[PATCH] ASoC: max9759: fix underflow in speaker_gain_control_put()

[Dan Carpenter <dan.carpenter@oracle.com>]

Check for negative values of "priv->gain" to prevent an out of bounds
access.  The concern is that these might come from the user via:
  -> snd_ctl_elem_write_user()
    -> snd_ctl_elem_write()
      -> kctl->put()

Fixes: fa8d915172b8 ("ASoC: max9759: Add Amplifier Driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
From static analysis.  Not tested.

This patch is obviously harmless but I sometimes get confused about
these sound get()/put() functions.  I have some code in Smatch which is
supposed to manually suppress warnings from snd_ctl_elem_write() but it
was four years old and has bitrotted so that's how I got this warning.

So I remember these as being false positives where Smatch gets confused
but when I searched my mailbox I just see similar patches which were
applied.

 sound/soc/codecs/max9759.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/sound/soc/codecs/max9759.c b/sound/soc/codecs/max9759.c
index d75fd61b9032..bc57d7687f16 100644
--- a/sound/soc/codecs/max9759.c
+++ b/sound/soc/codecs/max9759.c
@@ -64,7 +64,8 @@ static int speaker_gain_control_put(struct snd_kcontrol *kcontrol,
 	struct snd_soc_component *c = snd_soc_kcontrol_component(kcontrol);
 	struct max9759 *priv = snd_soc_component_get_drvdata(c);
 
-	if (ucontrol->value.integer.value[0] > 3)
+	if (ucontrol->value.integer.value[0] < 0 ||
+	    ucontrol->value.integer.value[0] > 3)
 		return -EINVAL;
 
 	priv->gain = ucontrol->value.integer.value[0];
-- 
2.20.1