linux-kernel - Re: [PATCH v2] x86/xen/efi: Initialize UEFI secure boot state during dom0 boot

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Date:   Wed, 4 Apr 2018 12:38:24 +0200
From:   Daniel Kiper <daniel.kiper@...cle.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, xen-devel@...ts.xenproject.org,
        ard.biesheuvel@...aro.org, boris.ostrovsky@...cle.com,
        eric.snowberg@...cle.com, hpa@...or.com, jgross@...e.com,
        konrad.wilk@...cle.com, mingo@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH v2] x86/xen/efi: Initialize UEFI secure boot state during
 dom0 boot

On Tue, Apr 03, 2018 at 10:00:52AM -0700, James Bottomley wrote:
> On Tue, 2018-04-03 at 18:07 +0200, Daniel Kiper wrote:
> > On Tue, Apr 03, 2018 at 08:44:41AM -0700, James Bottomley wrote:

[...]

> > > This looks like a bad idea: you're duplicating the secure boot
> > > check in
> > >
> > > drivers/firmware/efi/libstub/secureboot.c
> > >
> > > Which is an implementation of policy.  If we have to have policy in
> > > the kernel, it should really only be in one place to prevent drift;
> > > why can't you simply use the libstub efi_get_secureboot() so we're
> > > not duplicating the implementation of policy?
> >
> > Well, here is the first version of this patch:
> > https://lkml.org/lkml/2018/1/9/496 Ard did not like it. I was not
> > happy too. In general both approaches are not perfect. More you can
> > find in the discussion around this patchset. If you have better idea
> > how to do that I am happy to implement it.
>
> One way might be simply to have the pre exit-boot-services code lay
> down a variable containing the state which you pick up, rather than you

Do you mean variable in kernel proper or something like that? If yes this
is not possible. EFI Linux stub is not executed in Xen dom0. All UEFI
infrastructure is owned and operated by Xen. Dom0 kernel can access some
stuff in UEFI, including variables, via hypercall. However, when dom0
runs only UEFI runtime services are available.

> calling efi code separately and trying to use the insecure RT

I am not sure why they are insecure.

> variables.  That way there's a uniform view of the internal kernel
> secure boot state that everyone can use.

That would be perfect but I have a feeling that in form proposed above
it is not possible.

Daniel