[PATCH 6.1.y 3/3] mptcp: reset fallback status gracefully at disconnect() time

["Matthieu Baerts (NGI0)" <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>
[ Conflicts in protocol.c, because commit ebc1e08f01eb ("mptcp: drop
  last_snd and MPTCP_RESET_SCHEDULER") is not in this version and
  changed the context. The same modification can still be applied at the
  same place. ]
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 73e298f276a8..883efcbb8dfc 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3204,7 +3204,16 @@ static int mptcp_disconnect(struct sock *sk, int flags)
 	 */
 	mptcp_destroy_common(msk, MPTCP_CF_FASTCLOSE);
 	msk->last_snd = NULL;
+
+	/* 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