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ALSA: timer: Fix race between read and ioctl

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@google.com>
Tested-by: Alexander Potapenko <glider@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
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tiwai committed Jun 2, 2017
1 parent dd8038e commit d11662f4f798b50d8c8743f433842c3e40fe3378
Showing with 4 additions and 2 deletions.
  1. +4 −2 sound/core/timer.c
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@@ -1959,6 +1959,7 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
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) {
@@ -1974,7 +1975,9 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
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);
@@ -1994,7 +1997,6 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
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)))
@@ -2004,7 +2006,6 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
sizeof(struct snd_timer_read)))
err = -EFAULT;
}
mutex_unlock(&tu->ioctl_lock);
spin_lock_irq(&tu->qlock);
if (err < 0)
@@ -2014,6 +2015,7 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
}
_error:
spin_unlock_irq(&tu->qlock);
mutex_unlock(&tu->ioctl_lock);
return result > 0 ? result : err;
}

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