Reactive Peripheral Modeling for Faithful Firmware Rehosting

Rehosting enables tight control and introspection for firmware testing, but existing approaches largely fail to reach deeper application states and cannot drive embedded protocol stacks beyond early-stage initialization. This limitation reflects a broader weakness in current rehosting techniques: their inability to faithfully model complex peripheral semantics and dependencies. In particular, existing work typically relies on passive approximations of peripheral behavior and overlooks three key aspects: (i) the interplay among interrupts, MMIO, and DMA; (ii) implicit state transitions within peripherals; and (iii) interactions across multiple peripherals. To address this challenge, we propose Reactive Peripheral Modeling (RPM), an abstraction that models hardware peripherals as reactive and stateful systems. RPM captures peripheral behavior using event-condition-action semantics, enabling faithful representation of interrupt, MMIO, and DMA scheduling, implicit state transitions, and cross-peripheral interactions. We implement RPM in Bluezz for BLE firmware rehosting and fuzzing, and show that reactive modeling is necessary to reach deep protocol states. We evaluate Bluezz on representative BLE stacks, including NimBLE, Zephyr, and Nordic SoftDevice, a closed-source commercial stack. Across 18 targets, Bluezz achieves an average basic-block coverage more than 2.6 times that of prior state-of-the-art rehosting approaches. Unlike prior approaches, which remain largely confined to advertising and scanning logic, Bluezz reliably exercises connected BLE states and uncovers five previously unknown vulnerabilities that manifest only after connection establishment. Finally, we show that RPM generalizes beyond BLE to other embedded firmware running on different MCUs.