Commit a851e568 authored Jul 04, 2016 by Takashi Iwai Committed by Sasha Levin Jul 19, 2016

ALSA: timer: Fix negative queue usage by racy accesses



[ Upstream commit 3fa6993fef634e05d200d141a85df0b044572364 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>

parent b86ef7ef

sound/core/timer.c

+1 −1

Original line number	Original line	Diff line number	Diff line
	@@ -1959,6 +1959,7 @@ static ssize_t snd_timer_user_read(struct file file, char __user buffer,

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

	if (tu->tread) {		if (tu->tread) {
	@@ -1972,7 +1973,6 @@ static ssize_t snd_timer_user_read(struct file file, char __user buffer,
	}		}

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