linux-kernel - Re: [PATCH v6 04/14] x86: Secure Launch Resource Table header file

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Message-ID: <20230515212206.GA2162@srcf.ucam.org>
Date:   Mon, 15 May 2023 22:22:06 +0100
From:   Matthew Garrett <mjg59@...f.ucam.org>
To:     "Daniel P. Smith" <dpsmith@...rtussolutions.com>
Cc:     Ross Philipson <ross.philipson@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux-foundation.org,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        ardb@...nel.org, James.Bottomley@...senpartnership.com,
        luto@...capital.net, nivedita@...m.mit.edu,
        kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v6 04/14] x86: Secure Launch Resource Table header file

On Mon, May 15, 2023 at 05:15:15PM -0400, Daniel P. Smith wrote:
> On 5/12/23 06:55, Matthew Garrett wrote:
> > On Thu, May 04, 2023 at 02:50:13PM +0000, Ross Philipson wrote:
> > 
> > > +#define SLR_TABLE_MAGIC		0x4452544d
> > 
> >  From convention I'd expect this to be 0x534c5254, but not really an
> > issue.
> 
> Apologies, but which convention?

Tables in ACPI and UEFI tend to have magic that corresponds to their 
name, so a table called SLRT would tend to have magic that matches the 
ASCII values for that. In this case the SLRT has DRTM as its magic, 
which is a touch unexpected.

> > Oof. Having the kernel know about bootloaders has not worked out super
> > well for us in the past. If someone writes a new bootloader, are they
> > unable to Secure Launch any existing kernels? The pragmatic thing for
> > them to do would be to just pretend they're grub, which kind of defeats
> > the point of having this definition...
> 
> Actually, this is not for making the kernel know about bootloaders. This is
> dealing with the challenge created when the preamble was split for efi-stub,
> and similar use cases, where what sets up the preamble, ie. the bootloader,
> is separate from what invokes the dynamic launch, ie. the DLE handler. The
> reality is that even in the simplest implementation of the DLE handler, a
> remnant of GRUB for call back from efi-stub, there is information that is
> needed to cross the gap.

What if I don't use grub, but use something that behaves equivalently? 
Which value should be used here?

> We wrote the TrenchBoot Secure Launch general spec [1] with as much
> forethought as possible for the target environments. Specifically, the
> desire is to have a common approach for x86 (Intel and AMD), Arm, and
> perhaps down the road the POWER arch. In particular, I do not believe there
> is anything in the Arm DRTM beta spec that prohibits a mixed 32/64 bit
> environment. In the end it is better to for the spec to be safe for those
> environments then having to make changes to the spec later down the road.

Ok.