Measured boot involves generating cryptographic measurements of boot
components and configuration and using that to either control access to
a local secret (in the case of sealing secrets to a TPM) or proving to
another device (eg, a remote server or a local phone) what was booted.
We're shipping most of the infrastructure to do this, but we're still
left with a pretty fundamental problem - we need to know what the
expected values are in order to know whether something's been tampered
with or not.
For many components this isn't a huge problem (we build and distribute
the files - users can extract them and calculate the appropriate
measurements, and maybe long term we'll be able to ship the measurements
in a digestable way), but our initramfs images are generated on the user
system and include system-specific data. This makes it impractical to
know the expected measurements in advance.
I've been thinking about ways to solve this for a while, and I'm coming
to the conclusion that the best plan is probably to just ship pre-built
initramfs images. I can think of three main reasons to want to use
system-specific images:
1) They're smaller. By default we're already generating a generic image
for rescue purposes, so disk space isn't the concern here - we're
largely looking at losing boot speed. As machines have got faster this
is probably not a huge deal.
2) They contain machine-specific configuration. Some of this can be
passed on the kernel command line instead (eg, the machine ID), but we'd
need answers for the rest. I can think of a couple of solutions:
a) Stick the config in UEFI variables. It's small enough that we
wouldn't run out.
b) Extend grub to read some config files and synthesise an initramfs
image for them. If we measure the paths that those images use then
we don't need to worry about the contents as long as the tools that
read the config can't be subverted via that configuration.
3) User customisation, such as including extra tooling. grub supports
loading multiple initramfs images. Packages that right now install stuff
in the initramfs could instead ship a prebuilt image that grub could
append to the main initramfs. This would allow for things like
overriding Plymouth themes, and we could ship the measurements for these
pre-built images in order to allow them to be validated.
Any thoughts on this?
--
Matthew Garrett | mj...@srcf.ucam.org
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