ALSA: timer: Fix negative queue usage by racy accesses

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/timer.c b/sound/core/timer.c
index e722022..9a6157e 100644 (file)
--- 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;