Enabling Confidential VMs Across Diverse ARM Platforms by Retrofitting TrustZone

As cloud tenants increasingly adopt ARM servers, securing their sensitive workloads becomes a critical concern. Confidential virtualization, which shields data from privileged software by establishing isolated VMs, has emerged as a standard approach to address this concern. However, ARM's official solution, the Confidential Compute Architecture (CCA), fails to meet this demand. It lacks support on most deployed hardware, and its widespread adoption by major cloud providers remains distant. In this paper, we propose TwinVisor, a novel system that enables confidential virtualization on ARM platforms. TwinVisor innovatively retrofits the mature ARM TrustZone to enforce strong isolation for confidential VMs, while minimizing the TCB by decoupling resource management from security mechanisms. Specifically, TwinVisor introduces a tiny hypervisor in the secure world (calledS-visorin this paper) dedicated to protection, while reusing a full-fledged hypervisor (calledN-visor) in the normal world for resource management. By combining S-EL2 hardware virtualization features with software-based same-privilege protection techniques, TwinVisor achieves broad compatibility, enabling its deployment on almost all ARM platforms. We have implemented two prototypes of TwinVisor, and our evaluation demonstrates that it can effectively protect confidential VMs while imposing negligible performance overhead for real-world applications.