[PATCH 3.16 177/178] ALSA: timer: Fix race between read and ioctl

[Ben Hutchings]

3.16.46-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@xxxxxxx>

commit d11662f4f798b50d8c8743f433842c3e40fe3378 upstream.

The read from ALSA timer device, the function snd_timer_user_tread(),
may access to an uninitialized struct snd_timer_user fields when the
read is concurrently performed while the ioctl like
snd_timer_user_tselect() is invoked.  We have already fixed the races
among ioctls via a mutex, but we seem to have forgotten the race
between read vs ioctl.

This patch simply applies (more exactly extends the already applied
range of) tu->ioctl_lock in snd_timer_user_tread() for closing the
race window.

Reported-by: Alexander Potapenko <glider@xxxxxxxxxx>
Tested-by: Alexander Potapenko <glider@xxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 sound/core/timer.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1976,6 +1976,7 @@ static ssize_t snd_timer_user_read(struc
 
 	tu = file->private_data;
 	unit = tu->tread ? sizeof(struct snd_timer_tread) : sizeof(struct snd_timer_read);
+	mutex_lock(&tu->ioctl_lock);
 	spin_lock_irq(&tu->qlock);
 	while ((long)count - result >= unit) {
 		while (!tu->qused) {
@@ -1991,7 +1992,9 @@ static ssize_t snd_timer_user_read(struc
 			add_wait_queue(&tu->qchange_sleep, &wait);
 
 			spin_unlock_irq(&tu->qlock);
+			mutex_unlock(&tu->ioctl_lock);
 			schedule();
+			mutex_lock(&tu->ioctl_lock);
 			spin_lock_irq(&tu->qlock);
 
 			remove_wait_queue(&tu->qchange_sleep, &wait);
@@ -2011,7 +2014,6 @@ static ssize_t snd_timer_user_read(struc
 		tu->qused--;
 		spin_unlock_irq(&tu->qlock);
 
-		mutex_lock(&tu->ioctl_lock);
 		if (tu->tread) {
 			if (copy_to_user(buffer, &tu->tqueue[qhead],
 					 sizeof(struct snd_timer_tread)))
@@ -2021,7 +2023,6 @@ static ssize_t snd_timer_user_read(struc
 					 sizeof(struct snd_timer_read)))
 				err = -EFAULT;
 		}
-		mutex_unlock(&tu->ioctl_lock);
 
 		spin_lock_irq(&tu->qlock);
 		if (err < 0)
@@ -2031,6 +2032,7 @@ static ssize_t snd_timer_user_read(struc
 	}
  _error:
 	spin_unlock_irq(&tu->qlock);
+	mutex_unlock(&tu->ioctl_lock);
 	return result > 0 ? result : err;
 }