Formal modelling and analysis of Bitflips in ARM assembly code

摘要

Bitflips, or single-event upsets (SEUs) as they are more formally known, may occur for instance when a high-energy particle such as a proton strikes a CPU and thereby corrupting the contents of an on-chip register, e.g., by randomly flipping one or more bits in that register. Such random changes in central registers may lead to critical failure in the execution of a program, which is especially problematic for safety- or security-critical applications. Even though SEUs are well studied in the literature, relatively little attention have been given to the formal modelling of SEUs and the application of formal methods to mitigate the consequences of SEUs. In this paper we develop a formal semantic framework for easy formal modelling of a large variety of SEUs in a core assembly language capturing the essential features of the ARM assembly language. Based on the semantic framework, we derive and formally prove correct a static analysis that enforces so-called blue/green separation in a given program. Static blue/green separation is a language-based fault detection technique relying on inlined replication of critical code. A program that has proper blue/green separation is shown to be fault-tolerant with respect SEUs in data registers. However, this technique requires specialised hardware support in order to achieve full coverage. We therefore use our semantic framework to further develop so-called gadgets, essentially small code fragments that emulate the behaviour of blue/green instructions in a safe manner. The gadgets allow us to achieve partial blue/green separation without specialised hardware support. Finally, we show how our semantic framework can be used to extract timed-automata models of ARM assembler code programs. We then apply statistical model checking to these timed-automata models enabling us to model, analyse, and quantify program behaviour in the presence of fault models that go well beyond data-flow SEUs, e.g., bitflips in program counters or in the code itself. We use this approach to provide evidence that our suggested program modifications, i.e., the use of gadgets, significantly decrease the probability of such faults going undetected.