ALSA: timer: Fix negative queue usage by racy accesses

tiwai · tiwai · commit 3fa6993fef63 · 2016-07-04T14:02:15.000+02:00
The user timer tu-&gt;qused counter may go to a negative value when
multiple concurrent reads are performed since both the check and the
decrement of tu-&gt;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-&gt;qused counter into the
same spinlock context as the zero-check of the counter.

Cc: &lt;stable@vger.kernel.org&gt;
Signed-off-by: Takashi Iwai &lt;tiwai@suse.de&gt;
diff --git a/sound/core/timer.c b/sound/core/timer.c
@@ -1955,6 +1955,7 @@ 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) {
@@ -1968,7 +1969,6 @@ 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;
