From d11662f4f798b50d8c8743f433842c3e40fe3378 Mon Sep 17 00:00:00 2001
From: Takashi Iwai <tiwai@suse.de>
Date: Fri, 2 Jun 2017 15:03:38 +0200
Subject: [PATCH] ALSA: timer: Fix race between read and ioctl
Git-commit: d11662f4f798b50d8c8743f433842c3e40fe3378
Patch-mainline: 4.12-rc5
References: CVE-2017-1000380,bsc#1044125

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>

---
 sound/core/timer.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1864,6 +1864,7 @@ 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) {
@@ -1879,7 +1880,9 @@ 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);
@@ -1918,6 +1921,7 @@ static ssize_t snd_timer_user_read(struc
 	}
 	spin_unlock_irq(&tu->qlock);
  _error:
+	mutex_unlock(&tu->ioctl_lock);
 	return result > 0 ? result : err;
 }
 
