[PATCH] ALSA: usb-audio: Add the number of endpoints checked was 0

[Edward Adam Davis]

The user constructed a corrupted USB device, causing the USB device
enumeration phase to fail to resolve any endpoints. This resulted in
a null pointer dereference reported in [1] when the USB sound card
driver executed probe to initialize the mixer. 

To avoid the problem reported in [1], a check was added to ensure that
the number of endpoints contained in the interface was 0 when checking
the mixer status.

[1]
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: 0010:usb_endpoint_num include/uapi/linux/usb/ch9.h:479 [inline]
RIP: 0010:scarlett2_find_fc_interface sound/usb/mixer_scarlett2.c:8261 [inline]
RIP: 0010:scarlett2_init_private sound/usb/mixer_scarlett2.c:8295 [inline]
RIP: 0010:snd_scarlett2_controls_create sound/usb/mixer_scarlett2.c:8684 [inline]
RIP: 0010:snd_scarlett2_init.cold+0xbad/0x6c79 sound/usb/mixer_scarlett2.c:9407
Call Trace:
 snd_usb_mixer_apply_create_quirk+0x1c21/0x2b80 sound/usb/mixer_quirks.c:4446
 snd_usb_create_mixer+0x7a2/0x1910 sound/usb/mixer.c:3641
 usb_audio_probe+0xf6d/0x3a90 sound/usb/card.c:1033

Reported-by: syzbot+ae893a8901067fde2741@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=ae893a8901067fde2741 
Tested-by: syzbot+ae893a8901067fde2741@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
 sound/usb/mixer.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index ac8c71ba9483..bd28caec3004 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -3563,8 +3563,12 @@ static int snd_usb_mixer_status_create(struct usb_mixer_interface *mixer)
 	int buffer_length;
 	unsigned int epnum;
 
+	epnum = get_iface_desc(mixer->hostif)->bNumEndpoints;
+	if (epnum == 0)
+		return -EINVAL;
+
 	/* we need one interrupt input endpoint */
-	if (get_iface_desc(mixer->hostif)->bNumEndpoints < 1)
+	if (epnum < 1)
 		return 0;
 	ep = get_endpoint(mixer->hostif, 0);
 	if (!usb_endpoint_dir_in(ep) || !usb_endpoint_xfer_int(ep))
-- 
2.43.0