[PATCH v2] mptcp: push queued data on passive TFO subflows becoming established

[T S Rameshkumar <rameshsv06@gmail.com>]

With TCP Fast Open on an MPTCP listener, if the server application
writes data while the passive subflow is still in SYN_RECV (after
consuming the client's SYN data but before the MP_CAPABLE third ACK
arrives), __mptcp_subflow_active() refuses transmission and the data
is queued into the msk write queue.

When the MPC third ACK arrives, the subflow transitions to
TCP_ESTABLISHED, but because the third ACK carries no DSS data,
the queued bytes remain stranded until the peer sends more data.

Fix this in subflow_state_change() by checking if the subflow was doing
passive TFO (subflow->is_mptfo) and has reached TCP_ESTABLISHED. Acquire
mptcp_data_lock() and call __mptcp_check_push() to flush queued bytes,
clearing is_mptfo so subsequent state transitions are ignored.

Reported-by: Petar Sakic <petar.sakic@ink.fish>
Closes: https://lore.kernel.org/netdev/CAFPPu1gU2Y-D+d4i3F0MoNkYK+e1U+=X3qf6QycjfKBw+8snPg@mail.gmail.com/
Fixes: fb7084501a61 ("mptcp: add support for TCP_FASTOPEN sockopt")
Signed-off-by: T S Rameshkumar <rameshkumar.t@phytecembedded.in>
---
 net/mptcp/options.c | 7 +++++++
 net/mptcp/subflow.c | 7 +++++++
 2 files changed, 14 insertions(+)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index ce0de02f5..d5238fa11 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1042,6 +1042,13 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 
 	mptcp_data_lock((struct sock *)msk);
 	__mptcp_subflow_fully_established(msk, subflow, mp_opt);
+	/* Passive TFO: the application may have written data while the
+	 * subflow was still in SYN_RECV; __mptcp_subflow_active() refused
+	 * it then and nothing else spools the msk write queue when the
+	 * MPC third ack (no DSS) arrives. Push it now.
+	 */
+	if (subflow->is_mptfo)
+		__mptcp_check_push((struct sock *)msk, ssk);
 	mptcp_data_unlock((struct sock *)msk);
 
 check_notify:
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index f0a6725d2..c71122842 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1894,6 +1894,13 @@ static void subflow_state_change(struct sock *sk)
 	if (subflow->resetting)
 		return;
 
+	if (subflow->is_mptfo && sk->sk_state == TCP_ESTABLISHED) {
+		subflow->is_mptfo = 0;
+		mptcp_data_lock(parent);
+		__mptcp_check_push(parent, sk);
+		mptcp_data_unlock(parent);
+	}
+
 	/* as recvmsg() does not acquire the subflow socket for ssk selection
 	 * a fin packet carrying a DSS can be unnoticed if we don't trigger
 	 * the data available machinery here.
-- 
2.34.1