ALSA: usb-audio: Check mixer unit descriptors more strictly

We've had some sanity checks of the mixer unit descriptors but they
are too loose and some corner cases are overlooked.  Add more strict
checks in uac_mixer_unit_get_channels() for avoiding possible OOB
accesses by malformed descriptors.

This also changes the semantics of uac_mixer_unit_get_channels()
slightly.  Now it returns zero for the cases where the descriptor
lacks of bmControls instead of -EINVAL.  Then the caller side skips
the mixer creation for such unit while it keeps parsing it.
This corresponds to the case like Maya44.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -753,8 +753,9 @@ static int uac_mixer_unit_get_channels(struct mixer_build *state,
 				       struct uac_mixer_unit_descriptor *desc)
 {
 	int mu_channels;
+	void *c;
 
-	if (desc->bLength < 11)
+	if (desc->bLength < sizeof(*desc))
 		return -EINVAL;
 	if (!desc->bNrInPins)
 		return -EINVAL;
@@ -763,6 +764,8 @@ static int uac_mixer_unit_get_channels(struct mixer_build *state,
 	case UAC_VERSION_1:
 	case UAC_VERSION_2:
 	default:
+		if (desc->bLength < sizeof(*desc) + desc->bNrInPins + 1)
+			return 0; /* no bmControls -> skip */
 		mu_channels = uac_mixer_unit_bNrChannels(desc);
 		break;
 	case UAC_VERSION_3:
@@ -772,7 +775,11 @@ static int uac_mixer_unit_get_channels(struct mixer_build *state,
 	}
 
 	if (!mu_channels)
-		return -EINVAL;
+		return 0;
+
+	c = uac_mixer_unit_bmControls(desc, state->mixer->protocol);
+	if (c - (void *)desc + (mu_channels - 1) / 8 >= desc->bLength)
+		return 0; /* no bmControls -> skip */
 
 	return mu_channels;
 }
@@ -944,7 +951,7 @@ static int check_input_term(struct mixer_build *state, int id,
 				struct uac_mixer_unit_descriptor *d = p1;
 
 				err = uac_mixer_unit_get_channels(state, d);
-				if (err < 0)
+				if (err <= 0)
 					return err;
 
 				term->channels = err;
@@ -2118,7 +2125,7 @@ static int parse_audio_mixer_unit(struct mixer_build *state, int unitid,
 		if (err < 0)
 			continue;
 		/* no bmControls field (e.g. Maya44) -> ignore */
-		if (desc->bLength <= 10 + input_pins)
+		if (!num_outs)
 			continue;
 		err = check_input_term(state, desc->baSourceID[pin], &iterm);
 		if (err < 0)