CVE-2026-64419mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show()

Official description

In the Linux kernel, the following vulnerability has been resolved: mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() Reading the debugfs "count" file of a memcg-aware shrinker can sleep inside an RCU read-side critical section: BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:421 RCU nest depth: 1, expected: 0 css_rstat_flush mem_cgroup_flush_stats zswap_shrinker_count shrinker_debugfs_count_show shrinker_debugfs_count_show() invokes the ->count_objects() callback under rcu_read_lock(). The zswap callback flushes memcg stats via css_rstat_flush(), which may sleep, so it must not run under RCU. The RCU lock is not needed here. mem_cgroup_iter() takes RCU internally and returns a memcg holding a css reference (dropped on the next iteration or by mem_cgroup_iter_break()), so the memcg stays alive without it. The shrinker is kept alive by the open debugfs file: shrinker_free() removes the debugfs entries via debugfs_remove_recursive(), which waits for in-flight readers to drain, before call_rcu(..., shrinker_free_rcu_cb). The sibling "scan" handler already invokes the sleeping ->scan_objects() callback with no RCU section. Drop the rcu_read_lock()/rcu_read_unlock().

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/de5f69b8dae8698ac5e48dfcd30017887cdf4e5a
• https://git.kernel.org/stable/c/e441cbfbd0eaa6404278e985033c33caba4db767
• https://git.kernel.org/stable/c/2fed79f0fe8c8d28a972c290dbfd693c3546c8c4
• https://git.kernel.org/stable/c/560e21e8ccff813e84d05f6500907c549a3d6985
• https://git.kernel.org/stable/c/86237e56091e70f09c0fbf217f9d9c0e08f556c4
• https://git.kernel.org/stable/c/b902890c62d200b3509cb5e09cf1e0a66553c128