[PATCH 7.1 012/228] mptcp: fastopen: only mark MPTFO subflows with SYN data

7.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Wyatt Feng <[email protected]>

commit e00b63056fb4f261455b3e5df5268a1f8ce47a87 upstream.

Passive TCP Fast Open accepts a valid-cookie SYN even when it carries
no data. In that case the child socket's receive queue is intentionally
left empty.

mptcp_fastopen_subflow_synack_set_params() set is_mptfo before checking
for queued SYN data. That made data-less TFO SYNs hit a WARN and, if
the warning was non-fatal, left stale MPTFO state behind. The stale
flag could later trigger a state-confusion bug in
check_fully_established().

Only mark the subflow as MPTFO after confirming that an SKB was queued.
Return quietly when the receive queue is empty.

Note that mptcp_subflow_context's is_mptfo field is now not just about
subflows where the TFO was present, but about MPTFO subflow that
consumed SYN data. Only having a valid cookie but not carrying data is
not really "doing TFO".

Fixes: 36b122baf6a8 ("mptcp: add subflow_v(4,6)_send_synack()")
Cc: [email protected]
Reported-by: Yuan Tan <[email protected]>
Reported-by: Yifan Wu <[email protected]>
Reported-by: Juefei Pu <[email protected]>
Reported-by: Zhengchuan Liang <[email protected]>
Reported-by: Xin Liu <[email protected]>
Signed-off-by: Wyatt Feng <[email protected]>
Signed-off-by: Ren Wei <[email protected]>
Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://patch.msgid.link/20260803-net-mptcp-misc-fixes-7-2-rc6-v2-7-b8f496d71664@kernel.org
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 net/mptcp/fastopen.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/net/mptcp/fastopen.c
+++ b/net/mptcp/fastopen.c
@@ -24,12 +24,13 @@ void mptcp_fastopen_subflow_synack_set_p
 	sk = subflow->conn;
 	tp = tcp_sk(ssk);
 
-	subflow->is_mptfo = 1;
-
+	/* A valid TFO cookie does not guarantee SYN data. */
 	skb = skb_peek(&ssk->sk_receive_queue);
-	if (WARN_ON_ONCE(!skb))
+	if (!skb)
 		return;
 
+	subflow->is_mptfo = 1;
+
 	/* dequeue the skb from sk receive queue */
 	__skb_unlink(skb, &ssk->sk_receive_queue);
 	skb_ext_reset(skb);