FAILED: patch "[PATCH] mptcp: do not use Fast Open on MP_JOIN subflows" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 6d2cf411aba2aa0bd641b49be5b1964ac5097a50
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109202421-failed-5.15-6d2cf411aba2@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 6d2cf411aba2aa0bd641b49be5b1964ac5097a50 Mon Sep 17 00:00:00 2001
From: Yilin Zhang <yilinzhang@moonshot.ai>
Date: Fri, 30 Oct 2026 23:03:51 +0100
Subject: [PATCH] mptcp: do not use Fast Open on MP_JOIN subflows

tcp_fastopen_create_child() hands the SYN packet itself to
subflow_syn_recv_sock(). For an MP_JOIN request this takes the
fatal fallback: the cloned child is destroyed and handed back with
drop_req flagged, but tcp_fastopen_create_child() does not check
the flag and queues it, so accept() can expose the freed child.

MP_JOIN cannot use Fast Open: data on a subflow requires the
completed HMAC exchange (RFC 8684, sec. 3.2). Refuse it (only the
TFO path passes a SYN skb here) and let tcp_conn_request() fall
back to the regular MP_JOIN handshake; also strip the Fast Open
cookie from MP_JOIN SYN/ACKs.

Fixes: 90bf45134d55 ("mptcp: add new sock flag to deal with join subflows")
Cc: stable@vger.kernel.org
Reported-by: Kimi Security Team <bug-report@moonshot.ai>
Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Yilin Zhang <yilinzhang@moonshot.ai>
[ Moving code to avoid clearing TS and consolidating checks. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20261030-mptcp-tfo-mpj-v4-1-e6eff2027342@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/subflow.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 78e886b202585..00677fb3efcdc 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -379,6 +379,10 @@ static void subflow_prep_synack(const struct sock *sk, struct request_sock *req,
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
 	struct inet_request_sock *ireq = inet_rsk(req);
 
+	/* MP_JOIN cannot use TFO, do not send a cookie in the SYN/ACK */
+	if (foc && mptcp_subflow_rsk(req)->mp_join)
+		foc->len = -1;
+
 	/* clear tstamp_ok, as needed depending on cookie */
 	if (foc && foc->len > -1)
 		ireq->tstamp_ok = 0;
@@ -887,6 +891,13 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 			fallback = true;
 
 	} else if (subflow_req->mp_join) {
+		/* a SYN skb here comes from TFO, which MP_JOIN cannot use: just
+		 * fall back to the regular path.
+		 */
+		if ((TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_ACK)) ==
+		    TCPHDR_SYN)
+			return NULL;
+
 		mptcp_get_options(skb, &mp_opt);
 		if (!(mp_opt.suboptions & OPTION_MPTCP_MPJ_ACK))
 			fallback = true;
-- 
2.53.0