mptcp: use "middlebox interference" RST when no DSS

RFC8684 suggests use of "Middlebox interference (code 0x06)" in case of
fully established 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: #518
Signed-off-by: Davide Caratti <[email protected]>

net/mptcp/subflow.c

@@ -971,7 +971,8 @@ 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)
@@ -1128,8 +1129,9 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 			return MAPPING_EMPTY;
 		}
 
+		/* If the required DSS has likely been dropped by a middlebox */
 		if (!subflow->map_valid)
-			return MAPPING_INVALID;
+			return MAPPING_NODSS;
 
 		goto validate_seq;
 	}
@@ -1343,7 +1345,7 @@ 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)
@@ -1396,7 +1398,9 @@ 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;
+			subflow->reset_reason = status == MAPPING_NODSS ?
+						MPTCP_RST_EMIDDLEBOX :
+						MPTCP_RST_EMPTCP;
 
 reset:
 			WRITE_ONCE(ssk->sk_err, EBADMSG);