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Sample Application

We ran an existing TPM application to show that our virtual TPM implementation provides correct TPM functionality to virtual machines. As a sample application we chose IBM's open-source Integrity Measurement Architecture (IMA) for the Linux operating system [13].

IMA provides to a remote system verifiable evidence of what software is running on a measured system. It maintains a list of hash values covering all executable content loaded into a system since startup, including application binaries. It brings together measurements made by the BIOS, boot loader and OS, and it offers an interface to retrieve these hash values from a remote system. IMA returns its list of measurements as well as a quote of current PCR values signed by the TPM. The signed quote from the TPM proves the integrity of the measurements. The remote system can then compare the measurements against known values to determine what software was loaded on the measured system.

IMA was originally written to run in a non-virtualized environment, where the Linux kernel has direct access to a hardware TPM. As a test of our vTPM facility, we ran IMA in a Xen virtual machine with access to a vTPM instance.

The complete attestation sequence in our virtualized environment is as follows. The virtual TPM runs as a process in Xen's management virtual machine, domain-0. We boot the system using a trusted boot loader, Trusted GRUB [9,18]. We measure the Xen hypervisor executable, the domain-0 kernel and initial RAM disk, as well as the initial Xen access control policy [20], and extend a PCR in the hardware TPM with these measurements. The resulting hardware PCR value thus attests to the integrity of the vTPM's trusted computing base (TCB), namely the hypervisor plus the management virtual machine.

When a user virtual machine starts, we measure its kernel image and initial RAM disk, and extend a PCR in the virtual TPM with these measurements. This sequence of measurements is part of the setup process of the vTPM instance (see Section 4.4). As the user virtual machine continues to run, the IMA-enhanced kernel in that virtual machine also extends a virtual PCR with measurements of every application that is loaded.

IMA attests to the integrity of both the vTPM TCB and the user virtual machine by returning PCR values from both the hardware TPM and the virtual TPM. We achieve this with our vTPM by projecting the lower PCRs of the hardware TPM (e.g., PCRs 0 through 8) to all virtual TPMs. This means that if a user VM reads one of those PCRs, the vTPM facility actually fetches the value from the hardware TPM. Extending hardware PCRs from user VMs is therefore disabled since these registers are logically owned by the management VM as depicted in Figure 4. Upper PCRs are accessible by user VMs as usual.

Therefore, we have the management VM extend the lower PCRs with measurements of the vTPM TCB. We have the user VM extend the upper PCRs with measurements of the user VM itself. IMA reports then combine lower PCR values, higher PCR values, and the measurement list from both the user VM and the management VM to provide a comprehensive view of the system. To relay the names of applications measured into the hardware TPM, we implemented a small extension to Integrity Measurement Architecture that retrieves this information from the vTPM-hosting domain using the ReportEnvironment command. Other aspects of IMA were left unmodified.

next up previous
Next: Discussion and Future Work Up: vTPM: Virtualizing the Trusted Previous: Implementation
root 2006-05-12