mptcp: reset fallback status gracefully at disconnect() time

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: b29fcfb ("mptcp: full disconnect implementation")
Signed-off-by: Paolo Abeni <[email protected]>

Author: Paolo Abeni

net/mptcp/protocol.c

@@ -3143,7 +3143,16 @@ 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;
 	WRITE_ONCE(msk->can_ack, false);