中文摘要：

安全性和灵活性是各种改进的BLP模型追求的目标.如何在保持安全性的前提下增加BLP模型的灵活性,一直是安全操作系统研究人员研究的重点.安全模型是系统设计的基础,如果在系统中实现了不安全的“安全模型”,其后果是严重的.结合多级安全（MLS）的核心思想,通过实例列举的方式深入分析了两个改进的BLP模型——DBLP（dynamic BLP）和SLCF（security label common framework）.尽管这两个模型都提出了在系统运行时动态地调整主体安全级的规则,但是分析表明,它们还是不安全的.在这两个模型的规则控制下,特洛伊木马可以通过显式地读和写操作将高安全等级的信息泄漏给低安全等级的主体,从而违反了多级安全（MLS）策略.研究结果为人们避免选用不安全的模型提供了有意义的理论支持.

英文摘要：

The security and flexibility are two goals that various improved BLP models attempt to achieve. How to enhance the flexibility of BLP model is a challenging problem that security researchers try to solve. However, the implementation of an insecure ＂security model＂ in the system will result in an insecure system. In this paper, two improved BLP models, for short DBLP （dynamic BLP） and SLCF （security label common framework）, are analyzed. Although the designers of the two models claimed that their proposals can adjust the security level of the untrusted subject dynamically and accordingly improve the flexibility of the classical BLP model, the analytic results show that the two improved models are not secure at all. Under the rules of the two improved models a Trojan horse can ＂legally＂ read the high-level information and then write them to low-level objects, which violate the principle of multi-level security （MLS）. This effort provides a theoretical foundation for avoiding the choice of insecure MLS model.