The 'msk->write_seq' and 'msk->snd_nxt' are always updated under
the msk socket lock, except at MPC handshake completiont time.

Builds-up on the previous commit to move such init under the relevant
lock.

There are no known problems caused by the potential race, the
primary goal is consistency.

Fixes: 6d0060f600ad ("mptcp: Write MPTCP DSS headers to outgoing data packets")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c |  6 ++----
 net/mptcp/subflow.c  | 13 +++++++++++--
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c41f49532a0a..b7d1da7bcb50 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3481,10 +3481,8 @@ void mptcp_finish_connect(struct sock *ssk)
 	 * accessing the field below
 	 */
 	WRITE_ONCE(msk->local_key, subflow->local_key);
-	WRITE_ONCE(msk->write_seq, subflow->idsn + 1);
-	WRITE_ONCE(msk->snd_nxt, msk->write_seq);
-	WRITE_ONCE(msk->snd_una, msk->write_seq);
-	WRITE_ONCE(msk->wnd_end, msk->snd_nxt + tcp_sk(ssk)->snd_wnd);
+	WRITE_ONCE(msk->snd_una, subflow->idsn + 1);
+	WRITE_ONCE(msk->wnd_end, subflow->idsn + 1 + tcp_sk(ssk)->snd_wnd);
 
 	mptcp_pm_new_connection(msk, ssk, 0);
 }
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 23f639e0d8c3..fe5334cee477 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -421,12 +421,21 @@ static bool subflow_use_different_dport(struct mptcp_sock *msk, const struct soc
 
 void __mptcp_sync_state(struct sock *sk, int state)
 {
+	struct mptcp_subflow_context *subflow;
 	struct mptcp_sock *msk = mptcp_sk(sk);
+	struct sock *ssk = msk->first;
 
-	__mptcp_propagate_sndbuf(sk, msk->first);
+	subflow = mptcp_subflow_ctx(ssk);
+	__mptcp_propagate_sndbuf(sk, ssk);
 	if (!msk->rcvspace_init)
-		mptcp_rcv_space_init(msk, msk->first);
+		mptcp_rcv_space_init(msk, ssk);
+
 	if (sk->sk_state == TCP_SYN_SENT) {
+		/* subflow->idsn is always available is TCP_SYN_SENT state,
+		 * even for the FASTOPEN scenarios
+		 */
+		WRITE_ONCE(msk->write_seq, subflow->idsn + 1);
+		WRITE_ONCE(msk->snd_nxt, msk->write_seq);
 		mptcp_set_state(sk, state);
 		sk->sk_state_change(sk);
 	}
-- 
2.43.0