linux-cve-announce - CVE-2025-21896: fuse: revert back to __readahead

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Message-ID: <2025040132-CVE-2025-21896-395c@gregkh>
Date: Tue,  1 Apr 2025 16:25:32 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2025-21896: fuse: revert back to __readahead_folio() for readahead

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

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

fuse: revert back to __readahead_folio() for readahead

In commit 3eab9d7bc2f4 ("fuse: convert readahead to use folios"), the
logic was converted to using the new folio readahead code, which drops
the reference on the folio once it is locked, using an inferred
reference on the folio. Previously we held a reference on the folio for
the entire duration of the readpages call.

This is fine, however for the case for splice pipe responses where we
will remove the old folio and splice in the new folio (see
fuse_try_move_page()), we assume that there is a reference held on the
folio for ap->folios, which is no longer the case.

To fix this, revert back to __readahead_folio() which allows us to hold
the reference on the folio for the duration of readpages until either we
drop the reference ourselves in fuse_readpages_end() or the reference is
dropped after it's replaced in the page cache in the splice case.
This will fix the UAF bug that was reported.

The Linux kernel CVE team has assigned CVE-2025-21896 to this issue.

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

	Issue introduced in 6.13 with commit 3eab9d7bc2f4ae7f3f9c9c7852ff61600df79856 and fixed in 6.13.6 with commit 60db11f1b7fba4a66b117ea998d965818784a98d
	Issue introduced in 6.13 with commit 3eab9d7bc2f4ae7f3f9c9c7852ff61600df79856 and fixed in 6.14 with commit 0c67c37e1710b2a8f61c8a02db95a51fe577e2c1

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-2025-21896
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/fuse/dev.c
	fs/fuse/file.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/60db11f1b7fba4a66b117ea998d965818784a98d
	https://git.kernel.org/stable/c/0c67c37e1710b2a8f61c8a02db95a51fe577e2c1