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

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-74337 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.10 with commit 3a08c2fd763450a927d1130de078d6f9e74944fb and fixed in 6.18.40 with commit 8b0510cc3a4a000d4ed1a56cd96231f3d3ba94c5
	Issue introduced in 4.10 with commit 3a08c2fd763450a927d1130de078d6f9e74944fb and fixed in 7.1.5 with commit 440a2fdbb40608d55a7b11f2be53592a3785131d
	Issue introduced in 4.10 with commit 3a08c2fd763450a927d1130de078d6f9e74944fb and fixed in 7.2-rc1 with commit 89edbdfc5d0308cef57b71359331de5c4ddbf763

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74337
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	kernel/bpf/bpf_lru_list.c
	kernel/bpf/bpf_lru_list.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/8b0510cc3a4a000d4ed1a56cd96231f3d3ba94c5
	https://git.kernel.org/stable/c/440a2fdbb40608d55a7b11f2be53592a3785131d
	https://git.kernel.org/stable/c/89edbdfc5d0308cef57b71359331de5c4ddbf763