CVE-2026-64031: erofs: fix managed cache race for unaligned extents

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

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

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

erofs: fix managed cache race for unaligned extents

After unaligned compressed extents were introduced, the following race
could occur:

[Thread 1]                                   [Thread 2]
(z_erofs_fill_bio_vec)
<handle a Z_EROFS_PREALLOCATED_FOLIO folio>
...
filemap_add_folio (1)
                                             (z_erofs_bind_cache)
                                             <the same folio is found..>
                                             ..
                                             ..
folio_attach_private (2)
                                             filemap_add_folio (3) again

Since (1) is executed but (2) hasn't been executed yet, it's possible
that another thread finds the same managed folio in z_erofs_bind_cache()
for a different pcluster and calls filemap_add_folio() again since
folio->private is still Z_EROFS_PREALLOCATED_FOLIO.

Fix this by explicitly clearing folio->private before making the folio
visible in the managed cache so that another pcluster can simply wait
on the locked managed folio as what we did for other shared cases [1].

This only impacts unaligned data compression (`-E48bit` with zstd,
for example).

[1] Commit 9e2f9d34dd12 ("erofs: handle overlapped pclusters out of
 crafted images properly") was originally introduced to handle crafted
 overlapped extents, but it addresses unaligned extents as well.

The Linux kernel CVE team has assigned CVE-2026-64031 to this issue.


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

	Issue introduced in 6.15 with commit 7361d1e3763baaf7b9349c576137851458ad38d1 and fixed in 6.18.34 with commit 425d32d6288d7d845e486af9419bbedccd8c9103
	Issue introduced in 6.15 with commit 7361d1e3763baaf7b9349c576137851458ad38d1 and fixed in 7.0.11 with commit 038166f873c4caf6e85cfd4ea0c5a5ba297b4e8b
	Issue introduced in 6.15 with commit 7361d1e3763baaf7b9349c576137851458ad38d1 and fixed in 7.1 with commit 649932fc3815eda2f24eb4de4b3a5e94886ee0b9

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-2026-64031
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/erofs/zdata.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/425d32d6288d7d845e486af9419bbedccd8c9103
	https://git.kernel.org/stable/c/038166f873c4caf6e85cfd4ea0c5a5ba297b4e8b
	https://git.kernel.org/stable/c/649932fc3815eda2f24eb4de4b3a5e94886ee0b9