[PATCH] doc: uefi: explicitly describe manual dtb update is required

[Takahiro Akashi]

Hi Heinrich,

On Mon, Jun 19, 2023 at 06:37:14AM +0200, Heinrich Schuchardt wrote:
> 
> 
> Am 19. Juni 2023 02:49:54 MESZ schrieb Takahiro Akashi <takahiro.akashi at linaro.org>:
> >On Sat, Jun 17, 2023 at 09:58:13PM +0200, Heinrich Schuchardt wrote:
> >> On 6/15/23 10:03, Masahisa Kojima wrote:
> >> > To enforce anti-rollback to any older version, dtb must be
> >> > always update manually. This should be described in the
> >> > documentation.
> >> > 
> >> > Signed-off-by: Masahisa Kojima <masahisa.kojima at linaro.org>
> >> > ---
> >> >   doc/develop/uefi/uefi.rst | 3 +++
> >> >   1 file changed, 3 insertions(+)
> >> > 
> >> > diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
> >> > index ffd13cebe9..d5f8c5f236 100644
> >> > --- a/doc/develop/uefi/uefi.rst
> >> > +++ b/doc/develop/uefi/uefi.rst
> >> > @@ -552,6 +552,9 @@ update using a capsule file with --fw-version of 5, the update will fail.
> >> >   When the --fw-version in the capsule file is updated, lowest-supported-version
> >> >   in the dtb might be updated accordingly.
> >> > 
> >> > +If user needs to enroce anti-rollback to any older version,
> >> > +the lowest-supported-version property in dtb must be always updated manually.
> >> 
> >> Thank you for updating the documentation.
> >> 
> >> Allowing to circumvent the rollback protection is a security issue. On a
> >> secure system you would probably want to disable console commands like
> >> mc and fdt. Shouldn't we provide an advice for safe settings?
> >
> >Is there any case where a user wants to use fdt for some reason,
> >for example, in CONFIG_PREBOOT or CONFIG_BOOTCOMMAND?
> >
> >-Takahiro Akashi
> 
> Dtb overlays can applied via the fdt command.

What I meant to say was that, if there is an useful use case of fdt
command, it would be too restrictive to recommend disabling the command.
(Questioning if a device tree is the right place to put the data.)

-Takahiro Akashi

> Best regards
> 
> Heinrich
> 
> 
> >
> >> E.g.
> >> 
> >> "If a user wanted to enable a rollback to a version forbidden by the
> >> lowest-supported-version property specified in U-Boot's control
> >> device-tree, they could change this property using the fdt command.
> >> Secure systems should not enable this command."
> >> 
> >> Best regards
> >> 
> >> Heinrich
> >> 
> >> > +
> >> >   To insert the lowest supported version into a dtb
> >> > 
> >> >   .. code-block:: console
> >> > 
> >> > base-commit: e350d0c60d413d441cbdfa9432ebadb56f625903
> >>