From: Davide Caratti <dcaratti@redhat.com>

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: https://github.com/multipath-tcp/mptcp_net-next/issues/518
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/subflow.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 860903e0642255cf9efb39da9e24c39f6547481f..07352b15f145832572a4203ab4d0427c37675e94 100644
--- a/net/mptcp/subflow.c
+++ b/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);

-- 
2.45.2