$\mathcal{P}\text{ARseL}$: Towards a Verified Root-of-Trust Over seL4

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
$(\mathcal{R}\mathrm{A})$
which allows a trusted entity to determine the current software integrity of an untrusted remote device. For higher-end devices,
$\mathcal{R}\mathrm{A}$
is achievable via secure hardware components. For low-end (bare metal) devices, minimalistic hybrid (hardware/-software)
$\mathcal{R}\mathrm{A}$
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
$\mathcal{R}\mathrm{A}$
. HYDRA [1] 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
$\mathcal{P}\text{ARseL}$
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
$\mathcal{P}\text{ARseL}$
, We use formal verification to obtain several security properties for the isolated
$\mathcal{R}\mathrm{A}$
TCB, including: memory safety, functional correctness, and secret independence. We implement
$\mathcal{P}\text{ARseL}$
in
$F^{\ast}$
and specify/prove expected properties using Hoare logic. Next, we automatically translate the
$F^{\ast}$
implementation to C using KaRaM eL, which preserves verified properties of
$\mathcal{P}\text{ARseL}$
, C implementation (atop seL4). Finally, we instantiate and evaluate
$\mathcal{P}\text{ARseL}$
on a commodity platform - a SabreLite embedded device.