CVE-2026-74339: ALSA: seq: Clear variable event pointer on read

CVE-2026-74339: ALSA: seq: Clear variable event pointer on read

[Greg Kroah-Hartman]

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

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

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

ALSA: seq: Clear variable event pointer on read

snd_seq_read() copies a queued variable-length event header to userspace
before expanding the payload. Queued variable-length events use
SNDRV_SEQ_EXT_CHAINED internally, and data.ext.ptr points at the first
extension cell.

The read side strips SNDRV_SEQ_EXT_* bits from data.ext.len before the
copy, but it leaves data.ext.ptr untouched. A userspace sequencer client
can therefore write a direct variable event to itself and read back the
extension-cell kernel address from the returned header.

Clear the temporary header pointer before copy_to_user(). The original
queued event remains unchanged and is still passed to
snd_seq_expand_var_event(), so payload expansion keeps using the
internal chain.

The Linux kernel CVE team has assigned CVE-2026-74339 to this issue.


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

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 5.15.212 with commit 14fe4f75fd5309d6b75f8e840ada88912b374207
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.1.178 with commit f29243c211928114f8b906e0a3fee77c236f14c8
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.6.145 with commit e0c3edd86414534cfd179fefe45b38c29c01ae7a
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.12.97 with commit 6b52211eba213c461f68922708a99d8190c1fcd5
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.18.40 with commit 74ac1ce1f4afdb3b80b6742fa28fb86c8c51d31b
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.1.5 with commit c2ac9287e89916da684c2a548798351e63eb59ee
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.2-rc1 with commit 705dd6dcbc0ea87351c660c1a6443f85f1001c76

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-2026-74339
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:
	sound/core/seq/seq_clientmgr.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/14fe4f75fd5309d6b75f8e840ada88912b374207
	https://git.kernel.org/stable/c/f29243c211928114f8b906e0a3fee77c236f14c8
	https://git.kernel.org/stable/c/e0c3edd86414534cfd179fefe45b38c29c01ae7a
	https://git.kernel.org/stable/c/6b52211eba213c461f68922708a99d8190c1fcd5
	https://git.kernel.org/stable/c/74ac1ce1f4afdb3b80b6742fa28fb86c8c51d31b
	https://git.kernel.org/stable/c/c2ac9287e89916da684c2a548798351e63eb59ee
	https://git.kernel.org/stable/c/705dd6dcbc0ea87351c660c1a6443f85f1001c76