From 6530d0788e7048a1f9879d476122c916c6f72a96 Mon Sep 17 00:00:00 2001 From: Takashi Iwai Date: Mon, 4 Jul 2016 14:02:15 +0200 Subject: [PATCH] ALSA: timer: Fix negative queue usage by racy accesses 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 Signed-off-by: Ben Hutchings --- sound/core/timer.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/sound/core/timer.c b/sound/core/timer.c index bc83a9653c76..3e36dbe52900 100644 --- a/sound/core/timer.c +++ b/sound/core/timer.c @@ -1923,6 +1923,7 @@ 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) { @@ -1936,7 +1937,6 @@ 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; -- 2.39.2