[PATCH v3] 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 and is marked fully established in
check_fully_established(), but because the third ACK carries no
DSS data, the queued bytes remain stranded until the peer sends
more data.

Fix this in check_fully_established() by checking if the subflow
was doing passive TFO (subflow->is_mptfo). Under mptcp_data_lock(),
flush queued bytes via __mptcp_check_push() and clear is_mptfo.

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>
---
v2 -> v3:
 - Move the push and is_mptfo reset directly into check_fully_established()
   under mptcp_data_lock(), avoiding any potential race in subflow_state_change()
   which could clear is_mptfo before check_fully_established() processes the 3rd ACK.
v1 -> v2:
 - Use __mptcp_check_push() under mptcp_data_lock() instead of mptcp_push_pending().

 net/mptcp/options.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index ce0de02f5..b865ee6af 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1042,6 +1042,15 @@ 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) {
+		subflow->is_mptfo = 0;
+		__mptcp_check_push((struct sock *)msk, ssk);
+	}
 	mptcp_data_unlock((struct sock *)msk);
 
 check_notify:
-- 
2.34.1