[PATCH 4.6 171/203] ALSA: timer: Fix negative queue usage by racy accesses

Greg Kroah-Hartman
2016-07-25T21:42:01Z

From: Takashi Iwai <tiwai@suse.de>

commit 3fa6993fef634e05d200d141a85df0b044572364 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
sound/core/timer.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1954,6 +1954,7 @@ static ssize_t snd_timer_user_read(struc

qhead = tu->qhead++;
tu->qhead %= tu->queue_size;
+ tu->qused--;
spin_unlock_irq(&tu->qlock);

if (tu->tread) {
@@ -1967,7 +1968,6 @@ static ssize_t snd_timer_user_read(struc
}

spin_lock_irq(&tu->qlock);
- tu->qused--;
if (err < 0)
goto _error;
result += unit;