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ALSA: timer: Fix negative queue usage by racy accesses

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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>
This commit is contained in:
Takashi Iwai 2016-07-04 14:02:15 +02:00
parent 046e9ceefa
commit 3fa6993fef
1 changed files with 1 additions and 1 deletions
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2
sound/core/timer.c
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@ -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;