The Fixes commit mentioned this:

An MPTCP firewall blackhole can be detected if the following SYN
retransmission after a fallback to "plain" TCP is accepted.

But in fact, this blackhole was detected if any following SYN
retransmissions after a fallback to TCP was accepted.

That's because 'mptcp_subflow_early_fallback()' will set 'request_mptcp'
to 0, and 'mpc_drop' will never be reset to 0 after.

This is an issue, because some not so unusual situations might cause the
kernel to detect a false-positive blackhole, e.g. a client trying to
connect to a server while the network is not ready yet, causing a few
SYN retransmissions, before reaching the end server.

Fixes: 27069e7cb3d1 ("mptcp: disable active MPTCP in case of blackhole")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/ctrl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index 3999e0ba2c35b50c36ce32277e0b8bfb24197946..2dd81e6c26bdb5220abed68e26d70d2dc3ab14fb 100644
--- a/net/mptcp/ctrl.c
+++ b/net/mptcp/ctrl.c

@@ -418,9 +418,9 @@ void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
 			MPTCP_INC_STATS(net, MPTCP_MIB_MPCAPABLEACTIVEDROP);
 			subflow->mpc_drop = 1;
 			mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
-		} else {
-			subflow->mpc_drop = 0;
 		}
+	} else if (ssk->sk_state == TCP_SYN_SENT) {
+		subflow->mpc_drop = 0;
 	}
 }
 

-- 
2.47.1