CVE-2025-40348 — slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts

Affected: Linux

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

slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts

If two competing threads enter alloc_slab_obj_exts() and one of them fails to allocate the object extension vector, it might override the valid slab->obj_exts allocated by the other thread with OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and expects a valid pointer to dereference a NULL pointer later on.

Update slab->obj_exts atomically using cmpxchg() to avoid slab->obj_exts overrides by racing threads.

Thanks for Vlastimil and Suren's help with debugging.

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CVE-2025-40348 — slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts