Re: [PATCH 0/2] RFC: add fdt_add_pubkey tool

[Simon Glass <sjg@chromium.org>]

Hi Jan,

On Tue, 9 Nov 2021 at 23:44, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>
> On 10.11.21 01:58, Simon Glass wrote:
> > Hi Jan,
> >
> > On Tue, 9 Nov 2021 at 03:07, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> >>
> >> On 09.11.21 10:37, Roman Kopytin wrote:
> >>> Can we have discussion with code lines? For me it is not very clear, because it isn't my code.
> >>>
> >>
> >> Please do not top-post.
> >>
> >>> -----Original Message-----
> >>> From: Jan Kiszka <jan.kiszka@siemens.com>
> >>> Sent: Tuesday, November 9, 2021 12:17 PM
> >>> To: Roman Kopytin <Roman.Kopytin@kaspersky.com>; u-boot@lists.denx.de; Rasmus Villemoes <rasmus.villemoes@prevas.dk>
> >>> Subject: Re: [PATCH 0/2] RFC: add fdt_add_pubkey tool
> >>>
> >>> On 08.11.21 16:28, Roman Kopytin wrote:
> >>>> In order to reduce the coupling between building the kernel and
> >>>> U-Boot, I'd like a tool that can add a public key to U-Boot's dtb
> >>>> without simultaneously signing a FIT image. That tool doesn't seem to
> >>>> exist, so I stole the necessary pieces from mkimage et al and put it
> >>>> in a single .c file.
> >>>>
> >>>> I'm still working on the details of my proposed "require just k out
> >>>> these n required keys" and how it should be implemented, but it will
> >>>> probably involve teaching this tool a bunch of new options. These
> >>>> patches are not necessarily ready for inclusion (unless someone else
> >>>> finds fdt_add_pubkey useful as is), but I thought I might as well send
> >>>> it out for early comments.
> >>>
> >>> I'd also like to see the usage of this hooked into the build process.
> >>>
> >>> And to my understanding of [1], that approach will provide a feature that permits hooking with the build but would expect the key as dtsi fragment. Can we consolidate the approaches?
> >>>
> >>> My current vision of a user interface would be a Kconfig option that takes a list of key files to be injected. Maybe make that three lists, one for "required=image", one for "required=conf", and one for optional keys (if that has a use case in practice, no idea).
> >>>
> >>> Jan
> >>>
> >>> [1]
> >>> https://lore.kernel.org/u-boot/20210928085651.619892-1-rasmus.villemoes@prevas.dk/
> >>>
> >>> --
> >>> Siemens AG, T RDA IOT
> >>> Corporate Competence Center Embedded Linux
> >>>
> >>
> >> For what would you like to have code? The kconfig addition?
> >>
> >> diff --git a/common/Kconfig.boot b/common/Kconfig.boot
> >> index d3a12be228..a9ed4d4ec4 100644
> >> --- a/common/Kconfig.boot
> >> +++ b/common/Kconfig.boot
> >> @@ -279,6 +279,14 @@ config SPL_FIT_GENERATOR
> >>
> >>  endif # SPL
> >>
> >> +config FIT_SIGNATURE_PUB_KEYS
> >> +       string "Public keys to use for FIT image verification"
> >> +       depends on FIT_SIGNATURE || SPL_FIT_SIGNATURE
> >> +       help
> >> +         Public keys, or certificate files to extract them from, that shall
> >> +         be used to verify signed FIT images. The keys will be embedded into
> >> +         the control device tree of U-Boot.
> >> +
> >>  endif # FIT
> >>
> >>  config LEGACY_IMAGE_FORMAT
> >>
> >>
> >> But note that we are in a design discussion here, and I'm at least
> >> reluctant to code up n-versions without having some common idea where
> >> things should move.
> >
> > I'm not sure we want this built into U-Boot. I see signing of a
> > firmware image as a final step, with the keys being added then, e.g.
> > by binman.
>
> This is not signing, this in embedding public key information into build
> artifacts before they can be signed. As pointed out in my other thread,
> not having an embedding feature is a major drawback of the current
> workflow. It easily forces you to rebuild existing build flows in
> out-of-tree scripts.

The public key is not needed for signing to work, right? I don't
understand what you are getting at here. If you want to add the public
key to the image before it is signed, that's fine. I just don't
understand why you want to do that. Why not have the signer do
everything?

Regards,
Simon