[U-Boot] Extract RSA Keys from image

[Douglas Zobel]

On Tue, Feb 19, 2019 at 9:31 AM <Jeridiah.Welti at bench.com> wrote:

> I am working on an application needing the ability to update to a verified
> image from the running kernel/application.
>
> We can follow the "normal" verified image boot sequence, where the chain
> of trust is verified from U-Boot to image to execution, etc, but unsure how
> to verify a new image after already running.
>
> Is there a way to extract the public key hash from the U-Boot image so
> that we can compute a hash on an upgrade image and verify a match?  Either
> an existing tool, or some means that is accessibly from a Linux kernel that
> we could use to grab this information.
>
> I've done a lot of googling, and I have not seen any means to get to this
> once the image is already booted and running.
>
> Thank you for any guidance you can provide for this.
>
> Jeridiah Welti
> _______________________________________________
> U-Boot mailing list
> U-Boot at lists.denx.de
> https://lists.denx.de/listinfo/u-boot


I'll take a stab at answering this since I recently went through
implementing verified boot.  The public key data used to verify the boot
images isn't passed on for use inside those images.  However it would be
possible to get them.  The public key is usually stored in the device tree
of the object doing the verification (SPL or u-boot).  This device tree is
stored as an object inside the FIT boot image.  The FIT boot image is
simply another device tree which contains configuration, device trees and
binary executables.

Within a running Linux image, you could read the u-boot FIT image (or SPL
FIT image if CONFIG_SPL_FIT_SIGNATURE is used) from wherever it's stored.
Use dtc tools to find the traverse to the /images/fdt-1/data object.  This
data object is the device tree containing the public key that u-boot used
to verify the kernel (or in case of SPL, this is the public key used to
verify u-boot).  Use dtc tools again to decode the device tree blob and
traverse to /signature/[signature name].  This node will have your public
key in the format:
        key-somekeyname {
                required = "image";
                algo = "sha256,rsa2048";
                rsa,r-squared = <0x1874a2f....>;
                rsa,modulus = <0x4a7b31eb....>;
                rsa,exponent = <0x0 0x10001>;
                rsa,n0-inverse = <0x93a4cd16>;
                rsa,num-bits = <0x800>;
                key-name-hint = "somekeyname";
        };

-Doug

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