ALSA: seq: Fix racy access for queue timer in proc read

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.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_timer.c

@@ -471,15 +471,19 @@ void snd_seq_info_timer_read(struct snd_info_entry *entry,
 		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);
  	}
 }