The Xen open-source repository [27] contains a limited virtual TPM implementation comprised of combined contributions by Intel Corporation and the authors of this paper. Our contributions to Xen so far include the virtual TPM driver pair (front- and back-end drivers), hotplug scripts, and changes to Xen's management tools. We kept this infrastructure modular so that different realizations of virtual TPMs can work with it. The virtual TPM design and implementation presented in this paper adds the following to what is currently available in Xen: support for migrating a vTPM instance alongside its associated virtual machine, support for attestation of the complete vTPM environment along with the contents of a virtual machine, and an entirely separate software implementation of the TPM specification. In addition, the virtual TPM now in Xen is a partial implementation based on version 1.1 of the TPM specification, while we have updated our virtual TPM to be a complete implementation of version 1.2.
Previous research in the area of trusted computing examined how data that is protected (sealed) by a hardware TPM can be moved to another platform. Kuehn et al. [17] proposed a protocol for migrating the key-related hardware TPM security state from one hardware platform to another involving a separate TPM Migration Authority (TMA). Our protocol differs from the one presented there in many significant ways. Most notably, we migrate the complete virtual TPM state, we do not require a third party for migration, we maintain associations of virtual TPMs to their VMs and the operating system, and we can seamlessly integrate our protocol into the automated VM migration process. In addition, the extensions we introduce to the TPM standard do not require changes to existing commands and semantics. Similar to their concern about security of the destination TPM, we have pointed out that secure migration relies on a decision process that determines the safety of migrating a key pair to another TPM based on trust in that other TPM implementation.
The Terra project [7] investigated trusted virtual machine monitors. They developed a prototype based on VMWare's GSX server product that performs attestation of virtual machines and applications launched therein. Their publications recognize the availability of TPM 1.1b, but do not describe the design of a virtual TPM to run their attestation scheme against. Terra could use something like our vTPM facility to make a virtual TPM instance available to each of their virtual machines.