TrustGuard: A Containment Architecture with Verified Output

Abstract

Computers today are so complex and opaque that a user cannot know everything occurring within the system. Most efforts toward computer security have focused on securing software. However, software security techniques implicitly assume correct execution by the underlying system, including the hardware. Securing these systems has been challenging due to their complexity and the proportionate attack surface they present during their design, manufacturing, deployment, and operation. Ultimately, the user’s trust in the system depends on claims made by each party supplying the system’s components.

This dissertation presents the Containment Architecture with Verified Output (CAVO) model in recognition of the reality that existing tools and techniques are insufficient to secure complex hardware components in modern computing systems. Rather than attempt to secure each complex hardware component individually, CAVO establishes trust in hardware using a single, simple, separately manufactured component, called the Sentry. The Sentry bridges a physical gap between the untrusted system and its external interfaces and contains the effects of malicious behavior by untrusted system components before the external manifestation of any such effects. Thus, only the Sentry and the physical gap must be secured in order to assure users of the containment of malicious behavior. The simplicity and pluggability of CAVO’s Sentry enable suppliers and consumers to take additional measures to secure it, including formal verification, supervised manufacture, and supply chain diversification.

This dissertation also presents TrustGuard—the first prototype CAVO design—to demonstrate the feasibility of the CAVO model. TrustGuard achieves containment by only allowing the communication of correctly executed results of signed software. The Sentry in TrustGuard leverages execution information obtained from the untrusted processor to enable efficient checking of the untrusted system’s work, even when the Sentry itself is simpler and much slower than the untrusted processor. Simulations show that TrustGuard can guarantee containment of malicious hardware components with a geomean of 8.5% decline in the processor’s performance, even when the Sentry operates at half the clock frequency of the complex, untrusted processor.