Re: [RFC PATCH v2] x86/boot: add .sbat section to the bzImage

Luca Boccassi
2023-07-21T13:10:56Z

On Fri, 21 Jul 2023 at 14:01, James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Fri, 2023-07-21 at 13:40 +0100, Luca Boccassi wrote:
> > On Fri, 21 Jul 2023 at 12:24, James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:
> > >
> > > On Fri, 2023-07-21 at 09:55 +0100, Luca Boccassi wrote:
> > > > On Fri, 21 Jul 2023 at 02:49, Eric Snowberg
> > > > <eric.snowberg@oracle.com> wrote:
> > > > > > On Jul 20, 2023, at 1:16 PM, Luca Boccassi <bluca@debian.org>
> > > > > > wrote:
> > > > > > On Thu, 20 Jul 2023 at 18:11, Eric Snowberg
> > > > > > <eric.snowberg@oracle.com> wrote:
> > > [...]
> > > > > > > I agree with James in the previous thread; adding the SBAT
> > > > > > > section to the kernel should be handled by the signing
> > > > > > > tools. It really doesn't need to be included in the
> > > > > > > mainline kernel code. I also agree with the sentiment that
> > > > > > > mainline and the
> > > > > > > stable branches should not have SBAT versions attached
> > > > > > > to them. These are things distros should be responsible for
> > > > > > > including in their kernel if they want to have SBAT
> > > > > > > support.
> > > > > >
> > > > > > Why would 'signing tools' handle that? It's just a text-based
> > > > > > PE section, it doesn't require access to private key
> > > > > > materials to be handled, nor it has any relationship with
> > > > > > signing.
> > > > >
> > > > > There is a relationship, the sbat information within the signed
> > > > > file can be used for revocation in lieu of revoking the hash or
> > > > > signing certificate at a later time.
> > > >
> > > > No, it is completely disjoint. In fact, the kernel doesn't even
> > > > have to be signed at all, but it still _must_ have a .sbat
> > > > section when it is used in a UKI.
> > >
> > > Just a minute, this is wrong. I was talking to Peter after all of
> > > this blew up about how we handle signed kernels with no sbat (since
> > > we need that still to work for developers who sign their own
> > > kernels). I thought he was planning to require an sbat section for
> > > all EFI binaries, but he says that's not true. The current way
> > > shim does the sbat check is that if the section doesn't exist the
> > > binary is processed as having an empty sbat section (i.e. no sbat
> > > level checking will be done because there's no named sbat level for
> > > anything and it will just work) and they're planning to keep it
> > > that way so that a signed but no sbat kernel will always "just
> > > work" without any special key handling in shim. So if we're
> > > planning to keep this no-sbat case in discrete kernels, even when
> > > the shim verifier checks sbat, the UKI kernel will need to work for
> > > this case as well.
> >
> > Are you sure that's not just about local signing?
>
> Well, my job is to be concerned about how individuals who want to own
> their own keys, either in MoK or db, participate in this, so I am
> mostly thinking about local signing. Whatever we decide, there must be
> a local workflow pathway.

Sure but for local signing via MoK that's obviously fine, as one gets
to keep the pieces. AFAIK it's a different flow in Shim whether
something is authorized by MoK, DB or the built-in cert, so having
different policies built-in for those different cases should be
doable. Actually at the moment even if Shim loads the image, if it
gets authorized by DB .sbat isn't checked at all.

> > IE, MoK vs embedded cert auth flow? As far as I know, the plan for
> > the 3rd party CA flow is to eventually (very eventually) require it.
> > I might have missed some development ofc.
>
> There is a thought to get sbat adopted by UEFI to solve the dbx
> problem, but if that were to happen, UEFI will likely be extremely
> concerned about backward compatibility (and as you have remarked, they
> and the OEMs adopt at a glacial pace), so, even if they eventually
> adopt it, I can't foresee them mandating refusing to execute signed EFI
> binaries with no sbat. I'll pretty much stake cash on the compromise
> being that for the foreseeable future no sbat gets revoked by dbx and
> the plan will be a gradual shift towards sbat ... but all this is
> contingent on UEFI adoption in the first place, which isn't a given.
> There are also unsolved problems around sbat, like how the master sbat
> lists are kept and how they're delivered which must be solved before a
> UEFI proposal could be made.

I meant Shim + 3rd party CA flow here. UEFI SB 2.0 is so far ahead I'm
pretty sure a good chunk of the folks currently talking about it will
be retired by the time it actually exist ;-)