Abstracting Faceted Execution

Faceted execution is a linguistic paradigm for dynamic information-flow control with the distinguishing feature that program values may be faceted. Such values represent multiple versions or facets at once, for different security labels. This enables policy-agnostic programming: a paradigm permitting expressive privacy policies to be declared, independent of program logic. Although faceted execution prevents information leakage at runtime, it does not guarantee the absence of failure due to policy violations. By contrast with static mechanisms (such as security type systems), dynamic information-flow control permits arbitrarily expressive and dynamic privacy policies but imposes significant runtime overhead and delays discovery of any possible violations. In this paper, we present the two different abstract interpretations for faceted execution in the presence of first-class policies. We first present an abstraction which allows one to reason statically about the shape of facets at each program point. This abstraction is useful for statically proving the absence of runtime errors and eliminating runtime checks related to facets. Reasoning statically about the contents of faceted values, however, is complicated by the presence of first-class security labels, especially because abstract labels may conflate more than one runtime label. To address these issues, we also develop a more precise abstraction that relies on an analysis tracking singleton heap abstractions. We present an implementation of our coarse abstraction in Racket and demonstrate its performance on several sample programs. We conclude by showing how our precise domain can be used to verify information-flow properties.