Reasoning about Control Flow in the Presence of Transient Faults

A transient fault is a temporary, one-time event that causes a change in state or erroneous signal transfer in a digital circuit. These faults do not cause permanent damage, but when they strike conventional processors, they may result in incorrect program execution. While detecting and correcting faults in first-order data may be accomplished relatively easily by adding redundancy, protecting against faults during control flow transfers is substantially more difficult. This paper analyzes the problem of maintaining the control-flow integrity of a program in the face of transient faults from a formal theoretical perspective. More specifically, we augment the operational semantics of an idealized assembly language with additional rules that model erroneous control-flow transfers. Next, we explain a strategy for detecting control-flow errors based on previous work by Oh [11] and Reis [16]. In order to reason about the correctness of the strategy relative to our fault model, we develop a new assembly-level type system designed to guarantee that any control flow transfer to an incorrect block will be caught before control leaves that block. The key technical result of the paper is a rigorous proof of this fundamental control-flow property for well-typed programs. We also prove that this new typed assembly language is sufficiently expressive to serve as a target for type-preserving compilation from a simple language of while programs.