linux-cve-announce - CVE-2025-38709: loop: Avoid updating block size under exclusive owner

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Message-ID: <2025090456-CVE-2025-38709-f62c@gregkh>
Date: Thu,  4 Sep 2025 17:33:14 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-38709: loop: Avoid updating block size under exclusive owner

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

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

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

loop: Avoid updating block size under exclusive owner

Syzbot came up with a reproducer where a loop device block size is
changed underneath a mounted filesystem. This causes a mismatch between
the block device block size and the block size stored in the superblock
causing confusion in various places such as fs/buffer.c. The particular
issue triggered by syzbot was a warning in __getblk_slow() due to
requested buffer size not matching block device block size.

Fix the problem by getting exclusive hold of the loop device to change
its block size. This fails if somebody (such as filesystem) has already
an exclusive ownership of the block device and thus prevents modifying
the loop device under some exclusive owner which doesn't expect it.

The Linux kernel CVE team has assigned CVE-2025-38709 to this issue.

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

	Fixed in 6.6.103 with commit ce8da5d13d8c2a7b30b2fb376a22e8eb1a70b8bb
	Fixed in 6.12.43 with commit 139a000d20f2f38ce34296feddd641d730fe1c08
	Fixed in 6.15.11 with commit b928438cc87c0bf7ae078e4b7b6e14261e84c5c5
	Fixed in 6.16.2 with commit 5d67b30aefeb7a949040bbb1b4e3b84c5d29a624
	Fixed in 6.17-rc1 with commit 7e49538288e523427beedd26993d446afef1a6fb

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-2025-38709
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:
	drivers/block/loop.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/ce8da5d13d8c2a7b30b2fb376a22e8eb1a70b8bb
	https://git.kernel.org/stable/c/139a000d20f2f38ce34296feddd641d730fe1c08
	https://git.kernel.org/stable/c/b928438cc87c0bf7ae078e4b7b6e14261e84c5c5
	https://git.kernel.org/stable/c/5d67b30aefeb7a949040bbb1b4e3b84c5d29a624
	https://git.kernel.org/stable/c/7e49538288e523427beedd26993d446afef1a6fb