FAILED: patch "[PATCH] mptcp: pm: kernel: use RCU to free endpoint listeners" failed to apply to 6.1-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.1-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-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 49b09ae0bf57b4230187d4cdd0a370558e44a6e8
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109201949-failed-6.1-49b09ae0bf57@kernel.org' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

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

From 49b09ae0bf57b4230187d4cdd0a370558e44a6e8 Mon Sep 17 00:00:00 2001
From: Kyle Zeng <kylebot@openai.com>
Date: Thu, 29 Oct 2026 21:22:55 -0700
Subject: [PATCH] mptcp: pm: kernel: use RCU to free endpoint listeners

mptcp_pm_nl_create_listen_socket() starts the TCP listener directly
instead of going through mptcp_listen(). It consequently misses setting
SOCK_RCU_FREE on the parent MPTCP socket.

tcp_child_process() can invoke the listener's subflow_data_ready()
callback without holding the MPTCP parent lock or a reference to the
parent. An authenticated MP_JOIN racing with endpoint teardown can
therefore access the parent after its last reference has been dropped.
The ADD_ADDR error path can also close an already published listener.

Set SOCK_RCU_FREE before __inet_listen_sk() publishes the TCP listener,
matching mptcp_listen(). This protects the parent on every subsequent
close path, including a failed endpoint addition.

Fixes: 1729cf186d8a ("mptcp: create the listening socket for new port")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20261030042255.74216-1-kylebot@openai.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/pm_kernel.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index 1a77508132354..f7396628490e3 100644
--- a/net/mptcp/pm_kernel.c
+++ b/net/mptcp/pm_kernel.c
@@ -906,6 +906,7 @@ static int mptcp_pm_nl_create_listen_socket(struct sock *sk,
 	 * old status is known (TCP_CLOSE).
 	 */
 	inet_sk_state_store(newsk, TCP_LISTEN);
+	sock_set_flag(newsk, SOCK_RCU_FREE);
 	lock_sock(ssk);
 	WRITE_ONCE(mptcp_subflow_ctx(ssk)->pm_listener, true);
 	err = __inet_listen_sk(ssk, backlog);
-- 
2.53.0