mptcp: process MP_CAPABLE data option

This patch implements the handling of MP_CAPABLE + data option, as per
RFC 6824 bis / RFC 8684: MPTCP v1.

On the server side we can receive the remote key after that the connection
is established. We need to explicitly track the 'missing remote key'
status and avoid emitting a mptcp ack until we get such info.

When a late/retransmitted/OoO pkt carrying MP_CAPABLE[+data] option
is received, we have to propagate the mptcp seq number info to
the msk socket. To avoid ABBA locking issue, explicitly check for
that in recvmsg(), where we own msk and subflow sock locks.

The above also means that an established mp_capable subflow - still
waiting for the remote key - can be 'downgraded' to plain TCP.

Such change could potentially block a reader waiting for new data
forever - as they hook to msk, while later wake-up after the downgrade
will be on subflow only.

The above issue is not handled here, we likely have to get rid of
msk->fallback to handle that cleanly.

Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 35 insertions, 5 deletions

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 8892855f4f52..8cfa1d29d59c 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -61,6 +61,7 @@ static void subflow_init_req(struct request_sock *req,
 	mptcp_get_options(skb, &rx_opt);
 
 	subflow_req->mp_capable = 0;
+	subflow_req->remote_key_valid = 0;
 
 #ifdef CONFIG_TCP_MD5SIG
 	/* no MPTCP if MD5SIG is enabled on this socket or we may run out of
@@ -185,17 +186,28 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 
 	pr_debug("listener=%p, req=%p, conn=%p", listener, req, listener->conn);
 
-	/* if the sk is MP_CAPABLE, we need to fetch the client key */
+	/* if the sk is MP_CAPABLE, we try to fetch the client key */
 	subflow_req = mptcp_subflow_rsk(req);
 	if (subflow_req->mp_capable) {
+		if (TCP_SKB_CB(skb)->seq != subflow_req->ssn_offset + 1) {
+			/* here we can receive and accept an in-window,
+			 * out-of-order pkt, which will not carry the MP_CAPABLE
+			 * opt even on mptcp enabled paths
+			 */
+			goto create_child;
+		}
+
 		opt_rx.mptcp.mp_capable = 0;
 		mptcp_get_options(skb, &opt_rx);
-		if (!opt_rx.mptcp.mp_capable)
-			subflow_req->mp_capable = 0;
-		else
+		if (opt_rx.mptcp.mp_capable) {
 			subflow_req->remote_key = opt_rx.mptcp.sndr_key;
+			subflow_req->remote_key_valid = 1;
+		} else {
+			subflow_req->mp_capable = 0;
+		}
 	}
 
+create_child:
 	child = listener->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
 						     req_unhash, own_req);
 
@@ -377,6 +389,7 @@ static enum mapping_status get_mapping_status(struct sock *ssk)
 	subflow->map_subflow_seq = mpext->subflow_seq;
 	subflow->map_data_len = data_len;
 	subflow->map_valid = 1;
+	subflow->mpc_map = mpext->mpc_map;
 	pr_debug("new map seq=%llu subflow_seq=%u data_len=%u",
 		 subflow->map_seq, subflow->map_subflow_seq,
 		 subflow->map_data_len);
@@ -428,6 +441,19 @@ static bool subflow_check_data_avail(struct sock *ssk)
 		if (WARN_ON_ONCE(!skb))
 			return false;
 
+		/* if msk lacks the remote key, this subflow must provide an
+		 * MP_CAPABLE-based mapping
+		 */
+		if (unlikely(!READ_ONCE(msk->can_ack))) {
+			if (!subflow->mpc_map) {
+				ssk->sk_err = EBADMSG;
+				goto fatal;
+			}
+			WRITE_ONCE(msk->remote_key, subflow->remote_key);
+			WRITE_ONCE(msk->ack_seq, subflow->map_seq);
+			WRITE_ONCE(msk->can_ack, true);
+		}
+
 		old_ack = READ_ONCE(msk->ack_seq);
 		ack_seq = mptcp_subflow_get_mapped_dsn(subflow);
 		pr_debug("msk ack_seq=%llx subflow ack_seq=%llx", old_ack,
@@ -752,13 +778,17 @@ static void subflow_ulp_clone(const struct request_sock *req,
 		return;
 	}
 
+	/* see comments in subflow_syn_recv_sock(), MPTCP connection is fully
+	 * established only after we receive the remote key
+	 */
 	new_ctx->conn_finished = 1;
 	new_ctx->icsk_af_ops = old_ctx->icsk_af_ops;
 	new_ctx->tcp_data_ready = old_ctx->tcp_data_ready;
 	new_ctx->tcp_state_change = old_ctx->tcp_state_change;
 	new_ctx->tcp_write_space = old_ctx->tcp_write_space;
 	new_ctx->mp_capable = 1;
-	new_ctx->fourth_ack = 1;
+	new_ctx->fourth_ack = subflow_req->remote_key_valid;
+	new_ctx->can_ack = subflow_req->remote_key_valid;
 	new_ctx->remote_key = subflow_req->remote_key;
 	new_ctx->local_key = subflow_req->local_key;
 	new_ctx->token = subflow_req->token;