[PATCH 6.1.y] mptcp: reset msk state on early connect failure

[Sasha Levin <sashal@kernel.org>]

From: Quanye Yang <quanyeyang@proton.me>

[ Upstream commit 29afe67082b2d33ab2e90c56c337235d725f023f ]

mptcp_connect() can fall back before the subflow SYN is sent, for
example when the netns is in an MPTCP blackhole, token allocation fails,
or MD5SIG is in use. If the subsequent subflow connect() fails
immediately (EAFNOSUPPORT, ENETUNREACH, ...), __inet_stream_connect()
returns while the socket is still SS_UNCONNECTED and never calls
->disconnect(). This was reported by Clashiko [1].

The error path only dropped the token and moved the msk back to
TCP_CLOSE. MPTCP_FALLBACK_DONE, allow_subflows and
request_mptcp were left as after early fallback, so a later
connect() on the same fd stayed TCP-only.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/633 [1]
Fixes: 0235d075a592 ("mptcp: mark as fallback even early ones")
Cc: stable@vger.kernel.org
Signed-off-by: Quanye Yang <quanyeyang@proton.me>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20261013-net-mptcp-misc-fixes-7-3-rc8-v1-1-932c9b182fc7@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ sashal: retained the -EINPROGRESS exception required by the existing inet_stream_connect() error path ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/protocol.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index bdace644dbc6a..2084503567789 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3937,6 +3937,14 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 	 */
 	if (unlikely(err && err != -EINPROGRESS)) {
 		inet_sk_state_store(sk, inet_sk_state_load(ssock->sk));
+
+		spin_lock_bh(&msk->fallback_lock);
+		msk->allow_subflows = true;
+		msk->allow_infinite_fallback = true;
+		clear_bit(MPTCP_FALLBACK_DONE, &msk->flags);
+		spin_unlock_bh(&msk->fallback_lock);
+
+		mptcp_subflow_ctx_reset(subflow);
 		return err;
 	}
 
-- 
2.53.0