Proving Information Flow Security for Concurrent Programs

(Program) verification is the process of proving that a program satisfies some properties by using mathematical techniques and formal reasoning, rather than relying on testing the program with inputs. Program verification is typically used to prove functional correctness properties (e.g., proving that a sorting algorithm does not crash and correctly sorts inputs), but it can also be used to prove security properties such as information flow security, which ensures that the secret data manipulated by a program does not influence its observable output. Proving information flow security is especially challenging for concurrent programs, where operations on secret data may influence the execution time of a thread and, thereby, the interleaving between different threads. Such internal timing channels may affect the observable outcome of a program even if an attacker does not observe execution times. Existing verification techniques for information flow security in concurrent programs attempt to prove that secret data does not influence the relative timing of threads. However, these techniques are often restrictive (for instance because they disallow branching on secret data) and make strong assumptions about the execution platform (ignoring caching, processor instructions with data-dependent runtime, and other common features that affect execution time).

In this talk, we present a novel verification technique for secure information flow in concurrent programs that lifts these restrictions and does not make any assumptions about timing behavior. The key idea is to prove that all mutating operations performed on shared data commute, such that different thread interleavings do not influence its final value. Crucially, commutativity is required only for an abstraction of the shared data that contains the information that will be leaked to a public output. Abstract commutativity is satisfied by many more operations than standard commutativity, which makes our technique widely applicable. We formalize our technique in CommCSL, a relational concurrent separation logic with support for commutativity-based reasoning, and prove its soundness in Isabelle/HOL. We implemented CommCSL in HyperViper, an automated verifier based on the Viper verification infrastructure, and demonstrate its ability to verify challenging examples.