.. SPDX-License-Identifier: GPL-2.0+

Measured Boot
=====================

U-Boot can perform a measured boot, the process of hashing various components
of the boot process, extending the results in the TPM and logging the
component's measurement in memory for the operating system to consume.

By default, U-Boot will measure the operating system (linux) image, the
initrd image, and the "bootargs" environment variable. By enabling
CONFIG_MEASURE_DEVICETREE, U-Boot will also measure the devicetree image.

The operating system typically would verify that the hashes found in the
TPM PCRs match the contents of the event log. This can further be checked
against the hash results of previous boots.

Requirements
---------------------

* A hardware TPM 2.0 supported by the U-Boot drivers
* CONFIG_TPM=y
* CONFIG_MEASURED_BOOT=y
* Device-tree configuration of the TPM device to specify the memory area
  for event logging. The TPM device node must either contain a phandle to
  a reserved memory region or "linux,sml-base" and "linux,sml-size"
  indicating the address and size of the memory region. An example can be
  found in arch/sandbox/dts/test.dts
* The operating system must also be configured to use the memory regions
  specified in the U-Boot device-tree in order to make use of the event
  log.