Commit 3fa6993f authored Jul 04, 2016 by Takashi Iwai

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>

parent 046e9cee

sound/core/timer.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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;