[PATCH 0/2] RFC: add fdt_add_pubkey tool

Jan Kiszka jan.kiszka at siemens.com
Wed Nov 10 21:51:09 CET 2021

On 10.11.21 20:36, Simon Glass wrote:
> 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
>>>>>> +       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.

The public key is signed when it is placed into an image that is signed.
That is the case, e.g., when injecting it into the SPL control FDT and
then signing the SPL image. Or when injecting it into the main control
FDT and signing U-Boot proper afterwards.


Siemens AG, T RDA IOT
Corporate Competence Center Embedded Linux

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