ALSA: timer: Fix negative queue usage by racy accesses

commit 3fa6993f upstream.

The user timer tu->qused counter may go to a negative value when
multiple concurrent reads are performed since both the check and the
decrement of tu->qused are done in two individual locked contexts.
This results in bogus read outs, and the endless loop in the
user-space side.

The fix is to move the decrement of the tu->qused counter into the
same spinlock context as the zero-check of the counter.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

sound/core/timer.c

@@ -1923,6 +1923,7 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 
 		qhead = tu->qhead++;
 		tu->qhead %= tu->queue_size;
+		tu->qused--;
 		spin_unlock_irq(&tu->qlock);
 
 		if (tu->tread) {
@@ -1936,7 +1937,6 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 		}
 
 		spin_lock_irq(&tu->qlock);
-		tu->qused--;
 		if (err < 0)
 			goto _error;
 		result += unit;