Although cloud computing has emerged as a promising future computing model, security concerns due to malicious tenants have been deterring its fast adoption. In cloud computing, multiple tenants may share physical systems by using virtualization techniques. In such a virtualized system, a software hypervisor creates virtual machines (VMs) from the physical system, and provides each user with an isolated VM. However, the hypervisor, with a full control over hardware resources, can access the memory pages of guest VMs without any restriction. By compromising the hypervisor, a malicious user can access the memory contents of the VMs used by other users.

In this paper, we propose a hardware-based mechanism to protect the memory of guest VMs from unauthorized accesses, even with an untrusted hypervisor. With this mechanism, memory isolation is provided by the secure hardware, which is much less vulnerable than the software hypervisor. The proposed mechanism extends the current hardware support for memory virtualization with a small extra hardware cost. The hypervisor can still flexibly allocate physical memory pages to virtual machines for efficient resource management. However, the hypervisor can update nested page tables only through the secure hardware mechanism, which verifies each mapping change. Using the hardware-oriented mechanism in each system securing guest VMs under a vulnerable hypervisor, this paper also proposes a cloud system architecture, which supports the authenticated launch and migration of guest VMs.