[PATCH v13 6/8] doc: Add measured boot documentation

[Heinrich Schuchardt]

On 10/19/23 18:21, Eddie James wrote:
> Briefly describe the feature and specify the requirements.
>
> Signed-off-by: Eddie James <eajames at linux.ibm.com>
> Reviewed-by: Simon Glass <sjg at chromium.org>
> ---
> Changes since v12:
>   - Add a bit of detail about OS usage and what pieces are measured
>
>   doc/usage/index.rst         |  1 +
>   doc/usage/measured_boot.rst | 31 +++++++++++++++++++++++++++++++
>   2 files changed, 32 insertions(+)
>   create mode 100644 doc/usage/measured_boot.rst
>
> diff --git a/doc/usage/index.rst b/doc/usage/index.rst
> index 98b4719c40..bf53bb6bda 100644
> --- a/doc/usage/index.rst
> +++ b/doc/usage/index.rst
> @@ -14,6 +14,7 @@ Use U-Boot
>      partitions
>      cmdline
>      semihosting
> +   measured_boot
>
>   Shell commands
>   --------------
> diff --git a/doc/usage/measured_boot.rst b/doc/usage/measured_boot.rst
> new file mode 100644
> index 0000000000..0aad590859
> --- /dev/null
> +++ b/doc/usage/measured_boot.rst
> @@ -0,0 +1,31 @@
> +.. 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

The OS image would mean the root file system? I guess you mean the
loaded EFI binary here. Measured boot is not Linux specific.

We should consider different scenarios:

* U-Boot loading kernel, initrd, device-tree
* U-Boot loading GRUB (or another boot manager), device-tree,
   GRUB loading initrd
* U-Boot loading GRUB and passing a device-tree to GRUB,
   GRUB loading initrd and a new device-tree

With the documentation the user should be enabled to understand what
U-Boot measures in each of these cases. Please, observe that upstream
GRUB uses LoadImage() while distros (e.g. Debian and Ubuntu) tend to
bypass LoadImage().

Arent't there some other objects measured?

- SMBIOS table
- ACPI tables (still to be implemented)
- EFI variables

Best regards

Heinrich

> +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.