CVE-2026-68201: ALSA: timer: drain a slave's callback before its master detaches it

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

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

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

ALSA: timer: drain a slave's callback before its master detaches it

snd_timer_close_locked() drains the closing instance's own in-flight
callback (IFLG_CALLBACK) before freeing it, but not its slaves'. When a
master instance is closed, remove_slave_links() clears each slave's
->timer; the slave's own close then reads timer == NULL and takes the
branch that skips the drain entirely (snd_timer_stop_slave() also no-ops
on a NULL timer). So a slave whose callback is still running when the
master is closed is freed underneath the live callback, leading to
use-after-free.

Drain the slaves too before remove_slave_links() severs them.
snd_timer_stop() has already taken this instance off the active list, so
no new slave callback can be queued. Take the slaves off the ack list so
a pending one can't fire either, then wait for any that is already in
flight.

The Linux kernel CVE team has assigned CVE-2026-68201 to this issue.


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

	Issue introduced in 6.12 with commit 37745918e0e7575bc40f38da93a99b9fa6406224 and fixed in 6.12.101 with commit cd461bcfcdf8d6b6b5365941c1d3859f8bc77aa0
	Issue introduced in 6.12 with commit 37745918e0e7575bc40f38da93a99b9fa6406224 and fixed in 6.18.42 with commit 426c0ff1c433d6030610ad4f9375746dfe931caa
	Issue introduced in 6.12 with commit 37745918e0e7575bc40f38da93a99b9fa6406224 and fixed in 7.1.6 with commit 2b298997786876b225cff2446e11a0fa6f602f6d
	Issue introduced in 6.12 with commit 37745918e0e7575bc40f38da93a99b9fa6406224 and fixed in 7.2-rc5 with commit bdefe1346a8e6b8dc8593406dc2617e985fcbcab

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-2026-68201
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/timer.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/cd461bcfcdf8d6b6b5365941c1d3859f8bc77aa0
	https://git.kernel.org/stable/c/426c0ff1c433d6030610ad4f9375746dfe931caa
	https://git.kernel.org/stable/c/2b298997786876b225cff2446e11a0fa6f602f6d
	https://git.kernel.org/stable/c/bdefe1346a8e6b8dc8593406dc2617e985fcbcab