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Re: Discuss support for the linux kernel's EFI Handover Protocol on x86

From: Ard Biesheuvel
Subject: Re: Discuss support for the linux kernel's EFI Handover Protocol on x86 and ARM
Date: Fri, 11 Jan 2019 15:17:54 +0100

On Fri, 11 Jan 2019 at 11:58, Leif Lindholm <address@hidden> wrote:
> On Thu, Jan 10, 2019 at 09:59:38AM +0100, Alexander Graf wrote:
> > > Am 10.01.2019 um 09:12 schrieb Michael Chang <address@hidden>:
> > >
> > > Hi,
> > >
> > > With the advent of new verifier framework and shim lock protocol support
> > > to the grub's community, we are driving to the world of UEFI Secure
> > > Boot, well, almost ..
> > >
> > > There is a missing piece in the puzzle remaining, that is booting linux
> > > kernel via it's own EFI Handover Protocol's entry.

I don't understand what this means.

> Strictly speaking,
> > > the interface is not part of the UEFI Secure Boot, but we have to use it
> > > to avoid problem of using UEFI LoadImage Protocol, which will not work
> > > with shim and it's Machine Owner Key (MOK) as they are not part of
> > > firmware's KEK and db.
> >

The 'problem' of using the UEFI LoadImage protocol is the whole point
of secure boot. Shim and GRUB essentially bypasses UEFI secure boot
entirely, but in a controlled way.

> > So really dumb question here: What if we didn't use the MS key? What
> > if instead, we just provide a SUSE/openSUSE key and give customers
> > the ability to sign their own grub+Linux binaries?
> >
> > Then we would only need to lobby with platform vendors to include
> > our public key in the delivered Keystore in parallel and everything
> > would "just work".
> >
> > The only reason shim needs to provide its own key management is that
> > on most x86 systems, we (and customers) don't have control over the
> > keystore, right? We can just push to not have that problem on ARM.
> Sure. That's a valid (and I think Ard would say preferable) decision,
> and should "just work" with upstream GRUB. But that's for each distro
> to decide.
> > Am I missing anything?
> As I understand it, there was a concern with the wording in UEFI
> 2.(3?, 4?) that made it possible to interpret it such that only one key
> had to be supported.
> It all comes down to who wants to make sure the key is already in
> shipped systems..

I will repeat the same thing I have been saying since 2013: carrying
over Shim to other architectures is a mistake. We could have a simple
and clean secure boot architecture on arm64, where the firmware
authenticates GRUB, and GRUB calls LoadImage() which authenticates the
kernel against the firmware keys. All we need for that is to ensure
that the distros get their act together, and work with the industry to
get Redhat, Canonical and Suse keys into the KEK and/or db databases
by default.

Instead, we are having this discussion again, how we can circumvent
authentication checks so that GRUB can load what are essentially
unverified binaries (from the POV of the firmware), authenticated
against certificates that are kept in unauthenticated UEFI variables.
Canonical is even shipping a GRUB with cosmic and disco now that is
signed with their master key, and happily boots anything if shim is
not loaded, which makes it impossible to ever move to a model where
the canonical key is in the UEFI db rather than in the MOK database.

So I strongly suggest that you make things work without shim, relying
on a monolithic distro signed GRUB which authenticates against the
UEFI database only. Should the need ever arise, we can always
introduce shim at a later date.

In fact, if I were running a shrink wrapped distro and did not have to
rely on MS signed option ROMs, I wouldn't even want the MS key in my
UEFI db if all I want to run is SUSE.

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