linux-kernel - Re: [PATCH] tpm: Require that all digests are present in TCG_PCR

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Message-ID: <20200630194855.mfctxgveltvphgnc@redhat.com>
Date:   Tue, 30 Jun 2020 15:48:56 -0400
From:   Peter Jones <pjones@...hat.com>
To:     Tyler Hicks <tyhicks@...ux.microsoft.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Petr Vandrovec <petr@...are.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Thirupathaiah Annapureddy <thiruan@...rosoft.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: Require that all digests are present in
 TCG_PCR_EVENT2 structures

On Tue, Jun 30, 2020 at 02:23:22PM -0500, Tyler Hicks wrote:
> > > I am all for stringent checks, but this could potentially break
> > > measured boot on systems that are working fine today, right?
> > 
> > Seems like in that case our measurement is unreliable and can't really
> > be trusted.  That said, having things that were using the measurements
> > before this suddenly stop being able to access sealed secrets is not a
> > great experience for the user who unwittingly bought the junk hardware.
> 
> I haven't seen where anyone has suggested that such junk hardware
> exists. Do you know of or expect any firmware that has mismatched
> TCG_PCR_EVENT2.digests.count and TCG_EfiSpecIdEvent.numberOfAlgorithms
> values?

If nobody has seen any hardware that actually produces the values you're
excluding, then I don't have a strong objection.
 
> I would think that the userspace code that's parsing
> /sys/kernel/security/tpm0/binary_bios_measurements would also have
> issues with such an event log.
> 
> > Same with the zero-supported-hashes case.
> 
> Small but important correction: it is a zero-hashes case, not a
> zero-supported-hashes case
> 
> There's no handshake involved or anything like that. This would only
> cause problems if the firmware provided no hashes, which means the
> firmware event log is unusable, anyways.

Indeed.

-- 
        Peter