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>

diff --git a/sound/core/timer.c b/sound/core/timer.c
index 2f836ca09860e83995630de7be6546731523489a..1118bd8e2d3c273376f7b795d26136af4881c17f 100644 (file)
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -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;
 }