From: Paolo Abeni <pabeni@redhat.com>

commit da9b2fc7b73d147d88abe1922de5ab72d72d7756 upstream.

mptcp_disconnect() clears the fallback bit unconditionally, without
touching the associated flags.

The bit clear is safe, as no fallback operation can race with that --
all subflow are already in TCP_CLOSE status thanks to the previous
FASTCLOSE -- but we need to consistently reset all the fallback related
status.

Also acquire the relevant lock, to avoid fouling static analyzers.

Fixes: b29fcfb54cd7 ("mptcp: full disconnect implementation")
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/20250714-net-mptcp-fallback-races-v1-3-391aff963322@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/protocol.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index d95fc113d317..e3f09467b36b 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3208,7 +3208,16 @@ static int mptcp_disconnect(struct sock *sk, int flags)
 	 * subflow
 	 */
 	mptcp_destroy_common(msk, MPTCP_CF_FASTCLOSE);
+
+	/* The first subflow is already in TCP_CLOSE status, the following
+	 * can't overlap with a fallback anymore
+	 */
+	spin_lock_bh(&msk->fallback_lock);
+	msk->allow_subflows = true;
+	msk->allow_infinite_fallback = true;
 	WRITE_ONCE(msk->flags, 0);
+	spin_unlock_bh(&msk->fallback_lock);
+
 	msk->cb_flags = 0;
 	msk->recovery = false;
 	msk->can_ack = false;
-- 
2.50.0

This is a note to let you know that I've just added the patch titled

    mptcp: reset fallback status gracefully at disconnect() time

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mptcp-reset-fallback-status-gracefully-at-disconnect-time.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From stable+bounces-164898-greg=kroah.com@vger.kernel.org Mon Jul 28 11:15:34 2025
From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>
Date: Mon, 28 Jul 2025 11:14:51 +0200
Subject: mptcp: reset fallback status gracefully at disconnect() time
To: mptcp@lists.linux.dev, stable@vger.kernel.org, gregkh@linuxfoundation.org
Cc: Paolo Abeni <pabeni@redhat.com>, sashal@kernel.org, "Matthieu Baerts (NGI0)" <matttbe@kernel.org>, Jakub Kicinski <kuba@kernel.org>
Message-ID: <20250728091448.3494479-8-matttbe@kernel.org>

From: Paolo Abeni <pabeni@redhat.com>

commit da9b2fc7b73d147d88abe1922de5ab72d72d7756 upstream.

mptcp_disconnect() clears the fallback bit unconditionally, without
touching the associated flags.

The bit clear is safe, as no fallback operation can race with that --
all subflow are already in TCP_CLOSE status thanks to the previous
FASTCLOSE -- but we need to consistently reset all the fallback related
status.

Also acquire the relevant lock, to avoid fouling static analyzers.

Fixes: b29fcfb54cd7 ("mptcp: full disconnect implementation")
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/20250714-net-mptcp-fallback-races-v1-3-391aff963322@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 |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3208,7 +3208,16 @@ static int mptcp_disconnect(struct sock
 	 * subflow
 	 */
 	mptcp_destroy_common(msk, MPTCP_CF_FASTCLOSE);
+
+	/* The first subflow is already in TCP_CLOSE status, the following
+	 * can't overlap with a fallback anymore
+	 */
+	spin_lock_bh(&msk->fallback_lock);
+	msk->allow_subflows = true;
+	msk->allow_infinite_fallback = true;
 	WRITE_ONCE(msk->flags, 0);
+	spin_unlock_bh(&msk->fallback_lock);
+
 	msk->cb_flags = 0;
 	msk->recovery = false;
 	msk->can_ack = false;


Patches currently in stable-queue which might be from matttbe@kernel.org are

queue-6.6/selftests-mptcp-connect-also-cover-alt-modes.patch
queue-6.6/selftests-mptcp-connect-also-cover-checksum.patch
queue-6.6/mptcp-plug-races-between-subflow-fail-and-subflow-creation.patch
queue-6.6/mptcp-make-fallback-action-and-fallback-decision-atomic.patch
queue-6.6/mptcp-reset-fallback-status-gracefully-at-disconnect-time.patch