A New Security Threat in MCUs -- SoC-wide timing side channels and how to find them

Microarchitectural timing side channels have been thoroughly investigated as a security threat in hardware designs featuring shared buffers (e.g., caches) and/or parallelism between attacker and victim task execution. Contradicting common intuitions, recent activities demonstrate, however, that this threat is real also in microcontroller SoCs without such features. In this paper, we describe SoC-wide timing side channels previously neglected by security analysis and present a new formal method to close this gap. In a case study with the RISC-V Pulpissimo SoC platform, our method found a vulnerability to a so far unknown attack variant that allows an attacker to obtain information about a victim's memory access behavior. After implementing a conservative fix, we were able to verify that the SoC is now secure w.r.t. timing side channels.