PASAN: Detecting Peripheral Access Concurrency Bugs within Bare-metal Embedded Applications
Proceedings of the 30th USENIX Security Symposium (Security) 2021.
Concurrency bugs might be one of the most challenging software defects to detect and debug due to their non-deterministic triggers caused by task scheduling and interrupt handling. While different tools have been proposed to address concurrency issues, protecting peripherals in embedded systems from concurrent accesses imposes unique challenges. A naïve lock protection on a certain memory-mapped I/O (MMIO) address still allows concurrent accesses to other MMIO addresses of a peripheral. Meanwhile, embedded peripherals such as sensors often employ some internal state machines to achieve certain functionalities. As a result, improper locking can lead to the corruption of peripherals' on-going jobs (we call transaction corruption) thus corrupted sensor values or failed jobs.
In this paper, we propose a static analysis tool namely PASAN to detect peripheral access concurrency issues for embedded systems. PASAN automatically finds the MMIO address range of each peripheral device using the parser-ready memory layout documents, extracts the peripheral's internal state machines using the corresponding device drivers, and detects concurrency bugs of peripheral accesses automatically. We evaluate PASAN on seven different embedded platforms, including multiple real time operating systems (RTOSes) and robotic aerial vehicles (RAVs). PASAN found 17 true positive concurrency bugs in total from three different platforms with the bug detection rates ranging from 40% to 100%. We have reported all our findings to the corresponding parties. To the best of our knowledge, PASAN is the first static analysis tool detecting the intrinsic problems in concurrent peripheral accesses for embedded systems.