Re: shim 16 breaking systemd stub and next steps

[Lennart Poettering <lennart@xxxxxxxxxxxxxx>]

On Do, 20.03.25 09:38, Mate Kukri (mate.kukri@xxxxxxxxxxxxx) wrote:

> Hello,
>
> A new version of the rhboot secure boot shim was released yesterday
> https://github.com/rhboot/shim/releases/tag/16.0.
>
> This version contains an implementation of the
> LoadImage/StartImage/Exit/etc API set, which is exposed both via
> SystemTable hooks, and a new protocol called the shim loader protocol.
> This allows second stage bootloaders to load and execute shim signed
> PE binaries the same way as ones signed by firmware keys.
>
> Unfortunately this also means that systemd-stub will no longer be able
> to load its embedded kernel due to relying on overriding the non-UEFI
> standard SECURITY2_ARCH_PROTOCOL to avoid verification which the shim
> LoadImage implementation of course does not consult.
>
> I really hope the solution to this won't be another copy pasted PE
> loader inside the stub (as one of the big goals of the loader protocol
> work was to avoid the multiplication of PE loaders...)
> One possible solution is to add a new API to shim to allow loading
> previously verified images such as the embedded kernel without further
> verification.
>
> I am looking to hear your thoughts on how to fix this issue.

Alex Graf suggested to me that we really should not go through UEFI at
all for jumping into the inner image, already. The stuff we currently
do is just too brittle: we use BS->LoadImage() and then patch around
the protocols to turn off the security stuff around it, because we
don't want another authentication, and another measurement. It's
horrible if you ask me.

We have the inner image in memory anyway, we might as well jump
directly into it, bypassing BS->LoadImage(). it doesn't need to know
about how we arrange our images. We do not want the stuff it provides
to us mostly anyway.

I think that would both result in a more robust system (no more
interfering with firmware PE loader mechanisms), and would address
this shim mess too.

There is prior art around. Most importantly this:

https://github.com/u-boot/u-boot/blob/master/lib/efi_loader/efi_image_loader.c

It was written 10y ago by Alex. We'd need a dumbed down version of that,
i.e. without the Authenticode/pkcs7 stuff. And our inner image is
already in memory, hence we don't need that either.

We already have PE parsers in our EFI code, we use it to find the UKI
sections after all, hence this wouldn't even add completely new PE
logic to our tree. It would just have to do the section relocation,
and then jump to the PE entrypoint of the inner image.

Would love a patch for that!

Lennart

--
Lennart Poettering, Berlin