CVE-2026-89752: mm: memcg: stop reclaim when a limit update is superseded

CVE-2026-89752: mm: memcg: stop reclaim when a limit update is superseded

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

mm: memcg: stop reclaim when a limit update is superseded

kernfs serializes file operations only per open file, so separate open
files can update the same memory.high or memory.max file concurrently. 
Both handlers store the new limit before synchronous reclaim, but continue
to use the writer's local target in the reclaim loop.  If another writer
raises or removes the limit, the first writer can continue reclaiming
toward a stale target.

For memory.max, this can leave the writer looping indefinitely once
reclaim retries are exhausted.  The OOM path sees sufficient margin under
the current limit and returns true without killing, while the writer still
compares usage against its stale target and records another OOM event.

Check the current limit at the start of each reclaim iteration and stop if
it no longer matches the writer's target.

Reproducer:

Populate a cgroup with anonymous memory and disable swapping.  Lower
memory.max from one open file, then restore it to "max" through another
open file after the new limit becomes visible.

Without the patch, the first writer remains blocked and repeatedly
increments the OOM event counter.  With the patch, it returns normally.

This was not motivated by a reported production workload.  We found it
through automated randomized testing for our cgroup observability work
and reduced it to the reproducer above.

The Linux kernel CVE team has assigned CVE-2026-89752 to this issue.


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

	Issue introduced in 4.6 with commit b6e6edcfa40561e9c8abe5eecf1c96f8e5fd9c6f and fixed in 6.12.109 with commit 39ec1e4183a718f448b2e9de681dcbdda18fce42
	Issue introduced in 4.6 with commit b6e6edcfa40561e9c8abe5eecf1c96f8e5fd9c6f and fixed in 6.18.50 with commit 95d87030cae7764805aa7c621c52871139297cb0
	Issue introduced in 4.6 with commit b6e6edcfa40561e9c8abe5eecf1c96f8e5fd9c6f and fixed in 7.2.4 with commit aab391074de96ed601eeca32daef9a8507d436d9
	Issue introduced in 4.6 with commit b6e6edcfa40561e9c8abe5eecf1c96f8e5fd9c6f and fixed in 7.3-rc1 with commit 9477820c63cbf4d97114238f3d1ff10dfd6bee3f
	Issue introduced in 4.4.7 with commit 0ccab5b139971a2a3f48df24d1ee8be2dbf84042
	Issue introduced in 4.5.1 with commit 97729c37439040ec07807f0dbb71ea60ccbafd27

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-2026-89752
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/memcontrol.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/39ec1e4183a718f448b2e9de681dcbdda18fce42
	https://git.kernel.org/stable/c/95d87030cae7764805aa7c621c52871139297cb0
	https://git.kernel.org/stable/c/aab391074de96ed601eeca32daef9a8507d436d9
	https://git.kernel.org/stable/c/9477820c63cbf4d97114238f3d1ff10dfd6bee3f