[alsa-devel] [PATCH] ALSA: aloop: Fix access to not-yet-ready substream via cable

[Takashi Iwai]

In loopback_open() and loopback_close(), we assign and release the
substream object to the corresponding cable in a racy way.  It's
neither locked nor done in the right position.  The open callback
assigns the substream before its preparation finishes, hence the other
side of the cable may pick it up, which may lead to the invalid memory
access.

This patch addresses these: move the assignment to the end of the open
callback, and wrap with cable->lock for avoiding concurrent accesses.

Cc: <stable at vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/drivers/aloop.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index 0a08e63e9c66..1063a4377502 100644
--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -666,7 +666,9 @@ static void free_cable(struct snd_pcm_substream *substream)
 		return;
 	if (cable->streams[!substream->stream]) {
 		/* other stream is still alive */
+		spin_lock_irq(&cable->lock);
 		cable->streams[substream->stream] = NULL;
+		spin_unlock_irq(&cable->lock);
 	} else {
 		/* free the cable */
 		loopback->cables[substream->number][dev] = NULL;
@@ -705,7 +707,6 @@ static int loopback_open(struct snd_pcm_substream *substream)
 		loopback->cables[substream->number][dev] = cable;
 	}
 	dpcm->cable = cable;
-	cable->streams[substream->stream] = dpcm;
 
 	snd_pcm_hw_constraint_integer(runtime, SNDRV_PCM_HW_PARAM_PERIODS);
 
@@ -737,6 +738,11 @@ static int loopback_open(struct snd_pcm_substream *substream)
 		runtime->hw = loopback_pcm_hardware;
 	else
 		runtime->hw = cable->hw;
+
+	spin_lock_irq(&cable->lock);
+	cable->streams[substream->stream] = dpcm;
+	spin_unlock_irq(&cable->lock);
+
  unlock:
 	if (err < 0) {
 		free_cable(substream);
-- 
2.16.2