[PATCH 5.15.y] mptcp: close subflow when receiving TCP+FIN

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

commit f09b0ad55a1196f5891663f8888463c0541059cb upstream.

When a peer decides to close one subflow in the middle of a connection
having multiple subflows, the receiver of the first FIN should accept
that, and close the subflow on its side as well. If not, the subflow
will stay half closed, and would even continue to be used until the end
of the MPTCP connection or a reset from the network.

The issue has not been seen before, probably because the in-kernel
path-manager always sends a RM_ADDR before closing the subflow. Upon the
reception of this RM_ADDR, the other peer will initiate the closure on
its side as well. On the other hand, if the RM_ADDR is lost, or if the
path-manager of the other peer only closes the subflow without sending a
RM_ADDR, the subflow would switch to TCP_CLOSE_WAIT, but that's it,
leaving the subflow half-closed.

So now, when the subflow switches to the TCP_CLOSE_WAIT state, and if
the MPTCP connection has not been closed before with a DATA_FIN, the
kernel owning the subflow schedules its worker to initiate the closure
on its side as well.

This issue can be easily reproduced with packetdrill, as visible in [1],
by creating an additional subflow, injecting a FIN+ACK before sending
the DATA_FIN, and expecting a FIN+ACK in return.

Fixes: 40947e13997a ("mptcp: schedule worker when subflow is closed")
Cc: stable@vger.kernel.org
Link: https://github.com/multipath-tcp/packetdrill/pull/154 [1]
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240826-net-mptcp-close-extra-sf-fin-v1-1-905199fe1172@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ No conflicts but 'sk' is not available in __mptcp_close_subflow in
  this version. It would require b6985b9b8295 ("mptcp: use the workqueue
  to destroy unaccepted sockets") which has not been backported to this
  version. It is easier to get 'sk' by casting 'msk' into a 'struct
  sock'. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/protocol.c | 5 ++++-
 net/mptcp/subflow.c  | 8 ++++++--
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 5afd49bf4750..da2a1a150bc6 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2361,8 +2361,11 @@ static void __mptcp_close_subflow(struct mptcp_sock *msk)
 
 	list_for_each_entry_safe(subflow, tmp, &msk->conn_list, node) {
 		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+		int ssk_state = inet_sk_state_load(ssk);
 
-		if (inet_sk_state_load(ssk) != TCP_CLOSE)
+		if (ssk_state != TCP_CLOSE &&
+		    (ssk_state != TCP_CLOSE_WAIT ||
+		     inet_sk_state_load((struct sock *)ssk) != TCP_ESTABLISHED))
 			continue;
 
 		/* 'subflow_data_ready' will re-sched once rx queue is empty */
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 5f514943721f..e71082dd6484 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1131,12 +1131,16 @@ static void mptcp_subflow_discard_data(struct sock *ssk, struct sk_buff *skb,
 /* sched mptcp worker to remove the subflow if no more data is pending */
 static void subflow_sched_work_if_closed(struct mptcp_sock *msk, struct sock *ssk)
 {
-	if (likely(ssk->sk_state != TCP_CLOSE))
+	struct sock *sk = (struct sock *)msk;
+
+	if (likely(ssk->sk_state != TCP_CLOSE &&
+		   (ssk->sk_state != TCP_CLOSE_WAIT ||
+		    inet_sk_state_load(sk) != TCP_ESTABLISHED)))
 		return;
 
 	if (skb_queue_empty(&ssk->sk_receive_queue) &&
 	    !test_and_set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags))
-		mptcp_schedule_work((struct sock *)msk);
+		mptcp_schedule_work(sk);
 }
 
 static bool subflow_can_fallback(struct mptcp_subflow_context *subflow)
-- 
2.45.2