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

From: Michael Chang
Subject: Re: Discuss support for the linux kernel's EFI Handover Protocol on x86 and ARM
Date: Mon, 14 Jan 2019 15:30:00 +0800
User-agent: Mutt/1.10.1 (2018-07-13)

On Fri, Jan 11, 2019 at 03:17:54PM +0100, Ard Biesheuvel wrote:
> 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.

>From me it means 'maybe' we have to consider common linuxefi loader for
ARM and x86 architectures to boot in UEFI Secure Boot with shim-lock
protocol. It doesn't mean switching over from linux to linuxefi
completely, just offering it as another boot command (like linux16 for
legacy pc bios), and let the distribution choose what to do.

> > 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.

By far we don't know what UEFI Secure Boot support in ARM will be like.
There is rumor that Microsoft will also host signing service for ARM
secure boot, so the situation is simialr to the beginning of x86, and is
reasonle to relate it to shim since it was requested to satisfy that.

> > > 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.

I agree that technically it results better and clean boot stack. The
challege is on that do we consider to host central authority responsible
for the key signing and code review in lieu of vendor? Or do we agree to
trust whatever key giving out to the vendor? For x86, I think currently
microsoft takes the responsiblity to code review and authenicate the
identity of key owner and that costs a lot effort.

> 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.

The point of having MOK is that once anything goes wrong with grub, we
can just revoke MOK and we don't need to walk through the nightmare of
revoking firmware's key. 

IMHO we also need to think about misc shim features can be moved to
grub2 if necessary.

> 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.

OK, that seems to answer my question above. And again I think what's
missing for current grub is efi handover protocol support, which doesn't
conflict with existing LoadImage boot entry. (we run in circle again).

> 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.

Yes, same here. :) That's why in openSUSE we provided option to disable
shim installation and use pristine grub2-install, of course in this case
users are on their own when things are not working.


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