ALSA: pcm: Fix potential deadlock in OSS emulation

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@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/oss/pcm_oss.c b/sound/core/oss/pcm_oss.c
index 0e73d03..ebc9fdf 100644 (file)
--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -835,7 +835,8 @@ 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;
@@ -849,7 +850,10 @@ 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 = kzalloc(sizeof(*sw_params), GFP_KERNEL);
        params = kmalloc(sizeof(*params), GFP_KERNEL);
@@ -1092,7 +1096,7 @@ 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;
                }
@@ -1132,7 +1136,7 @@ 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;
        }
@@ -2163,7 +2167,7 @@ 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;
@@ -2804,7 +2808,12 @@ 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