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 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.

This vulnerability is rated as Moderate severity for Red Hat Enterprise Linux due to requiring local system access coupled with strict, microsecond-level race timing to trigger inconsistent slab states. In standard production environments, the required preemption sequence during concurrent allocation and free cycles is difficult to reliably achieve without crashing the system or failing to corrupt memory. Exploitation cannot be conducted remotely and is primarily limited to inducing system instability or kernel crashes.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.