Security vulnerabilities are software bugs that are exploited by an attacker. Systems software is at high risk of exploitation: attackers commonly exploit security vulnerabilities to gain control over a system, remotely, over the internet. Bug-checking tools have been used with fair success in recent years to automatically find bugs in software. However, for finding software bugs that can cause security vulnerabilities, a bug checking tool must determine whether the software bug can be controlled by user-input.
In this paper we introduce a static program analysis for computing user-input dependencies. This analysis is used as a pre-processing filter to our static bug checking tool, currently under development, to identify bugs that can be exploited as security vulnerabilities. Runtime speed and scalability of the user-input dependence analysis is of key importance if the analysis is used for large commercial systems software.
Our user-input dependency analysis takes both data and control dependencies into account. We extend Static Single Assignment (SSA) form by augmenting phi-nodes with control dependencies of its arguments. A formal definition of user-input dependency is expressed in a dataflow analysis framework as a Meet-Over-all-Paths (MOP) solution. We reduce the equation system to a sparse equation system exploiting the properties of SSA. The sparse equation system is solved as a reachability problem that results in a fast algorithm for computing user-input dependencies. We have implemented a call-insensitive and a call-sensitive version of the analysis. The paper compares their efficiency for various systems codes.