XOMZone: DMA-aware code protection using TrustZone-M

Abstract

Considering the expertise and effort required to develop an embedded system, protecting software intellectual properties (IP) is the foremost task for many embedded system vendors. However, with the connectivity of modern embedded devices and the tendency of using low-level languages, defeating embedded system IP thefts have become a challenging goal. While execute-only memory (XOM) is a promising solution, the feature is not widely adopted for low-end ARM embedded processors (i.e., Microcontrollers (MCUs)). Therefore, a recent study proposed a technique to realize software-based XOM on ARM MCUs. However, the study disregards threats based on DMAs and thus limiting the applicability.

This paper proposes XOMZone, a code protection method that extends the software-based XOM for achieving DMA-aware code protection on ARM MCUs. XOMZone protects code from DMAs using the isolation provided by TrustZone-M, a lightweight TrustZone for ARM MCUs. Further, XOMZone maintains the compatibility with trusted applications. The current prototype of XOMZone shows performance and code size overhead of 13.1% and 18.9%, respectively, on BEEBS benchmarks. The evaluation indicates that XOMZone could be a practical code protection solution with further implementation optimizations.