[PATCH 6.1.y] mptcp: pm: kernel: use RCU to free endpoint listeners

[Sasha Levin <sashal@kernel.org>]

From: Kyle Zeng <kylebot@openai.com>

[ Upstream commit 49b09ae0bf57b4230187d4cdd0a370558e44a6e8 ]

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>
[ sashal: adjusted listener-creation context for kernel_listen() instead of __inet_listen_sk() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/pm_netlink.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index 05fb4d43187ad..a618ebbc42b09 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -1261,6 +1261,7 @@ static int mptcp_pm_nl_create_listen_socket(struct sock *sk,
 	}
 
 	inet_sk_state_store(newsk, TCP_LISTEN);
+	sock_set_flag(newsk, SOCK_RCU_FREE);
 	WRITE_ONCE(mptcp_subflow_ctx(ssock->sk)->pm_listener, true);
 	err = kernel_listen(ssock, backlog);
 	if (err) {
-- 
2.53.0