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Operating System / Hypervisor Layer

Moving up the stack, the second layer is the OS/hypervisor. To participate in a measured launch, an OS or hypervisor has to be enabled for TXT. The changes related to TXT are in the initialization code, and also during termination and shutdown. Additionally, basic enablement means that the operating system or hypervisor can invoke the secure launch process. This entails including a pre-kernel module that can ensure the right SINIT (authenticated code modules from Intel) module is selected and assure the orderly evaluation of the launch components of the software. Intel provides a reference implementation called Trusted Boot (tboot) for the pre-kernel module that can be integrated into OS/hypervisors toward enabling for Intel TXT, and it is the maintainer of this open-source “tboot” project.

Tboot is by far the most widely used mechanism offered by software vendors to enable their OS or hypervisor. SINIT modules on server platforms are generally embedded in the platform BIOS, and are processorand chipset generation-specific. The tboot components provided by Intel are integrated into the operating systems or hypervisors (by the respective ISVs) and work across multiple generations of platforms.

This makes sense, as it allows the most qualified party (in this case, the ISV) to determine which modules are essential for the trusted compute base (TCB) of their software, and therefore which modules to include in the measured launch and in which order.

Tboot technology is included in multiple open-source operating system/hypervisor environments from Linux, to Xen/KVM, to a number of commercial products, such as Red Hat and Citrix XenServer. Other vendors, like VMware, have implemented their own tboot-like functions. It is interesting to note that the percentage of TCB measured by vendors as part of the launch process varies significantly. As of this writing, VMware by far has the most coverage of the TCB. Other OS/VMM vendors have the core kernel and few modules measured. All of these vendors have been actively working toward increasing the amount of TCB that they measure. For detailed coverage of the measured launch environments (MLE) developer guidance, check out the book Intel Trusted Execution Technology for Server Platforms from Apress.

With TXT and TPM correctly configured and enabled in hardware, when a

TXT-enabled OS/hypervisor is launched, the platform goes through a measured D-RTM launch. Just to refresh your understanding of the TXT launch process, when a TXT launch happens, what you have is a measured launch of the firmware, BIOS, and controlling software like an OS or VMM. These measurements (which are the identities of the various components), as part of the launch process are stored in the various registers in the TPM (RTS and RTR) called PCRs (platform configuration registers) and are verified with an attestation system. TCG PC Spec provides the semantics for where the various measurements are stored in the TPM.

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