mptcp: parse and emit MP_CAPABLE option according to v1 spec

This implements MP_CAPABLE options parsing and writing according
to RFC 6824 bis / RFC 8684: MPTCP v1.

Local key is sent on syn/ack, and both keys are sent on 3rd ack.
MP_CAPABLE messages len are updated accordingly. We need the skbuff to
correctly emit the above, so we push the skbuff struct as an argument
all the way from tcp code to the relevant mptcp callbacks.

When processing incoming MP_CAPABLE + data, build a full blown DSS-like
map info, to simplify later processing.  On child socket creation, we
need to record the remote key, if available.

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

1 files changed, 12 insertions, 2 deletions

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 9fb3eb87a20f..8892855f4f52 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -77,7 +77,6 @@ static void subflow_init_req(struct request_sock *req,
 		if (err == 0)
 			subflow_req->mp_capable = 1;
 
-		subflow_req->remote_key = rx_opt.mptcp.sndr_key;
 		subflow_req->ssn_offset = TCP_SKB_CB(skb)->seq;
 	}
 }
@@ -180,11 +179,22 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 					  bool *own_req)
 {
 	struct mptcp_subflow_context *listener = mptcp_subflow_ctx(sk);
+	struct mptcp_subflow_request_sock *subflow_req;
+	struct tcp_options_received opt_rx;
 	struct sock *child;
 
 	pr_debug("listener=%p, req=%p, conn=%p", listener, req, listener->conn);
 
-	/* if the sk is MP_CAPABLE, we already received the client key */
+	/* if the sk is MP_CAPABLE, we need to fetch the client key */
+	subflow_req = mptcp_subflow_rsk(req);
+	if (subflow_req->mp_capable) {
+		opt_rx.mptcp.mp_capable = 0;
+		mptcp_get_options(skb, &opt_rx);
+		if (!opt_rx.mptcp.mp_capable)
+			subflow_req->mp_capable = 0;
+		else
+			subflow_req->remote_key = opt_rx.mptcp.sndr_key;
+	}
 
 	child = listener->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
 						     req_unhash, own_req);