Bluetooth: check for l2cap header in start fragment

BLUETOOTH SPECIFICATION Version 4.0 [Vol 3] page 36 mentioned
"Note: Start Fragments always begin with the Basic L2CAP header
of a PDU."
Signed-off-by: NAndrei Emeltchenko <andrei.emeltchenko@nokia.com>
Acked-by: NMarcel Holtmann <marcel@holtmann.org>
Signed-off-by: NGustavo F. Padovan <padovan@profusion.mobi>

net/bluetooth/l2cap.c

@@ -4675,7 +4675,8 @@ static int l2cap_recv_acldata(struct hci_conn *hcon, struct sk_buff *skb, u16 fl
 			l2cap_conn_unreliable(conn, ECOMM);
 		}
 
-		if (skb->len < 2) {
+		/* Start fragment always begin with Basic L2CAP header */
+		if (skb->len < L2CAP_HDR_SIZE) {
 			BT_ERR("Frame is too short (len %d)", skb->len);
 			l2cap_conn_unreliable(conn, ECOMM);
 			goto drop;