mptcp: use "middlebox interference" for MP_TCPRST in case map_valid i…

…s false

RFC8684 suggests use of "Middlebox interference (code 0x06)" in case of
MPJ subflow that carries data at TCP level with no DSS sub-option. This
is generally the case when mpext is NULL or mpext->use_map is 0: use a
dedicated value of 'mapping_status' and use it before closing the socket
in subflow_check_data_avail().

Link: multipath-tcp/mptcp_net-next#518 (comment)
Signed-off-by: Davide Caratti <dcaratti@redhat.com>

net/mptcp/subflow.c

@@ -960,7 +960,8 @@ enum mapping_status {
 	MAPPING_EMPTY,
 	MAPPING_DATA_FIN,
 	MAPPING_DUMMY,
-	MAPPING_BAD_CSUM
+	MAPPING_BAD_CSUM,
+	MAPPING_NODSS
 };
 
 static void dbg_bad_map(struct mptcp_subflow_context *subflow, u32 ssn)
@@ -1118,7 +1119,7 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 		}
 
 		if (!subflow->map_valid)
-			return MAPPING_INVALID;
+			return MAPPING_NODSS;
 
 		goto validate_seq;
 	}
@@ -1332,7 +1333,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 		status = get_mapping_status(ssk, msk);
 		trace_subflow_check_data_avail(status, skb_peek(&ssk->sk_receive_queue));
 		if (unlikely(status == MAPPING_INVALID || status == MAPPING_DUMMY ||
-			     status == MAPPING_BAD_CSUM))
+			     status == MAPPING_BAD_CSUM || status == MAPPING_NODSS))
 			goto fallback;
 
 		if (status != MAPPING_OK)
@@ -1385,7 +1386,13 @@ static bool subflow_check_data_avail(struct sock *ssk)
 			 * subflow_error_report() will introduce the appropriate barriers
 			 */
 			subflow->reset_transient = 0;
-			subflow->reset_reason = MPTCP_RST_EMPTCP;
+			switch (status) {
+			case MAPPING_NODSS:
+				subflow->reset_reason = MPTCP_RST_EMIDDLEBOX;
+				break;
+			default:
+				subflow->reset_reason = MPTCP_RST_EMPTCP;
+			}
 
 reset:
 			WRITE_ONCE(ssk->sk_err, EBADMSG);