PARseL: Towards a Verified Root-of-Trust over seL4
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Widespread adoption and growing popularity of embedded/IoT/CPS devices make them attractive attack targets. On low-to-mid-range devices, security features are typically few or none due to various constraints. Such devices are thus subject to malware-based compromise. One popular defensive measure is Remote Attestation (RA) which allows a trusted entity to determine the current software integrity of an untrusted remote device. For higher-end devices, RA is achievable via secure hardware components. For low-end (bare metal) devices, minimalistic hybrid (hardware/software) RA is effective, which incurs some hardware modifications. That leaves certain mid-range devices (e.g., ARM Cortex-A family) equipped with standard hardware components, e.g., a memory management unit (MMU) and perhaps a secure boot facility. In this space, seL4 (a verified microkernel with guaranteed process isolation) is a promising platform for attaining RA. HYDRA made a first step towards this, albeit without achieving any verifiability or provable guarantees. This paper picks up where HYDRA left off by constructing a PARseL architecture, that separates all user-dependent components from the TCB. This leads to much stronger isolation guarantees, based on seL4 alone, and facilitates formal verification. In PARseL, We use formal verification to obtain several security properties for the isolated RA TCB, including: memory safety, functional correctness, and secret independence. We implement PARseL in F* and specify/prove expected properties using Hoare logic. Next, we automatically translate the F* implementation to C using KaRaMeL, which preserves verified properties of PARseL C implementation (atop seL4). Finally, we instantiate and evaluate PARseL on a commodity platform – a SabreLite embedded device.
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