[PATCH 1/2] ALSA: usb-audio: Bound MIDI endpoint descriptor scans

[Cássio Gabriel]

snd_usbmidi_get_ms_info() validates the internal MIDIStreaming endpoint
descriptor size before using baAssocJackID[], but the descriptor walker can
still return a class-specific endpoint descriptor whose bLength exceeds the
remaining bytes in the endpoint-extra scan.

That leaves later flexible-array reads bounded by bLength, but not by the
remaining bytes in the endpoint-extra scan.

Stop walking when bLength is zero or
extends past the remaining endpoint-extra scan.

Fixes: 5c6cd7021a05 ("ALSA: usb-audio: Fix case when USB MIDI interface has more than one extra endpoint descriptor")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@xxxxxxxxx>
---
 sound/usb/midi.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/sound/usb/midi.c b/sound/usb/midi.c
index 0a5b8941ebda..d87e3f357cf7 100644
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -1951,15 +1951,17 @@ static struct usb_ms_endpoint_descriptor *find_usb_ms_endpoint_descriptor(
 	while (extralen > 3) {
 		struct usb_ms_endpoint_descriptor *ms_ep =
 				(struct usb_ms_endpoint_descriptor *)extra;
+		int length = ms_ep->bLength;
 
-		if (ms_ep->bLength > 3 &&
+		if (!length || length > extralen)
+			break;
+
+		if (length > 3 &&
 		    ms_ep->bDescriptorType == USB_DT_CS_ENDPOINT &&
 		    ms_ep->bDescriptorSubtype == UAC_MS_GENERAL)
 			return ms_ep;
-		if (!extra[0])
-			break;
-		extralen -= extra[0];
-		extra += extra[0];
+		extralen -= length;
+		extra += length;
 	}
 	return NULL;
 }

-- 
2.54.0