Subject: CVE-2021-47281: ALSA: seq: Fix race of snd_seq_timer_open()

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

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

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

ALSA: seq: Fix race of snd_seq_timer_open()

The timer instance per queue is exclusive, and snd_seq_timer_open()
should have managed the concurrent accesses.  It looks as if it's
checking the already existing timer instance at the beginning, but
it's not right, because there is no protection, hence any later
concurrent call of snd_seq_timer_open() may override the timer
instance easily.  This may result in UAF, as the leftover timer
instance can keep running while the queue itself gets closed, as
spotted by syzkaller recently.

For avoiding the race, add a proper check at the assignment of
tmr->timeri again, and return -EBUSY if it's been already registered.

The Linux kernel CVE team has assigned CVE-2021-47281 to this issue.

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

	Fixed in 5.10.44 with commit bd7d88b0874f
	Fixed in 5.12.11 with commit 536a7646c00a
	Fixed in 5.13 with commit 83e197a8414c

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-2021-47281
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_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/bd7d88b0874f82f7b29d1a53e574cedaf23166ba
	https://git.kernel.org/stable/c/536a7646c00a0f14fee49e5e313109e5da2f6031
	https://git.kernel.org/stable/c/83e197a8414c0ba545e7e3916ce05f836f349273