[3.16.y-ckt stable] Patch "ALSA: pcm: Fix potential deadlock in OSS emulation" has been added to the 3.16.y-ckt tree

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    ALSA: pcm: Fix potential deadlock in OSS emulation

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt25.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

---8<------------------------------------------------------------

>From b9b54e197d3c7bea47ea2c0458e4e43a18aeaf94 Mon Sep 17 00:00:00 2001
From: Takashi Iwai <tiwai at suse.de>
Date: Sun, 31 Jan 2016 10:32:37 +0100
Subject: ALSA: pcm: Fix potential deadlock in OSS emulation

commit b248371628aad599a48540962f6b85a21a8a0c3f upstream.

There are potential deadlocks in PCM OSS emulation code while
accessing read/write and mmap concurrently.  This comes from the
infamous mmap_sem usage in copy_from/to_user().  Namely,

   snd_pcm_oss_write() ->
     &runtime->oss.params_lock ->
        copy_to_user() ->
          &mm->mmap_sem
  mmap() ->
    &mm->mmap_sem ->
      snd_pcm_oss_mmap() ->
        &runtime->oss.params_lock

Since we can't avoid taking params_lock from mmap code path, use
trylock variant and aborts with -EAGAIN as a workaround of this AB/BA
deadlock.

BugLink: http://lkml.kernel.org/r/CACT4Y+bVrBKDG0G2_AcUgUQa+X91VKTeS4v+wN7BSHwHtqn3kQ@mail.gmail.com
Reported-by: Dmitry Vyukov <dvyukov at google.com>
Signed-off-by: Takashi Iwai <tiwai at suse.de>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 sound/core/oss/pcm_oss.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/sound/core/oss/pcm_oss.c b/sound/core/oss/pcm_oss.c
index ada69d7a8d70..f29f1ce4a455 100644
--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -834,7 +834,8 @@ static int choose_rate(struct snd_pcm_substream *substream,
 	return snd_pcm_hw_param_near(substream, params, SNDRV_PCM_HW_PARAM_RATE, best_rate, NULL);
 }

-static int snd_pcm_oss_change_params(struct snd_pcm_substream *substream)
+static int snd_pcm_oss_change_params(struct snd_pcm_substream *substream,
+				     bool trylock)
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	struct snd_pcm_hw_params *params, *sparams;
@@ -848,7 +849,10 @@ static int snd_pcm_oss_change_params(struct snd_pcm_substream *substream)
 	struct snd_mask sformat_mask;
 	struct snd_mask mask;

-	if (mutex_lock_interruptible(&runtime->oss.params_lock))
+	if (trylock) {
+		if (!(mutex_trylock(&runtime->oss.params_lock)))
+			return -EAGAIN;
+	} else if (mutex_lock_interruptible(&runtime->oss.params_lock))
 		return -EINTR;
 	sw_params = kmalloc(sizeof(*sw_params), GFP_KERNEL);
 	params = kmalloc(sizeof(*params), GFP_KERNEL);
@@ -1093,7 +1097,7 @@ static int snd_pcm_oss_get_active_substream(struct snd_pcm_oss_file *pcm_oss_fil
 		if (asubstream == NULL)
 			asubstream = substream;
 		if (substream->runtime->oss.params) {
-			err = snd_pcm_oss_change_params(substream);
+			err = snd_pcm_oss_change_params(substream, false);
 			if (err < 0)
 				return err;
 		}
@@ -1133,7 +1137,7 @@ static int snd_pcm_oss_make_ready(struct snd_pcm_substream *substream)
 		return 0;
 	runtime = substream->runtime;
 	if (runtime->oss.params) {
-		err = snd_pcm_oss_change_params(substream);
+		err = snd_pcm_oss_change_params(substream, false);
 		if (err < 0)
 			return err;
 	}
@@ -2164,7 +2168,7 @@ static int snd_pcm_oss_get_space(struct snd_pcm_oss_file *pcm_oss_file, int stre
 	runtime = substream->runtime;

 	if (runtime->oss.params &&
-	    (err = snd_pcm_oss_change_params(substream)) < 0)
+	    (err = snd_pcm_oss_change_params(substream, false)) < 0)
 		return err;

 	info.fragsize = runtime->oss.period_bytes;
@@ -2801,7 +2805,12 @@ static int snd_pcm_oss_mmap(struct file *file, struct vm_area_struct *area)
 		return -EIO;

 	if (runtime->oss.params) {
-		if ((err = snd_pcm_oss_change_params(substream)) < 0)
+		/* use mutex_trylock() for params_lock for avoiding a deadlock
+		 * between mmap_sem and params_lock taken by
+		 * copy_from/to_user() in snd_pcm_oss_write/read()
+		 */
+		err = snd_pcm_oss_change_params(substream, true);
+		if (err < 0)
 			return err;
 	}
 #ifdef CONFIG_SND_PCM_OSS_PLUGINS