--- /dev/null
+From 7a1b3490f47e88ec4cbde65f1a77a0f4bc972282 Mon Sep 17 00:00:00 2001
+From: Davide Caratti <dcaratti@redhat.com>
+Date: Fri, 29 Mar 2024 13:08:52 +0100
+Subject: mptcp: don't account accept() of non-MPC client as fallback to TCP
+
+From: Davide Caratti <dcaratti@redhat.com>
+
+commit 7a1b3490f47e88ec4cbde65f1a77a0f4bc972282 upstream.
+
+Current MPTCP servers increment MPTcpExtMPCapableFallbackACK when they
+accept non-MPC connections. As reported by Christoph, this is "surprising"
+because the counter might become greater than MPTcpExtMPCapableSYNRX.
+
+MPTcpExtMPCapableFallbackACK counter's name suggests it should only be
+incremented when a connection was seen using MPTCP options, then a
+fallback to TCP has been done. Let's do that by incrementing it when
+the subflow context of an inbound MPC connection attempt is dropped.
+Also, update mptcp_connect.sh kselftest, to ensure that the
+above MIB does not increment in case a pure TCP client connects to a
+MPTCP server.
+
+Fixes: fc518953bc9c ("mptcp: add and use MIB counter infrastructure")
+Cc: stable@vger.kernel.org
+Reported-by: Christoph Paasch <cpaasch@apple.com>
+Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/449
+Signed-off-by: Davide Caratti <dcaratti@redhat.com>
+Reviewed-by: Mat Martineau <martineau@kernel.org>
+Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
+Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
+Link: https://lore.kernel.org/r/20240329-upstream-net-20240329-fallback-mib-v1-1-324a8981da48@kernel.org
+Signed-off-by: Jakub Kicinski <kuba@kernel.org>
+Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ net/mptcp/protocol.c | 3 ---
+ net/mptcp/subflow.c | 3 +++
+ tools/testing/selftests/net/mptcp/mptcp_connect.sh | 7 +++++++
+ 3 files changed, 10 insertions(+), 3 deletions(-)
+
+--- a/net/mptcp/protocol.c
++++ b/net/mptcp/protocol.c
+@@ -3018,9 +3018,6 @@ static struct sock *mptcp_accept(struct
+ sock_hold(new_mptcp_sock);
+ newsk = new_mptcp_sock;
+ MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_MPCAPABLEPASSIVEACK);
+- } else {
+- MPTCP_INC_STATS(sock_net(sk),
+- MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK);
+ }
+
+ out:
+--- a/net/mptcp/subflow.c
++++ b/net/mptcp/subflow.c
+@@ -725,6 +725,9 @@ create_child:
+ goto dispose_child;
+ }
+
++ if (fallback)
++ SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK);
++
+ subflow_drop_ctx(child);
+ goto out;
+ }
+--- a/tools/testing/selftests/net/mptcp/mptcp_connect.sh
++++ b/tools/testing/selftests/net/mptcp/mptcp_connect.sh
+@@ -451,6 +451,7 @@ do_transfer()
+ local stat_cookierx_last=$(get_mib_counter "${listener_ns}" "TcpExtSyncookiesRecv")
+ local stat_csum_err_s=$(get_mib_counter "${listener_ns}" "MPTcpExtDataCsumErr")
+ local stat_csum_err_c=$(get_mib_counter "${connector_ns}" "MPTcpExtDataCsumErr")
++ local stat_tcpfb_last_l=$(get_mib_counter "${listener_ns}" "MPTcpExtMPCapableFallbackACK")
+
+ timeout ${timeout_test} \
+ ip netns exec ${listener_ns} \
+@@ -516,6 +517,7 @@ do_transfer()
+ local stat_cookietx_now=$(get_mib_counter "${listener_ns}" "TcpExtSyncookiesSent")
+ local stat_cookierx_now=$(get_mib_counter "${listener_ns}" "TcpExtSyncookiesRecv")
+ local stat_ooo_now=$(get_mib_counter "${listener_ns}" "TcpExtTCPOFOQueue")
++ local stat_tcpfb_now_l=$(get_mib_counter "${listener_ns}" "MPTcpExtMPCapableFallbackACK")
+
+ expect_synrx=$((stat_synrx_last_l))
+ expect_ackrx=$((stat_ackrx_last_l))
+@@ -560,6 +562,11 @@ do_transfer()
+ fi
+ fi
+
++ if [ ${stat_ooo_now} -eq 0 ] && [ ${stat_tcpfb_last_l} -ne ${stat_tcpfb_now_l} ]; then
++ printf "[ FAIL ]\nunexpected fallback to TCP"
++ rets=1
++ fi
++
+ if [ $retc -eq 0 ] && [ $rets -eq 0 ]; then
+ printf "[ OK ]"
+ fi