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

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

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().

The Linux kernel CVE team has assigned CVE-2026-64419 to this issue.


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

	Issue introduced in 6.0 with commit 5035ebc644aec92d55d1bbfe042f35341e4bffb5 and fixed in 6.1.178 with commit de5f69b8dae8698ac5e48dfcd30017887cdf4e5a
	Issue introduced in 6.0 with commit 5035ebc644aec92d55d1bbfe042f35341e4bffb5 and fixed in 6.6.145 with commit e441cbfbd0eaa6404278e985033c33caba4db767
	Issue introduced in 6.0 with commit 5035ebc644aec92d55d1bbfe042f35341e4bffb5 and fixed in 6.12.96 with commit 2fed79f0fe8c8d28a972c290dbfd693c3546c8c4
	Issue introduced in 6.0 with commit 5035ebc644aec92d55d1bbfe042f35341e4bffb5 and fixed in 6.18.39 with commit 560e21e8ccff813e84d05f6500907c549a3d6985
	Issue introduced in 6.0 with commit 5035ebc644aec92d55d1bbfe042f35341e4bffb5 and fixed in 7.1.4 with commit 86237e56091e70f09c0fbf217f9d9c0e08f556c4
	Issue introduced in 6.0 with commit 5035ebc644aec92d55d1bbfe042f35341e4bffb5 and fixed in 7.2-rc1 with commit b902890c62d200b3509cb5e09cf1e0a66553c128

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-2026-64419
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:
	mm/shrinker_debug.c


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/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