mptcp: do not reset MP_CAPABLE subflow on mapping errors

When some mapping related errors occurs we close the main
MPC subflow with a RST. We should instead fallback gracefully
to TCP, and do the reset only for MPJ subflows.

Fixes: d22f498 ("mptcp: process MP_CAPABLE data option")
Closes: #192
Reported-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/subflow.c

@@ -1011,21 +1011,11 @@ 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 (status == MAPPING_INVALID) {
- ssk->sk_err = EBADMSG;
- goto fatal;
- }
- if (status == MAPPING_DUMMY) {
- __mptcp_do_fallback(msk);
- skb = skb_peek(&ssk->sk_receive_queue);
- subflow->map_valid = 1;
- subflow->map_seq = READ_ONCE(msk->ack_seq);
- subflow->map_data_len = skb->len;
- subflow->map_subflow_seq = tcp_sk(ssk)->copied_seq -
- subflow->ssn_offset;
- subflow->data_avail = MPTCP_SUBFLOW_DATA_AVAIL;
- return true;
- }
+ if (unlikely(status == MAPPING_INVALID))
+ goto fallback;
+
+ if (unlikely(status == MAPPING_DUMMY))
+ goto fallback;
 
  if (status != MAPPING_OK)
  goto no_data;
@@ -1038,10 +1028,8 @@ static bool subflow_check_data_avail(struct sock *ssk)
  * MP_CAPABLE-based mapping
  */
  if (unlikely(!READ_ONCE(msk->can_ack))) {
- if (!subflow->mpc_map) {
- ssk->sk_err = EBADMSG;
- goto fatal;
- }
+ if (!subflow->mpc_map)
+ goto fallback;
  WRITE_ONCE(msk->remote_key, subflow->remote_key);
  WRITE_ONCE(msk->ack_seq, subflow->map_seq);
  WRITE_ONCE(msk->can_ack, true);
@@ -1069,17 +1057,31 @@ static bool subflow_check_data_avail(struct sock *ssk)
 no_data:
  subflow_sched_work_if_closed(msk, ssk);
  return false;
-fatal:
- /* fatal protocol error, close the socket */
- /* This barrier is coupled with smp_rmb() in tcp_poll() */
- smp_wmb();
- ssk->sk_error_report(ssk);
- tcp_set_state(ssk, TCP_CLOSE);
- subflow->reset_transient = 0;
- subflow->reset_reason = MPTCP_RST_EMPTCP;
- tcp_send_active_reset(ssk, GFP_ATOMIC);
- subflow->data_avail = 0;
- return false;
+
+fallback:
+ /* RFC 8684 section 3.7. */
+ if (subflow->mp_join || subflow->fully_established) {
+ /* fatal protocol error, close the socket.
+ * subflow_error_report() will introduce the appropriate barriers
+ */
+ ssk->sk_err = EBADMSG;
+ ssk->sk_error_report(ssk);
+ tcp_set_state(ssk, TCP_CLOSE);
+ subflow->reset_transient = 0;
+ subflow->reset_reason = MPTCP_RST_EMPTCP;
+ tcp_send_active_reset(ssk, GFP_ATOMIC);
+ subflow->data_avail = 0;
+ return false;
+ }
+
+ __mptcp_do_fallback(msk);
+ skb = skb_peek(&ssk->sk_receive_queue);
+ subflow->map_valid = 1;
+ subflow->map_seq = READ_ONCE(msk->ack_seq);
+ subflow->map_data_len = skb->len;
+ subflow->map_subflow_seq = tcp_sk(ssk)->copied_seq - subflow->ssn_offset;
+ subflow->data_avail = MPTCP_SUBFLOW_DATA_AVAIL;
+ return true;
 }
 
 bool mptcp_subflow_data_available(struct sock *sk)