类Unix文件系统中TOCTTOU缺陷的静态分析方法

Static Analysis of TOCTTOU Vulnerabilities in Unix-Style File System

文件系统中的TOCTTOU缺陷是类Unix操作系统面临的一个严重安全问题,现有的静态检测方法具有很高的误报率.原因有2点:一是对导致TOCTTOU缺陷的函数对缺乏精确定义和分析;二是分析过程过度抽象,忽略了很多重要的程序信息.因此,首先对TOCTTOU缺陷进行了分类,并系统分析了C标准库中可以导致TOCTTOU缺陷的函数对.在此基础上,提出了一种TOCTTOU缺陷的静态分析方法,利用有限状态安全属性刻画TOCTTOU缺陷,分析精度达到了过程内路径敏感、过程间流敏感.实验结果表明,该方法能够有效检测C程序中的TOCTTOU缺陷,相比现有方法,有效降低了误报率.

TOCTTOU is a serious threat to Unix-style file systems. All the existing static detection methods have high false positive rate. There are two reasons., firstly, the function pairs which may cause TOCTTOU vulnerabilities are not defined and enumerated accurately; and secondly, the methods make an o^eer-approximation of the program and omit a lot of useful information. In this paper, we first systematically examine the TOCTTOU pairs in the standard C library. On this basis, a static analysis method is presented to detect the TOCTTOU vulnerabilities. Vulnerability is expressed as a finite safety state property. At each program point, a value is associated to a set of states. To make the analysis more precise, the algorithm is inter-procedurally flow sensitive and intra- procedurally path sensitive. To achieve scalability, the safety state property of each procedural is analyzed independently and the inter-procedurally analysis is summary based. The experimental results show that this method can effectively find TOCTTOU vulnerabilities in C programs. In comparison with other static methods, this method can effectively reduce false positive rate.