[PATCH 4.19 280/306] ALSA: usb-audio: Fix NULL dereference at parsing BADD

[Greg Kroah-Hartman]

From: Takashi Iwai <tiwai@xxxxxxx>

commit 9435f2bb66874a0c4dd25e7c978957a7ca2c93b1 upstream.

snd_usb_mixer_controls_badd() that parses UAC3 BADD profiles misses a
NULL check for the given interfaces.  When a malformed USB descriptor
is passed, this may lead to an Oops, as spotted by syzkaller.
Skip the iteration if the interface doesn't exist for avoiding the
crash.

Fixes: 17156f23e93c ("ALSA: usb: add UAC3 BADD profiles support")
Reported-by: syzbot+a36ab65c6653d7ccdd62@xxxxxxxxxxxxxxxxxxxxxxxxx
Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20191122112840.24797-1-tiwai@xxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 sound/usb/mixer.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -2949,6 +2949,9 @@ static int snd_usb_mixer_controls_badd(s
 			continue;
 
 		iface = usb_ifnum_to_if(dev, intf);
+		if (!iface)
+			continue;
+
 		num = iface->num_altsetting;
 
 		if (num < 2)