[PATCH 5.4 023/222] ALSA: seq: Fix racy access for queue timer in proc read

Greg Kroah-Hartman
2020-01-22T09:45:15Z

From: Takashi Iwai <tiwai@suse.de>

commit 60adcfde92fa40fcb2dbf7cc52f9b096e0cd109a upstream.

snd_seq_info_timer_read() reads the information of the timer assigned
for each queue, but it's done in a racy way which may lead to UAF as
spotted by syzkaller.

This patch applies the missing q->timer_mutex lock while accessing the
timer object as well as a slight code change to adapt the standard
coding style.

Reported-by: syzbot+2b2ef983f973e5c40943@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200115203733.26530-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
sound/core/seq/seq_timer.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

--- a/sound/core/seq/seq_timer.c
+++ b/sound/core/seq/seq_timer.c
@@ -465,15 +465,19 @@ void snd_seq_info_timer_read(struct snd_
q = queueptr(idx);
if (q == NULL)
continue;
- if ((tmr = q->timer) == NULL ||
- (ti = tmr->timeri) == NULL) {
- queuefree(q);
- continue;
- }
+ mutex_lock(&q->timer_mutex);
+ tmr = q->timer;
+ if (!tmr)
+ goto unlock;
+ ti = tmr->timeri;
+ if (!ti)
+ goto unlock;
snd_iprintf(buffer, "Timer for queue %i : %s\n", q->queue, ti->timer->name);
resolution = snd_timer_resolution(ti) * tmr->ticks;
snd_iprintf(buffer, " Period time : %lu.%09lu\n", resolution / 1000000000, resolution % 1000000000);
snd_iprintf(buffer, " Skew : %u / %u\n", tmr->skew, tmr->skew_base);
+unlock:
+ mutex_unlock(&q->timer_mutex);
queuefree(q);
}
}