M-Step: A Single-Stepping Framework for Side-Channel Analysis on TrustZone-M

Trusted Execution Environments (TEEs) have become a key technology for isolating sensitive enclave applications from untrusted operating systems. Extensive research on high-end platforms like Intel SGX and TDX, AMD SEV, and Arm TrustZone-A has exposed their limitations in terms of software-based side-channel analysis, amplified by specialized single-stepping attack frameworks that exploit privileged timer interrupts to execute enclaves one instruction at a time. Meanwhile, TEEs are increasingly deployed on resource-constrained IoT devices, with Arm TrustZone-M emerging as a leading solution, which, however, remains largely unexplored for high-resolution, software-based side channels. This paper presents M-Step, an open and extensible single-stepping attack framework for TrustZone-M. While Cortex-M microcontrollers feature precise timers and deterministic behavior, achieving precise, instruction-level stepping remains challenging due to (i) the absence of virtual memory and page tables used in high-end frameworks; and (ii) Cortex-M's unique interrupt behavior, where certain multi-cycle instructions are abandoned or paused to reduce latency. To overcome these challenges, we extensively profile interrupt handling CPU behavior and develop a novel approach that uses previously dismissed interrupt-latency leakage to dynamically adjust the timer interrupts. We demonstrate M-Step's improved resolution and practicality by discovering previously unknown vulnerabilities in the latest Arm Mbed TLS library that enable single-trace, deterministic attacks recovering full RSA keys from a TrustZone enclave.