[PATCH 3.16 176/254] ALSA: aloop: Fix inconsistent format due to incomplete rule

[Ben Hutchings]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@xxxxxxx>

commit b088b53e20c7d09b5ab84c5688e609f478e5c417 upstream.

The extra hw constraint rule for the formats the aloop driver
introduced has a slight flaw, where it doesn't return a positive value
when the mask got changed.  It came from the fact that it's basically
a copy&paste from snd_hw_constraint_mask64().  The original code is
supposed to be a single-shot and it modifies the mask bits only once
and never after, while what we need for aloop is the dynamic hw rule
that limits the mask bits.

This difference results in the inconsistent state, as the hw_refine
doesn't apply the dependencies fully.  The worse and surprisingly
result is that it causes a crash in OSS emulation when multiple
full-duplex reads/writes are performed concurrently (I leave why it
triggers Oops to readers as a homework).

For fixing this, replace a few open-codes with the standard
snd_mask_*() macros.

Reported-by: syzbot+3902b5220e8ca27889ca@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: b1c73fc8e697 ("ALSA: snd-aloop: Fix hw_params restrictions and checking")
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 sound/drivers/aloop.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -39,6 +39,7 @@
 #include <sound/core.h>
 #include <sound/control.h>
 #include <sound/pcm.h>
+#include <sound/pcm_params.h>
 #include <sound/info.h>
 #include <sound/initval.h>
 
@@ -623,14 +624,12 @@ static int rule_format(struct snd_pcm_hw
 {
 
 	struct snd_pcm_hardware *hw = rule->private;
-	struct snd_mask *maskp = hw_param_mask(params, rule->var);
+	struct snd_mask m;
 
-	maskp->bits[0] &= (u_int32_t)hw->formats;
-	maskp->bits[1] &= (u_int32_t)(hw->formats >> 32);
-	memset(maskp->bits + 2, 0, (SNDRV_MASK_MAX-64) / 8); /* clear rest */
-	if (! maskp->bits[0] && ! maskp->bits[1])
-		return -EINVAL;
-	return 0;
+	snd_mask_none(&m);
+	m.bits[0] = (u_int32_t)hw->formats;
+	m.bits[1] = (u_int32_t)(hw->formats >> 32);
+	return snd_mask_refine(hw_param_mask(params, rule->var), &m);
 }
 
 static int rule_rate(struct snd_pcm_hw_params *params,