tboot 1.8.0 and UEFI

Version 1.8.0 of tboot was released a while back. This is a pretty big deal as the EFI support has been a long time coming. Anyone wanting to use tboot on a modern piece of hardware using EFI has been out of luck till now.

For the past week or so I’ve been slowly figuring out how to build an OE image with grub-efi, building the new version of tboot and then debugging an upgrade in meta-measured. My idea of a good time for sure.

As always the debugging was the hardest part, building the software was easy. For the most part tboot EFI “just worked” … after I figured out all the problems with kernel version and grub configuration. Hard parts were

  • realizing the Linux kernel image had to be the latest 3.14 version
  • debugging new kernel version
  • configuring grub
  • which modules needed to be built into grub

If you want the details you can see the full history on the meta-meausred github. The highlights are pretty simple:

multiboot2 in oe-core grub-efi

The grub-efi recipe in oe-core is a bit rigid. I’ve pushed a patch upstream that allows another layer (like meta-measured) to modify which grub modules are built into the grub EFI executable. It’s a tiny change but it makes all of the difference:

http://lists.openembedded.org/pipermail/openembedded-core/2014-April/091768.html

This lets us add modules to the grub EFI executable. I also had to cobble together a working grub multiboot2 configuration.

linux-yocto v3.14

Pairing this with the older 3.10 Yocto Linux kernel image will allow you to get through grub and tboot but the kernel will panic very early in the boot process. The newer 3.14 doesn’t suffer from this limitation.

The measured reference image in meta-measured used aufs to keep from having to mount the rootfs read/write. This is to keep the rootfs hash from changing across boots. I wrote the whole thing up a while back: http://twobit.us/blog/2013/01/meta-measured/. Anyways aufs doesn’t work in 3.14 so I took the extra few minutes to migrate the image to use the read-only-rootfs IMAGE_FEATURE. This is a good thing regardless, aufs was being used as a shortcut. I hadn’t had the drive to fix this till it broke. Problem solved.

rough edges

I still haven’t figured out all of the details in grub and it’s configuration. The current configuration in meta-measured is sufficient to boot but something gets screwed up in setting up VGA output for tboot and the early kernel output. Currently grub displays an error message indicating that tboot won’t get a console and no VGA output will be shown till the kernel loads the DRM driver. Output is still available on the serial console so if you’ve got a reasonable test setup you can get all the data you need for debugging.

No lies, I’m a bit afraid of grub, guess I’ll have to get over it. The measured-image-bootimg has a menuentry for tboot and a normal linux boot. Booting the kernel using the linux and initrd grub commands provide normal VGA output but the multiboot2 config required by tboot does not. I take this to mean that grub is capable of doing all of the necessary VGA stuff but that it can’t pass this data through to tboot via multiboot2. More to come on this soon hopefully.

Till then, if you build this stuff and have feedback leave it here.

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