CVE-2022-49700

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 dont 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 slabs 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 havent actually tested these cases though, this is just based on looking at the code. Writing testcases for this stuff seems like itd 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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/0515cc9b6b24877f59b222ade704bfaa42caa2a6
• https://git.kernel.org/stable/c/197e257da473c725dfe47759c3ee02f2398d8ea5
• https://git.kernel.org/stable/c/308c6d0e1f200fd26c71270c6e6bfcf0fc6ff082
• https://git.kernel.org/stable/c/6c32496964da0dc230cea763a0e934b2e02dabd5
• 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/eeaa345e128515135ccb864c04482180c08e3259