[PATCH] ALSA: usx2y: us144mkii: fix NULL deref on missing interface 0

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

A malicious USB device with the TASCAM US-144MKII device id can have a
configuration containing bInterfaceNumber=1 but no interface 0.  USB
configuration descriptors are not required to assign interface numbers
sequentially, so usb_ifnum_to_if(dev, 0) returns will NULL, which will
then be dereferenced directly.

Fix this up by checking the return value properly.

Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Fixes: dee1bcf28a3d ("ALSA: usx2y: Add native ALSA driver for TASCAM US-144MKII")
Cc: stable <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 sound/usb/usx2y/us144mkii.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sound/usb/usx2y/us144mkii.c b/sound/usb/usx2y/us144mkii.c
index 0cf4fa74e210..94553b61013c 100644
--- a/sound/usb/usx2y/us144mkii.c
+++ b/sound/usb/usx2y/us144mkii.c
@@ -420,7 +420,11 @@ static int tascam_probe(struct usb_interface *intf,
 
 	/* The device has two interfaces; we drive both from this driver. */
 	if (intf->cur_altsetting->desc.bInterfaceNumber == 1) {
-		tascam = usb_get_intfdata(usb_ifnum_to_if(dev, 0));
+		struct usb_interface *intf_zero = usb_ifnum_to_if(dev, 0);
+
+		if (!intf_zero)
+			return -ENODEV;
+		tascam = usb_get_intfdata(intf_zero);
 		if (tascam) {
 			usb_set_intfdata(intf, tascam);
 			tascam->iface1 = intf;
-- 
2.53.0