linux-kernel - Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch early measurements

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Date:   Fri, 12 May 2023 20:12:03 +0100
From:   Matthew Garrett <mjg59@...f.ucam.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Eric Biggers <ebiggers@...nel.org>,
        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,
        dpsmith@...rtussolutions.com, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, James.Bottomley@...senpartnership.com,
        luto@...capital.net, nivedita@...m.mit.edu,
        kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch
 early measurements

On Fri, May 12, 2023 at 08:17:21PM +0200, Thomas Gleixner wrote:
> On Fri, May 12 2023 at 17:13, Matthew Garrett wrote:
> > On Fri, May 12, 2023 at 03:24:04PM +0200, Thomas Gleixner wrote:
> >> On Fri, May 12 2023 at 12:28, Matthew Garrett wrote:
> >> > Unless we assert that SHA-1 events are unsupported, it seems a bit odd 
> >> > to force a policy on people who have both banks enabled. People with 
> >> > mixed fleets are potentially going to be dealing with SHA-1 measurements 
> >> > for a while yet, and while there's obviously a security benefit in using 
> >> > SHA-2 instead it'd be irritating to have to maintain two attestation 
> >> > policies.
> >> 
> >> Why?
> >> 
> >> If you have a mixed fleet then it's not too much asked to provide two
> >> data sets. On a TPM2 system you can enforce SHA-2 and only fallback to
> >> SHA-1 on TPM 1.2 hardware. No?
> >
> > No, beause having TPM2 hardware doesn't guarantee that your firmware 
> > enables SHA-2 (which also means this is something that could change with 
> > firmware updates, which means that refusing to support SHA-1 if the 
> > SHA-2 banks are enabled could result in an entirely different policy 
> > being required (and plausibly one that isn't implemented in their 
> > existing tooling)
> 
> It's not rocket science to have both variants supported in tooling,
> really.

People who are currently using tboot are only getting SHA-1, so there's 
no obvious reason for them to have added support yet. *My* tooling all 
supports SHA-2 so I'm completely fine here, but either we refuse to 
support a bunch of hardware or we have to support SHA-1 anyway, and if 
we have to support it the only reason not to implement it even in the 
"SHA-2 is supported" case is because we have opinions about how other 
people implement their security.