At the edge of modern cyber-physical systems, Micro-Controller Units (MCUs) are responsible for safety-critical sensing/actuation. However, MCU cost constraints rule out the usual security mechanisms of general-purpose computers. Thus, various low-cost security architectures have been proposed to remotely verify MCU software integrity. Control Flow Attestation (CFA) enables a Verifier $(\mathcal{V}{\text{rf}})$ to remotely assess the run-time behavior of a prover MCU $(\mathcal{P}rv)$, generating an authenticated trace of all of $\mathcal{P}{\text{rv}}$ control flow transfers (CFLog). Further, Control Flow Auditing architectures augment CFA by guaranteeing the delivery of evidence to $\mathcal{V}{\text{rf}}$.Unfortunately, a limitation of existing CFA lies in the cost to store and transmit CFLog, as even simple MCU software may generate large traces. Given these issues, prior work has proposed static (context-insensitive) optimizations. However, they do not support configurable program-specific optimizations. In this work, we note that programs may produce unique predictable control flow sub-paths and argue that program-specific predictability can be leveraged to dynamically optimize CFA while retaining all security guarantees. Therefore, we propose SpecCFA: an approach for dynamic sub-path speculation in CFA. SpecCFA allows $\mathcal{V}{\text{rf}}$ to securely speculate on likely control flow sub-paths for each attested program. At run-time, when a sub-path in CFLogmatches a pre-defined speculation, the entire sub-path is replaced by a reserved symbol. SpecCFA can speculate on multiple variable-length control flow sub-paths simultaneously. We implement SpecCFA atop two open-source control flow auditing architectures: one based on a custom hardware design [1] and one based on a commodity Trusted Execution Environment (ARM TrustZone-M) [2]. In both cases, SpecCFA significantly lowers storage/performance costs that are critical to resource-constrained MCUs.

Micro-controller units (MCUs) implement the de facto interface between the physical and digital worlds. As a consequence, they appear in a variety of sensing/actuation applications from smart personal spaces to complex industrial control systems and safety-critical medical equipment. While many of these devices perform safety- and time-critical tasks, they often lack support for security features compatible with their importance to overall system functions. This lack of architectural support leaves them vulnerable to run-time attacks that can remotely alter their intended behavior, with potentially catastrophic consequences. In particular, we note that, MCU software often includes untrusted third-party libraries (some of them closed-source) that are blindly used within MCU programs, without proper isolation from the rest of the system. In turn, a single vulnerability (or intentional backdoor) in one such third-party software can often compromise the entire MCU software state. In this article, we tackle this problem by proposing, demonstrating security, and formally verifying the implementation of UCCA: anUntrustedCodeCompartmentArchitecture. UCCA provides flexible hardware-enforced isolation of untrusted code sections (e.g., third-party software modules) in resource-constrained and time-critical MCUs. To demonstrate UCCA’s practicality, we implement an open-source version of the design on a real resource-constrained MCU: the well-known TI MSP430. Our evaluation shows that UCCA incurs little overhead and is affordable even to lowest-end MCUs, requiring significantly less overhead and assumptions than the prior related work.