FAILED: patch "[PATCH] mptcp: reset msk state on early connect failure" failed to apply to 6.1-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 29afe67082b2d33ab2e90c56c337235d725f023f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107055040-failed-6.1-29afe67082b2@kernel.org' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 29afe67082b2d33ab2e90c56c337235d725f023f Mon Sep 17 00:00:00 2001
From: Quanye Yang <quanyeyang@proton.me>
Date: Tue, 13 Oct 2026 16:29:19 +0200
Subject: [PATCH] mptcp: reset msk state on early connect failure

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>
---
 net/mptcp/protocol.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index e89a69ab927c9..e6f73a5ee630a 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -4212,9 +4212,16 @@ static int mptcp_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 	 * subflow_finish_connect()
 	 */
 	if (unlikely(err)) {
-		/* avoid leaving a dangling token in an unconnected socket */
 		mptcp_token_destroy(msk);
 		mptcp_set_state(sk, TCP_CLOSE);
+
+		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(mptcp_subflow_ctx(ssk));
 		return err;
 	}
 
-- 
2.53.0