Bluetooth: Check packet FCS earlier

This way, if FCS is enabled and the packet is corrupted, we just drop it
without read it len, which could be corrupted.
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap.c

@@ -4166,25 +4166,25 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
 		skb_pull(skb, 2);
 		len = skb->len;
 
+		/*
+		 * We can just drop the corrupted I-frame here.
+		 * Receiver will miss it and start proper recovery
+		 * procedures and ask retransmission.
+		 */
+		if (l2cap_check_fcs(pi, skb))
+			goto drop;
+
 		if (__is_sar_start(control) && __is_iframe(control))
 			len -= 2;
 
 		if (pi->fcs == L2CAP_FCS_CRC16)
 			len -= 2;
 
-		/*
-		 * We can just drop the corrupted I-frame here.
-		 * Receiver will miss it and start proper recovery
-		 * procedures and ask retransmission.
-		 */
 		if (len > pi->mps) {
 			l2cap_send_disconn_req(pi->conn, sk);
 			goto drop;
 		}
 
-		if (l2cap_check_fcs(pi, skb))
-			goto drop;
-
 		req_seq = __get_reqseq(control);
 		req_seq_offset = (req_seq - pi->expected_ack_seq) % 64;
 		if (req_seq_offset < 0)
@@ -4224,6 +4224,9 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
 		skb_pull(skb, 2);
 		len = skb->len;
 
+		if (l2cap_check_fcs(pi, skb))
+			goto drop;
+
 		if (__is_sar_start(control))
 			len -= 2;
 
@@ -4233,9 +4236,6 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
 		if (len > pi->mps || len < 0 || __is_sframe(control))
 			goto drop;
 
-		if (l2cap_check_fcs(pi, skb))
-			goto drop;
-
 		tx_seq = __get_txseq(control);
 
 		if (pi->expected_tx_seq == tx_seq)