From: Takashi Iwai Date: Mon, 4 Jul 2016 12:02:15 +0000 (+0200) Subject: ALSA: timer: Fix negative queue usage by racy accesses X-Git-Tag: v3.16.37~38 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=cb2955d6be6202b181a3598f32013ae68cf31b53;p=thirdparty%2Fkernel%2Fstable.git 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 --- diff --git a/sound/core/timer.c b/sound/core/timer.c index 17538961e3570..7dfb741a91b4d 100644 --- a/sound/core/timer.c +++ b/sound/core/timer.c @@ -1965,6 +1965,7 @@ 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) { @@ -1978,7 +1979,6 @@ 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;