AW: secure boot, mkimage with external signing server

[Rasmus Villemoes]

On Wed, Jan 22 2025, "Rosenschild, Klaus" <Klaus.Rosenschild at hilti.com> wrote:

> Hi Rasmus,
> thank you for pointing to this solution.
> I think this is the best way to do this.
>
> However, our signing server is very well protected and making changes there is a long and complex process.
> Right now, it only provides the following two functions:
>
>   1.
> generation of a signature of a sha256 hash using the private key
>   2.
> providing the public key, the pure key, not the certificate
>
> I found a workaround to determine the hash that mkimage uses to create the signature of the configuration without re-implementing the internal algorithm that mkimage uses:
>
>   1.
> create a temporary rsa private and public key
>   2.
> run mkimage to create a FIT image with signature: mkimage -k keys-f fitImage-sign-orig.its -r fitImage-sign
>   3.
> extract the signature from the FIT image
>   4.
> re-generate the hash from the signature and the public key: openssl pkeyutl -verifyrecover -in signFile.hash.sign.bin -pubin -inkey ../keys/build.pub -asn1parse
>   5.
> now, I can send the hash to the signing server, get the correct signature back and re-enter it into the FIT image (e.g. via python libfdt)
>

Urgh, it shouldn't be that complicated, and I would consider it quite
reasonable if mkimage could be instructed to emit the hash it actually
signs along with the signature.

But, I do think you should be able to create a pkcs#11 module which
simply takes that sha256 as input from the higher layers and does
whatever it needs to do to talk to the server, getting the signature
back.

> However, there is now another problem. I also need to put the public key into the device tree file. So, I have to run a slightly different mkimage command (with  -K option):
> mkimage -k keys -f fitImage-sign-orig.its -r -K bcm2711-rpi-4-b.dtb fitImage-sign
>
> However, the -K options requires a certificate and not just the
> public.

Yeah, that is a fundamental design flaw of mkimage; one shouldn't need
to sign any image in order to get the public key data embedded in
u-boot's control dtb.

Fortunately, you can ignore what most tutorials tell you about that -K
option. There is a simple script in the u-boot repo, tools/key2dtsi.py,
which you can apply to just the public key, and you get a .dtsi fragment
that you can include when you build u-boot's control dtb. Either you can
use the CONFIG_DEVICE_TREE_INCLUDES mechanism for making sure that .dtsi
gets picked up during build, or you can simply copy-paste the contents
into your board's .dts file.

Rasmus