ALSA: timer: Add missing mutex lock for compat ioctls

The races among ioctl and other operations were protected by the
commit af36802 ("ALSA: timer: Fix race among timer ioctls") and
later fixes, but one code path was forgotten in the scenario: the
32bit compat ioctl.  As syzkaller recently spotted, a very similar
use-after-free may happen with the combination of compat ioctls.

The fix is simply to apply the same ioctl_lock to the compat_ioctl
callback, too.

Fixes: af36802 ("ALSA: timer: Fix race among timer ioctls")
Reference: http://lkml.kernel.org/r/089e082686ac9b482e055c832617@google.com
Reported-by: syzbot <bot+e5f3c9783e7048a74233054febbe9f1bdf54b6da@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/timer_compat.c

@@ -133,7 +133,8 @@ enum {
 #endif /* CONFIG_X86_X32 */
 };
 
-static long snd_timer_user_ioctl_compat(struct file *file, unsigned int cmd, unsigned long arg)
+static long __snd_timer_user_ioctl_compat(struct file *file, unsigned int cmd,
+					  unsigned long arg)
 {
 	void __user *argp = compat_ptr(arg);
 
@@ -153,7 +154,7 @@ static long snd_timer_user_ioctl_compat(struct file *file, unsigned int cmd, uns
 	case SNDRV_TIMER_IOCTL_PAUSE:
 	case SNDRV_TIMER_IOCTL_PAUSE_OLD:
 	case SNDRV_TIMER_IOCTL_NEXT_DEVICE:
-		return snd_timer_user_ioctl(file, cmd, (unsigned long)argp);
+		return __snd_timer_user_ioctl(file, cmd, (unsigned long)argp);
 	case SNDRV_TIMER_IOCTL_GPARAMS32:
 		return snd_timer_user_gparams_compat(file, argp);
 	case SNDRV_TIMER_IOCTL_INFO32:
@@ -167,3 +168,15 @@ static long snd_timer_user_ioctl_compat(struct file *file, unsigned int cmd, uns
 	}
 	return -ENOIOCTLCMD;
 }
+
+static long snd_timer_user_ioctl_compat(struct file *file, unsigned int cmd,
+					unsigned long arg)
+{
+	struct snd_timer_user *tu = file->private_data;
+	long ret;
+
+	mutex_lock(&tu->ioctl_lock);
+	ret = __snd_timer_user_ioctl_compat(file, cmd, arg);
+	mutex_unlock(&tu->ioctl_lock);
+	return ret;
+}