Product Programs In The Wild: Retrofitting Program Verifiers To Check Information Flow Security

Most existing program verifiers check trace properties such as functional correctness, but do not support the verification of hyperproperties, in particular, information flow security. In principle, product programs allow one to reduce the verification of hyperproperties to trace properties and, thus, apply standard verifiers to check them; in practice, product constructions are usually defined only for simple programming languages without features like dynamic method binding or concurrency and, consequently, cannot be directly applied to verify information flow security in a full-fledged language. However, many existing verifiers encode programs from source languages into simple intermediate verification languages, which opens up the possibility of constructing a product program on the intermediate language level, reusing the existing encoding and drastically reducing the effort required to develop new verification tools for information flow security.In this paper, we explore the potential of this approach along three dimensions: (1) Soundness: We show that the combination of an encoding and a product construction that are individually sound can still be unsound, and identify a novel condition on the encoding that ensures overall soundness. (2) Concurrency: We show how sequential product programs on the intermediate language level can be used to verify information flow security of concurrent source programs. (3) Performance: We implement a product construction in Nagini, a Python verifier built upon the Viper intermediate language, and evaluate it on a number of challenging examples. We show that the resulting tool offers acceptable performance, while matching or surpassing existing tools in its combination of language feature support and expressiveness.