Cloud computing relies on hypervisors to enforce isolation between co-tenanted virtual machines. Hypervisors are therefore critical security infrastructure, and assurance of their correctness is paramount. Traditional engineering techniques - code review, testing, fuzzing - provide strong assurance but cannot exhaustively verify that isolation holds across all possible execution paths. Formal verification extends and complements these approaches by establishing mathematical guarantees about system behaviour. This talk presents our experience applying interactive theorem proving to verify a production hypervisor component: the Nitro Isolation Engine. This is a trusted, minimalist computing base written in Rust, enforcing isolation between virtual machines on AWS Graviton5 EC2 instances. Designed for verification from inception, we have specified the intended behaviour of this component and verified correctness in the Isabelle/HOL interactive theorem prover, producing approximately 330,000 lines of machine-checked models and proofs, and establishing three key classes of property: 1) Functional correctness: The system behaves as specified for all operations including virtual machine creation, memory mapping, and abort handling. Our total verification approach additionally establishes memory-safety, termination, and absence of runtime errors. 2) Confidentiality: A noninterference-style property demonstrates that guest virtual machine state remains hidden from an expansive definition of observer monitoring system actions, formalised as indistinguishability preservation up to permitted declassification flows. 3) Integrity: Guest virtual machine private state is unaffected by operations on distinct virtual machines. Currently, our proof coverage extends to verification of the core virtual machine-management hypercalls, guest power management, various utility hypercalls, and a subset of data, instruction, and asynchronous abort handling, and will continue to expand to cover more functionality including PCI device management and virtual GIC (Generic Interrupt Controller) handling. The talk will discuss the verification approach, key proof techniques, and challenges in applying formal methods to production systems. Note that this work builds on decades of academic research across interactive theorem proving, formal specification, separation logic and its automation, and programming language semantics.

We present Arm's efforts in verifying the specification and prototype reference implementation of the Realm Management Monitor (RMM), an essential firmware component of Arm Confidential Computing Architecture (Arm CCA), the recently-announced Confidential Computing technologies incorporated in the Armv9-A architecture. Arm CCA introduced the Realm Management Extension (RME), an architectural extension for Armv9-A, and a technology that will eventually be deployed in hundreds of millions of devices. Given the security-critical nature of the RMM, and its taxing threat model, we use a combination of interactive theorem proving, model checking, and concurrency-aware testing to validate and verify security and safety properties of both the specification and a prototype implementation of the RMM. Crucially, our verification efforts were, and are still being, developed and refined contemporaneously with active development of both specification and implementation, and have been adopted by Arm's product teams. We describe our major achievements, realized through the application of formal techniques, as well as challenges that remain for future work. We believe that the work reported in this paper is the most thorough application of formal techniques to the design and implementation of any current commercially-viable Confidential Computing implementation, setting a new high-water mark for work in this area.