On the identification of covert storage channels in secure systems

A practical method for the identification of covert storage channels is presented and its application to the source code of the Secure Xenix kernel is illustrated. The method is based on the identification of all visible/alterable kernel variables by using information-flow analysis of language code. The method also requires that, after the sharing relationships among the kernel primitives and the visible/alterable variables are determined, the nondiscretionary access rules implemented by each primitive be applied to identify the potential storage channels. The method can be generalized to other implementation languages, and has the following advantages: it helps discover all potential storage channels is kernel code, thereby helping determine whether the nondiscretionary access rules are implemented correctly; it helps avoid discovery of false flow violations and their unnecessary analysis; and it helps identify the kernel locations where audit code and time-delay variables need to be placed for covert-channel handling