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

[Ilias Apalodimas]

Hi Heinrich

On Thu, 19 Oct 2023 at 21:22, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> 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

Eddie's part is trying to describe how 'bootm' would measure binaries.
I'll update it with the EFI part once we merge this.

Regards
/Ilias

>
> 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.
>