BUSted!!! Microarchitectural side-channel attacks on the MCU bus interconnect

Spectre and Meltdown have pushed the research community toward an otherwise-unavailable understanding of the security implications of processors’ microarchitecture. Notwithstanding, research efforts have concentrated on highend processors (e.g., Intel, AMD, Arm Cortex-A), and very little has been done for microcontrollers (MCU) that power billions of small embedded and IoT devices. In this paper, we present BUSted. BUSted is a novel side-channel attack that explores the side effects of the MCU bus interconnect arbitration logic to bypass security guarantees enforced by memory protection primitives. Side-channel attacks on MCUs pose incremental and unforeseen challenges, which are strictly tied to the resource-constrained nature of these systems (e.g., single-core CPU, stateless bus). We devise a unique approach that relies on the concept of hardware gadgets. We present practical attacks on state-of-the-art Armv8-M MCUs with TrustZone-M, running the Trusted Firmware-M (TF-M). In contrast to the Nemesis attack, our attack is practical on Arm Cortex-M MCUs, and our findings suggest that it can scale across the full MCU spectrum.We would like to thank the reviewers for their valuable feedback and suggestions. Cristiano Rodrigues was supported by FCT grant 2020.08729.BD. Daniel Oliveira was supported by FCT grant 2020.04585.BD. This work is supported by FCT – Fundac¸ao para a Cência e Tecnologia within the R&D Units Project Scope UIDB/00319/2020, and partially by the European Union’s Horizon Europe research and innovation program under grant agreement No 101070537, project CROSSCON (Cross-platform Open Security Stack for Connected Devices)