[PATCH 0/2] RFC: add fdt_add_pubkey tool

[Simon Glass]

Hi Jan,

On Wed, 10 Nov 2021 at 09:48, Jan Kiszka <jan.kiszka at siemens.com> wrote:
>
> On 10.11.21 17:31, Simon Glass wrote:
> > Hi Jan,
> >
> > On Tue, 9 Nov 2021 at 23:44, Jan Kiszka <jan.kiszka at 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 at 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 at siemens.com>
> >>>>> Sent: Tuesday, November 9, 2021 12:17 PM
> >>>>> To: Roman Kopytin <Roman.Kopytin at kaspersky.com>; u-boot at lists.denx.de; Rasmus Villemoes <rasmus.villemoes at 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?
>
> A) Because sensitive signing environments will not run arbitrary logic.
>    They will hand out the public key, but they may not give you the
>    chance to run mkimage with the private key, like you would do during
>    development.

That's OK, so long as there is a way to get the data to be signed in,
and the public key and signature out.

>
> B) It avoids having to run the signing process in a specific order
>    because it already embeds the public key during build, thus
>    generates everything that shall be signed upfront.

The public key is not signed though. Whether it is present at the
start or not is not important.

Regards,
Simon