mptcp: fix DSS map generation on fin retransmission

The RFC 8684 mandates that no-data DATA FIN packets should carry
a DSS with 0 sequence number and data len equal to 1. Currently,
on FIN retransmission we re-use the existing mapping; if the previous
fin transmission was part of a partially acked data packet, we could
end-up writing in the egress packet a non-compliant DSS.

The above will be detected by a "Bad mapping" warning on the receiver
side.

This change addresses the issue explicitly checking for 0 len packet
when adding the DATA_FIN option.

Fixes: 6d0060f ("mptcp: Write MPTCP DSS headers to outgoing data packets")
Reported-by: syzbot+42a07faa5923cfaeb9c9@syzkaller.appspotmail.com
Tested-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -449,9 +449,9 @@ static bool mptcp_established_options_mp(struct sock *sk, struct sk_buff *skb,
 }
 
 static void mptcp_write_data_fin(struct mptcp_subflow_context *subflow,
-				 struct mptcp_ext *ext)
+				 struct sk_buff *skb, struct mptcp_ext *ext)
 {
-	if (!ext->use_map) {
+	if (!ext->use_map || !skb->len) {
 		/* RFC6824 requires a DSS mapping with specific values
 		 * if DATA_FIN is set but no data payload is mapped
 		 */
@@ -503,7 +503,7 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 			opts->ext_copy = *mpext;
 
 		if (skb && tcp_fin && subflow->data_fin_tx_enable)
-			mptcp_write_data_fin(subflow, &opts->ext_copy);
+			mptcp_write_data_fin(subflow, skb, &opts->ext_copy);
 		ret = true;
 	}