[PATCH 5.15.y] mptcp: fix premature close in case of fallback

[Sasha Levin <sashal@kernel.org>]

From: Paolo Abeni <pabeni@redhat.com>

[ Upstream commit 17393fa7b7086664be519e7230cb6ed7ec7d9462 ]

I'm observing very frequent self-tests failures in case of fallback when
running on a CONFIG_PREEMPT kernel.

The root cause is that subflow_sched_work_if_closed() closes any subflow
as soon as it is half-closed and has no incoming data pending.

That works well for regular subflows - MPTCP needs bi-directional
connectivity to operate on a given subflow - but for fallback socket is
race prone.

When TCP peer closes the connection before the MPTCP one,
subflow_sched_work_if_closed() will schedule the MPTCP worker to
gracefully close the subflow, and shortly after will do another schedule
to inject and process a dummy incoming DATA_FIN.

On CONFIG_PREEMPT kernel, the MPTCP worker can kick-in and close the
fallback subflow before subflow_sched_work_if_closed() is able to create
the dummy DATA_FIN, unexpectedly interrupting the transfer.

Address the issue explicitly avoiding closing fallback subflows on when
the peer is only half-closed.

Note that, when the subflow is able to create the DATA_FIN before the
worker invocation, the worker will change the msk state before trying to
close the subflow and will skip the latter operation as the msk will not
match anymore the precondition in __mptcp_close_subflow().

Fixes: f09b0ad55a11 ("mptcp: close subflow when receiving TCP+FIN")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251118-net-mptcp-misc-fixes-6-18-rc6-v1-3-806d3781c95f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ sk -> ssk ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/protocol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 490fd8b188894..bc7e66cdc77f3 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2401,7 +2401,8 @@ static void __mptcp_close_subflow(struct mptcp_sock *msk)
 
 		if (ssk_state != TCP_CLOSE &&
 		    (ssk_state != TCP_CLOSE_WAIT ||
-		     inet_sk_state_load((struct sock *)ssk) != TCP_ESTABLISHED))
+		     inet_sk_state_load((struct sock *)ssk) != TCP_ESTABLISHED ||
+		     __mptcp_check_fallback(msk)))
 			continue;
 
 		/* 'subflow_data_ready' will re-sched once rx queue is empty */
-- 
2.51.0