μUSB: Practical and Safe USB Driver Reuse for Arm TrustZone
Abstract
TrustZone isolates device access with secure I/O, yet lacks support for USB devices, which constitute the most diverse peripherals. Existing approaches to integrate USB drivers into the TEE are impractical due to the complex USB protocols, high-frequency DMA accesses, and varying vendor implementations. In this paper, we present μUSB, a system that derives a functional, micro USB driver for TrustZone from the observable concrete execution traces of a full-fledged, complex USB driver, an approach we dub record, lift, and replay. Based on the key insight of kernel specialization and the deterministic nature of USB FSMs, μUSB proposes a lightweight mutational recorder and a novel program analysis technique, addressing two challenges: how to record USB execution sufficiently and analyze the traces deeply? Across four major USB classes and six diverse devices, μUSB generates performant, ready-to-use USB drivers for TrustZone, enabling in-TEE apps to, for the first time, leverage complex USB devices with near-native performance while still enjoying full TrustZone protection.