Bluetooth: Check the minimum {I,S}-frame size into L2CAP

All packets with size fewer than the minimum specified is dropped.
Note that the size of the l2cap basic header, FCS and SAR fields are
already subtracted of len at the moment of the size check.
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap.c

@@ -3627,10 +3627,17 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
 		if (l2cap_check_fcs(pi, skb))
 			goto drop;
 
-		if (__is_iframe(control))
+		if (__is_iframe(control)) {
+			if (len < 4)
+				goto drop;
+
 			l2cap_data_channel_iframe(sk, control, skb);
-		else
+		} else {
+			if (len != 0)
+				goto drop;
+
 			l2cap_data_channel_sframe(sk, control, skb);
+		}
 
 		goto done;
 
@@ -3645,7 +3652,8 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
 		if (pi->fcs == L2CAP_FCS_CRC16)
 			len -= 2;
 
-		if (len > L2CAP_DEFAULT_MAX_PDU_SIZE || __is_sframe(control))
+		if (len > L2CAP_DEFAULT_MAX_PDU_SIZE || len < 4
+				|| __is_sframe(control))
 			goto drop;
 
 		if (l2cap_check_fcs(pi, skb))