Tboot
Trusted Boot (tboot) is an open source, pre- kernel/VMM module that uses Intel(R) Trusted Execution Technology (Intel(R) TXT) to perform a measured and verified launch of an OS kernel/VMM.The first OS code loaded is the Trusted Boot (called TBOOT) code that sets up the platform for Intel TXT and initiates the measured launch before loading the OS kernel.In particular, the TBOOT module will setup the platform for a measured launch and then invoke the SENTER command. To better understand the role of the Tboot, consider the platform phases associated with measured launch, which are illustrated in Figure.
The phases are as follows:
1. Pre-boot phase is performed by the system firmware (BIOS/UEFI). One of the goals of an Intel TXT-enabled BIOS is to initialize the platform to a state that will support a “measured launch.” To do so, the firmware measures the Static Root of Trust and other platform components into PCRs 0 through 7. It also protects Intel TXT resources and locks the platform configuration.
2. IPL represents the normal boot process up until the time that the process would normally load and execute the kernel. The first module executed should now be the Trusted Boot (tBoot) module.
3. TBOOT Pre-Launch is the part of tBoot that determines whether a measured launch is possible and sets up the platform to perform the measured launch.
4. TBOOT Launch is the part of tBoot that starts the measured launch process by executing the GETSEC [SENTER] instruction. This execution starts the dynamic chain of trust measurements, extending the root of trust measurement into PCR 17 and measures tBoot Post Code into PCR 18.
5. TBOOT Post Launch is the code that executes as a result of the measured launch. Its purpose is to securely bring the platform to a protected, usable state. This is the first system code to be measured, and it starts the chain of measurements.
6. OS/VMM Post Launch includes the kernel and any other modules that need to be loaded. The kernel is responsible for measuring other modules before they are executed if they have not already been measured by the tBoot code.
7. Regular Operation commences after successful launch, when the OS/hypervisor performs its primary functionality (i.e., the same functionality as would occur in an environment without Intel TXT). However, there are some additional capabilities available to the OS/hypervisor, which must protect Intel TXT resources.
8. MLE Shutdown occurs before turning off or resetting the platform; there are certain steps the OS/hypervisor is required to take to exit the secure environment. While this phase could be followed by another measured launch, it is typically followed by a platform reset or power cycle.
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