Verifying Confidentiality Under Nondeterminism for Storage Systems

Abstract

Storage systems must often store confidential data for their users. It is important to ensure that confidentiality of the stored data is maintained in the presence of bugs and malicious adversaries. This thesis tackles this problem using formal verification, a technique that involves proving a software system always satisfy certain requirements.

There are numerous challenges in specifying what it means a system being confidential and proving that a system satisfies that specification: nondeterministic behavior, indirect leakage of the data, system complexity, and others. Nondeterminism in particular creates unique challenges by making probabilistic leakage possible. This dissertation introduces the following to address these challenges:

Two novel confidentiality specifications for storage systems with nondeterministic behavior: data nonleakage and relatively deterministic noninfluence. Both definitions accommodate discretionary access control and intentional disclosure of the system metadata.

Two techniques accompanying these specifications: sealed blocks and nondeterminism oracles. These techniques addressed the challenges encountered in proving the confidentiality of the systems, and also reduced the proof effort required for said proofs. These techniques are formalized and implemented in two frameworks: DiskSec and ConFrm. Both frameworks contain metatheory to help the developer to prove that their implementation satisfies the specification.

The first confidential, crash-safe, and formally verified file systems with machine-checkable proofs: SFSCQ and ConFs. SFSCQ uses data nonleakage and ConFs uses relatively deterministic noninfluence as their confidentiality specifications. Both are implemented and verified in Coq.

An evaluation shows relatively deterministic noninfluence has 9.2x proof overhead per line of implementation code. Experiments with multiple benchmarks show that our systems perform better compared to FSCQ verified file system but worse compared to ext4 file system.