Bug 2422710 (CVE-2025-40348)

Summary: CVE-2025-40348 kernel: slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: high    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A race condition was found in the Linux kernel's slab allocator. When two threads concurrently call alloc_slab_obj_exts() and one fails allocation, it may overwrite the other's valid obj_exts pointer with OBJEXTS_ALLOC_FAIL. The winning thread later dereferences this as a valid pointer, causing a null pointer dereference.
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Description OSIDB Bzimport 2025-12-16 14:05:34 UTC 
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.
Comment 1 Alexander B 2025-12-17 01:24:34 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025121635-CVE-2025-40348-4387@gregkh/T