limitation of the Bell-LaPadula Model?

limitation of the BLP model:
have no policies for changing access data control
Intended for systems with static security levels(Static in nature)
Contains covert channels: a low subject can detect the existence of a high object when it is denied access. Sometimes it is enough to hide the content of an object; also their existence may have to be hidden.(covert channels秘密通道）
Restricted to confidentiality.