Safety Checking of Machine Code

Zhichen Xu, Barton, P. Miller, and Thomas Reps
University of Wisconsin

We show how to determine statically whether it is safe for untrusted machine code to be loaded into a trusted host system.

Our safety-checking technique operates directly on the untrusted machine-code program, requiring only that the initial inputs to the ntrusted program be annotated with typestate information and linear constraints. This approach opens up the possibility of being able to certify code produced by any compiler from any source language, which gives code producers more freedom in choosing the language in which they write their programs. It eliminates the dependence of safety on the correctness of the compiler because the final product of the compiler is checked. It leads to the decoupling of the safety policy from the language in which the untrusted code is written, and consequently, makes it possible for safety checking to be performed with respect to an extensible set of safety properties that are specified on the host side.

We have implemented a prototype safety checker for SPARC machine-language programs, and applied the safety checker to several examples. The safety checker was able to either prove that an example met the necessary safety conditions, or identify the places where the safety conditions were violated. The checking times ranged from less than a second to 14 seconds on an UltraSPARC machine.

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