Information-flow control tackles two of the classic Confidentiality-Integrity-Availability (CIA) aspects of security. This paper presents a framework that applies IFC to the mCertiKOS microkernel in order to prove confidentiality properties. The proof consists of Coq proofs about individual system calls and a general framework relating lower-level “unwinding conditions” to the overall security properties. The proof covers both C and x86 assembly code in mCertiKOS and includes reasoning about page tables.
For those familiar with the mCertiKOS work, one of the remarkable things about this paper is the size of the proofs. While the refinement proofs (described elsewhere) are over 200K lines of proof, the security proofs in this paper are just over 6K lines of proof. I think that the reason that they are so small comes from two things.
mCertiKOS is deterministic and is isomorphic to its specification. This means that they only/mostly need to prove that the specification satisfies their security specification. This allows them to leverage their existing, voluminous refinement proofs that the implementation satisfies the functional specification.
The proof is limited to a restricted subset of mCertiKOS that omits inter-processor communication. Omitting IPC eliminates a lot of information flow which makes it simpler to state and prove the security properties. But omitting IPC also eliminates a lot of the useful behaviour of an operating system leaving you with little more than time-sharing.
The framework used is a variant of Sabelfield and Sand’s PER model and fits into the classic pattern of “non-interference” proofs. The framework used is more general than is required for the proof about mCertiKOS that seems to be based on a static labelling of each component of the state as being part of one process or another. This static labelling seems to cause some difficulty as shared pieces of state such as machine registers need to change labels on every context switch.