
SLUB will think that slab-1 is completely unused, and may free its page,
leading to use-after-free.

The Linux kernel CVE team has assigned CVE-2022-49700 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 4.9.323 with commit 308c6d0e1f200fd26c71270c6e6bfcf0fc6ff082
	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 4.14.288 with commit d6a597450e686d4c6388bd3cdcb17224b4dae7f0
	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 4.19.252 with commit e2b2f0e2e34d71ae6c2a1114fd3c525930e84bc7
	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 5.4.205 with commit e7e3e90d671078455a3a08189f89d85b3da2de9e
	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 5.10.130 with commit 6c32496964da0dc230cea763a0e934b2e02dabd5
	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 5.15.54 with commit 0515cc9b6b24877f59b222ade704bfaa42caa2a6
	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 5.18.8 with commit 197e257da473c725dfe47759c3ee02f2398d8ea5
	Issue introduced in 3.1 with commit 03e404af26dc2ea0d278d7a342de0aab394793ce and fixed in 5.19 with commit eeaa345e128515135ccb864c04482180c08e3259

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2022-49700
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	mm/slub.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/308c6d0e1f200fd26c71270c6e6bfcf0fc6ff082
	https://git.kernel.org/stable/c/d6a597450e686d4c6388bd3cdcb17224b4dae7f0
	https://git.kernel.org/stable/c/e2b2f0e2e34d71ae6c2a1114fd3c525930e84bc7
	https://git.kernel.org/stable/c/e7e3e90d671078455a3a08189f89d85b3da2de9e
	https://git.kernel.org/stable/c/6c32496964da0dc230cea763a0e934b2e02dabd5
	https://git.kernel.org/stable/c/0515cc9b6b24877f59b222ade704bfaa42caa2a6
	https://git.kernel.org/stable/c/197e257da473c725dfe47759c3ee02f2398d8ea5
	https://git.kernel.org/stable/c/eeaa345e128515135ccb864c04482180c08e3259

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