ALSA: usb-audio: Fix potential out-of-bounce access in MIDI EP parser

The recently introduced MIDI endpoint parser code has an access to the
field without the size validation, hence it might lead to
out-of-bounce access.  Add the sanity checks for the descriptor
sizes.

Fixes: eb596e0f ("ALSA: usb-audio: generate midi streaming substream names from jack names")
Link: https://lore.kernel.org/r/20210511090500.2637-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/midi.c

@@ -1750,7 +1750,7 @@ static struct usb_midi_in_jack_descriptor *find_usb_in_jack_descriptor(
 		struct usb_midi_in_jack_descriptor *injd =
 				(struct usb_midi_in_jack_descriptor *)extra;
 
-		if (injd->bLength > 4 &&
+		if (injd->bLength >= sizeof(*injd) &&
 		    injd->bDescriptorType == USB_DT_CS_INTERFACE &&
 		    injd->bDescriptorSubtype == UAC_MIDI_IN_JACK &&
 				injd->bJackID == jack_id)
@@ -1773,7 +1773,7 @@ static struct usb_midi_out_jack_descriptor *find_usb_out_jack_descriptor(
 		struct usb_midi_out_jack_descriptor *outjd =
 				(struct usb_midi_out_jack_descriptor *)extra;
 
-		if (outjd->bLength > 4 &&
+		if (outjd->bLength >= sizeof(*outjd) &&
 		    outjd->bDescriptorType == USB_DT_CS_INTERFACE &&
 		    outjd->bDescriptorSubtype == UAC_MIDI_OUT_JACK &&
 				outjd->bJackID == jack_id)
@@ -1820,7 +1820,8 @@ static void snd_usbmidi_init_substream(struct snd_usb_midi *umidi,
 			outjd = find_usb_out_jack_descriptor(hostif, jack_id);
 			if (outjd) {
 				sz = USB_DT_MIDI_OUT_SIZE(outjd->bNrInputPins);
-				iJack = *(((uint8_t *) outjd) + sz - sizeof(uint8_t));
+				if (outjd->bLength >= sz)
+					iJack = *(((uint8_t *) outjd) + sz - sizeof(uint8_t));
 			}
 		} else {
 			/* and out jacks connect to ins */