Verified Boot through U-Boot for RPi3 with OP-TEE

[thomas.gymer at bt.com]

Hello all,

We (Eden and I) have been assigned a summer project exploring the possibility of verified boot for Linux with a view to experiment with a TEE, and would like to share our outcomes from the project so far.

We've been working on condensing all the information and documentation on TF-A, OP-TEE, and U-Boot verified boot into a trivially workable form, a Makefile, for the Raspberry Pi 3B+. Our aim is a proof of concept pilot project to see what parts of the technology outlined in the NIST standards for secure boot on IoT is currently available, and thus what needs to be developed. This also leads us to an interest in full crypto algorithm agility to ensure quick adoption of the quantum safe algorithms as they come out.
I should note that our boot order here is TF-A to OP-TEE, then TF-A to U-Boot to Linux.
This has raised a few areas of note and a couple of issues on our end:

# Areas of note
- U-Boot and Linux DTB's being incompatible has minimal documentation
Newer versions of U-Boot do have `CONFIG_OF_UPSTREAM`, but I can't see that as a recommended solution. Perhaps a note in the Kconfig entry for `CONFIG_OF_CONTROL` would be sufficient
- U-Boot can fail silently if the DTB used is incompatible
No idea what the solution here could be; an invalid DTB inevitably means that the firmware cannot access any of the hardware correctly.
- U-Boot unable to find `CONFIG_OF_SEPARATE` DTB's when loaded by TF-A; this is a TF-A problem, but maybe check other possible places for a DTB on the U-Boot end?
- U-Boot itself appears to have good crypto agility (or will in the near future), with a macro and existing algorithms as examples for the addition of new algorithms
- `mkimage` appears to be lacking these same features for crypto agility, which would otherwise set U-Boot up to be ready for the new quantum safe algorithms' release
A similar macro system for `mkimage`, or some other schema for defining how to add algorithm keys to the FDT, would provide some amazing flexibility, already being able to define more than one signature as required. Adding keys to the FDT would need to be standardised in someway for this, I would imagine.

I am very aware that some of the below issues are not relevant to all projects, but thought it best to maintain a single list to ensure that the correct place to make a fix for each issue is not locked project wise too early.
# Issues
- buildroot build fails when the directory length is greater than ~80 characters
- TF-A does not pass on the DTB address unless `RPI3_DIRECT_LINUX_BOOT` is set, which also implies a define U-Boot doesn't need
- TF-A `LOG_LEVEL=50` flag has compatibility issue in xlat tables v2 when compiled for 64bit
- `mkimage` ECDSA key adding to FDT fails if there is no signature node pre-added and doesn't set `required` node when `-r` is used
- `fit_check_sig`fails to check an ECDSA signed FIT file with corresponding FDT due to `<null>` hash node
- `mkimage` expects ECDSA private key only in a `.pem` and not `.der`, which would be standard for a single private key

We intend to release the Makefile and accompanying scripts for this on Monday 12th August, likely ~5pm BST to give us time to finalise what we have currently; this will consist of two Makefiles: one for verified boot using `CONFIG_OF_EMBED`, and another using `CONFIG_OF_SEPARATE` on U-Boot v2024.07. We are currently planning on releasing this on GitHub under the same license as the involved projects, but what would be the most helpful way for us to release this to you?
We have, beside the points raised in the Issues section above, no specific changes for the exiting repos, focusing instead on Makefiles which pull together parts of the existing releases to produce a working PoC as quickly as possible. We intend to also release this to the Raspberry Pi community as a TEE solution they can experiment with today.

This email has been sent to:
- u-boot at lists.denx.de
- op-tee at lists.trustedfirmware.org
and the Makefiles will be sent to Raspberry Pi Foundation and community some point next week

Thomas Gymer (thomas.gymer at bt.com) & Eden Hamilton (eden.hamilton at bt.com)