Trapped by Your WORDs: (Ab)using Processor Exception for Generic Binary Instrumentation on Bare-metal Embedded Devices

Analyzing the security of closed-source drivers and libraries in embedded systems holds significant importance, given their fundamental role in the supply chain. Unlike x86, embedded platforms lack comprehensive binary manipulating tools, making it difficult for researchers and developers to effectively detect and patch security issues in such closed-source components. Existing works either depend on full-fledged operating system features or suffer from tedious corner cases, restricting their application to bare-metal firmware prevalent in embedded environments.

In this paper, we present PIFER (Practical Instrumenting Framework for Embedded fiRmware) that enables general and fine-grained static binary instrumentation for embedded bare-metal firmware. By abusing the built-in hardware exception-handling mechanism of the embedded processors, PIFER can perform instrumentation on arbitrary target addresses. Additionally, We propose an instruction translation-based scheme to guarantee the correct execution of the original firmware after patching. We evaluate PIFER against real-world, complex firmware, including Zephyr RTOS, CoreMark benchmark, and a close-sourced commercial product. The results indicate that PIFER correctly instrumented 98.9% of the instructions. Further, a comprehensive performance evaluation was conducted, demonstrating the practicality and efficiency of our work.