linux-cve-announce - CVE-2022-50668: ext4: fix deadlock due to mbcache entry corruption

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Message-ID: <2025120945-CVE-2022-50668-153a@gregkh>
Date: Tue,  9 Dec 2025 10:30:53 +0900
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2022-50668: ext4: fix deadlock due to mbcache entry corruption

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

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

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

ext4: fix deadlock due to mbcache entry corruption

When manipulating xattr blocks, we can deadlock infinitely looping
inside ext4_xattr_block_set() where we constantly keep finding xattr
block for reuse in mbcache but we are unable to reuse it because its
reference count is too big. This happens because cache entry for the
xattr block is marked as reusable (e_reusable set) although its
reference count is too big. When this inconsistency happens, this
inconsistent state is kept indefinitely and so ext4_xattr_block_set()
keeps retrying indefinitely.

The inconsistent state is caused by non-atomic update of e_reusable bit.
e_reusable is part of a bitfield and e_reusable update can race with
update of e_referenced bit in the same bitfield resulting in loss of one
of the updates. Fix the problem by using atomic bitops instead.

This bug has been around for many years, but it became *much* easier
to hit after commit 65f8b80053a1 ("ext4: fix race when reusing xattr
blocks").

The Linux kernel CVE team has assigned CVE-2022-50668 to this issue.


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

	Issue introduced in 4.6 with commit 6048c64b26097a0ffbd966866b599f990e674e9b and fixed in 4.19.270 with commit efaa0ca678f56d47316a08030b2515678cebbc50
	Issue introduced in 4.6 with commit 6048c64b26097a0ffbd966866b599f990e674e9b and fixed in 5.4.229 with commit af53065276376750dfac35a7248af18806404c5d
	Issue introduced in 4.6 with commit 6048c64b26097a0ffbd966866b599f990e674e9b and fixed in 5.10.163 with commit 1be16a0c2f10186df505e28b0cc92d7f3366e2a8
	Issue introduced in 4.6 with commit 6048c64b26097a0ffbd966866b599f990e674e9b and fixed in 5.15.87 with commit 5bc0b2fda4b47c86278f7c6d30c211f425bf51cf
	Issue introduced in 4.6 with commit 6048c64b26097a0ffbd966866b599f990e674e9b and fixed in 6.0.18 with commit 127b80cefb941a81255c72f11081123f3a705369
	Issue introduced in 4.6 with commit 6048c64b26097a0ffbd966866b599f990e674e9b and fixed in 6.1.4 with commit cc1538c693d25e282bed8c54b65c914a04023a78
	Issue introduced in 4.6 with commit 6048c64b26097a0ffbd966866b599f990e674e9b and fixed in 6.2 with commit a44e84a9b7764c72896f7241a0ec9ac7e7ef38dd

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-2022-50668
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:
	fs/ext4/xattr.c
	fs/mbcache.c
	include/linux/mbcache.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/efaa0ca678f56d47316a08030b2515678cebbc50
	https://git.kernel.org/stable/c/af53065276376750dfac35a7248af18806404c5d
	https://git.kernel.org/stable/c/1be16a0c2f10186df505e28b0cc92d7f3366e2a8
	https://git.kernel.org/stable/c/5bc0b2fda4b47c86278f7c6d30c211f425bf51cf
	https://git.kernel.org/stable/c/127b80cefb941a81255c72f11081123f3a705369
	https://git.kernel.org/stable/c/cc1538c693d25e282bed8c54b65c914a04023a78
	https://git.kernel.org/stable/c/a44e84a9b7764c72896f7241a0ec9ac7e7ef38dd