CVE-2026-74337 — bpf: Fix NMI/tracepoint re-entry deadlock on lru locks

Affected: Linux

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

bpf: Fix NMI/tracepoint re-entry deadlock on lru locks

NMI and tracepoint BPF programs can re-enter the per-CPU or global LRU lock that bpf_lru_pop_free()/push_free() already hold on the same CPU, AA-deadlocking. Lockdep reports "inconsistent {INITIAL USE} -> {IN-NMI}" on &l->lock (syzbot c69a0a2c816716f1e0d5) and "possible recursive locking detected" on &loc_l->lock (syzbot 18b26edb69b2e19f3b33).

Prior trylock and rqspinlock based fixes (see links) were nacked because compromised on reliability.

This patch converts every LRU lock site to rqspinlock_t and adds a recovery path for some failure windows to avoid node leaks.

Failure recovery:

- *_pop_free top-level: return NULL; prealloc_lru_pop() already treats that as no-free-element (-ENOMEM).

- Cross-CPU steal: skip the victim's locked loc_l, try next CPU.

- Post-steal local lock fail: publish stolen node to lockless per-CPU free_llist; next pop on this CPU picks it up.

- push_free fail: mark node pending_free=1. __local_list_flush(), __local_list_pop_pending() reclaim the node from pending_list. __bpf_lru_list_shrink_inactive() reclaims the node from inactive list. Nodes from active list are reclaimed by __bpf_lru_list_shrink() or after __bpf_lru_list_rotate_active() demotes it to the inactive.

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CVE-2026-74337 — bpf: Fix NMI/tracepoint re-entry deadlock on lru locks