On Tue, Feb 19, 2019 at 9:31 AM Jeridiah.Welti@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@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