CVE-2026-53162memcg: use round-robin victim selection in refill_stock

Official description

In the Linux kernel, the following vulnerability has been resolved: memcg: use round-robin victim selection in refill_stock Harry Yoo reported that get_random_u32_below() is not safe to call in the nmi context and memcg charge draining can happen in nmi context. More specifically get_random_u32_below() is neither reentrant- nor NMI-safe: it acquires a per-cpu local_lock via local_lock_irqsave() on the batched_entropy_u32 state. An NMI that lands on a CPU mid-update of the ChaCha batch state and recurses into the random subsystem would corrupt that state. The memcg_stock local_trylock prevents re-entry on the percpu stock itself, but cannot protect an unrelated subsystem's per-cpu lock. Replace the random pick with a per-cpu round-robin counter stored in memcg_stock_pcp and serialized by the same local_trylock that already guards cached[] and nr_pages[]. No atomics, no random calls, no extra locks needed.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/89bd8215e25aa6999cc51696da418e0d422bc5e0
• https://git.kernel.org/stable/c/00731bd7e18f182a32ca54d6b176eaa470b51ed7
• https://git.kernel.org/stable/c/c0cafe24d3f6534294c4b2bc2d47734ff7cbd313