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Measured Secure Boot

Purpose

To build a Measured Secure Boot (MSB) implementation that uses a hardware based Root of Trust for Measurement (RTM) from which to build a Root of Trust for Verification (RTV).

Background

To date the only supported Late Launch environment is the tboot project which is maintained by Intel for their Intel TXT capability. While tboot provides a complete capability, it is limited and has a number of deficiencies. While it would be possible to build a Measured Secure Boot solution around tboot, it will still be limited in comparison to approach provided by this blueprint.

Before detailing the approach and the improvements it provides, it is good to understand how the tboot environment works. Before tboot can run, it must be loaded into memory along with the target kernel it will launch into by either a legacy multiboot compliant bootloader or by an UEFI boot manager. Tboot itself is composed of two main parts, pre-launch and post-launch execution paths. Below is a top-level execution flow for tboot from boot loader/manager to the trusted kernel that will be given control.

                        |--          tboot             --|
boot ldr/mngr --load--> pre-launch --SENTER--> post-launch --launch--> trusted kernel

In terms of integrity policy, tboot policy can only be used to measure and verify any multiboot modules loaded by the boot loader/manager. Typically tboot policy is used in one of two manners, enforcing and non-enforcing. In both cases the policy is used to control what tboot will measure and which PCRs those measurements are stored. When an enforcing policy is in place, the measurements are compared with values populated in the policy and will take a selected action when the measurements do not match. With a non-enforcing policy, measurement comparison is not done and it is left to the trusted kernel to take action on the measurements. This means if advanced actions other than measurement verification is desired, then the trusted kernel must be aware and made to take the actions.

Approach

The x86 Late Launch capability will be used to establish an RTM using a Dynamic Root of Trust Measurement (DRTM) that will include the device owner's RSA public key. The Late Launch will start a TrenchBoot Security Engine (TSE) that is capable of simple measurement verification similar to tboot, but will be able to do advance actions such as a KMIP attestation for device encryption key.

MSB will consist of an enhancement to the GNU GRUB bootloader (grub), an extended Linux kernel, and the uroot initramfs environment. The enhancement to grub will be to add TPM 1.2/2.0 support along with relocators for AMD and Intel Late Launch instructions. The Linux kernel will be extend to function as a post-launch kernel that will run the TSE. Details for each of these are documented in their own respective blueprints. Finally, below is the execution flow for MSB for comparison with tboot's execution flow above.

grub --SENTER/SKINIT--> Linux/TSE --kexec--> trusted kernel

Last update: November 30, 2021