[PATCH v2] ALSA: scarlett2: 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 creating
mixer controls for the Focusrite Scarlett 2nd/3rd Gen USB sound card.

[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 
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
v1 -> v2: move the check to scarlett2

 sound/usb/mixer_scarlett2.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/sound/usb/mixer_scarlett2.c b/sound/usb/mixer_scarlett2.c
index ef3150581eab..4b300226f16c 100644
--- a/sound/usb/mixer_scarlett2.c
+++ b/sound/usb/mixer_scarlett2.c
@@ -9393,6 +9393,15 @@ int snd_scarlett2_init(struct usb_mixer_interface *mixer)
 		return 0;
 	}
 
+	if (get_iface_desc(mixer->hostif)->bNumEndpoints == 0) {
+		usb_audio_err(chip,
+			"%s: There are no endpoints for %04x:%04x\n",
+			__func__,
+			USB_ID_VENDOR(chip->usb_id),
+			USB_ID_PRODUCT(chip->usb_id));
+		return 0;
+	}
+
 	usb_audio_info(chip,
 		"Focusrite %s Mixer Driver enabled (pid=0x%04x); "
 		"report any issues to "
-- 
2.43.0