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

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Message-ID: <20200617230958.GC62794@linux.intel.com>
Date:   Thu, 18 Jun 2020 02:09:58 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Peter Jones <pjones@...hat.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 16, 2020 at 11:08:38AM +0200, Ard Biesheuvel wrote:
> (cc Matthew and Peter)
> 
> On Tue, 16 Jun 2020 at 01:28, Tyler Hicks <tyhicks@...ux.microsoft.com> wrote:
> >
> > Require that the TCG_PCR_EVENT2.digests.count value strictly matches the
> > value of TCG_EfiSpecIdEvent.numberOfAlgorithms in the event field of the
> > TCG_PCClientPCREvent event log header. Also require that
> > TCG_EfiSpecIdEvent.numberOfAlgorithms is non-zero.
> >
> > The TCG PC Client Platform Firmware Profile Specification section 9.1
> > (Family "2.0", Level 00 Revision 1.04) states:
> >
> >  For each Hash algorithm enumerated in the TCG_PCClientPCREvent entry,
> >  there SHALL be a corresponding digest in all TCG_PCR_EVENT2 structures.
> >  Note: This includes EV_NO_ACTION events which do not extend the PCR.
> >
> > Section 9.4.5.1 provides this description of
> > TCG_EfiSpecIdEvent.numberOfAlgorithms:
> >
> >  The number of Hash algorithms in the digestSizes field. This field MUST
> >  be set to a value of 0x01 or greater.
> >
> > Enforce these restrictions, as required by the above specification, in
> > order to better identify and ignore invalid sequences of bytes at the
> > end of an otherwise valid TPM2 event log. Firmware doesn't always have
> > the means necessary to inform the kernel of the actual event log size so
> > the kernel's event log parsing code should be stringent when parsing the
> > event log for resiliency against firmware bugs. This is true, for
> > example, when firmware passes the event log to the kernel via a reserved
> > memory region described in device tree.
> >
> 
> When does this happen? Do we have code in mainline that does this?
> 
> > Prior to this patch, a single bit set in the offset corresponding to
> > either the TCG_PCR_EVENT2.eventType or TCG_PCR_EVENT2.eventSize fields,
> > after the last valid event log entry, could confuse the parser into
> > thinking that an additional entry is present in the event log. This
> > patch raises the bar on how difficult it is for stale memory to confuse
> > the kernel's event log parser but there's still a reliance on firmware
> > to properly initialize the remainder of the memory region reserved for
> > the event log as the parser cannot be expected to detect a stale but
> > otherwise properly formatted firmware event log entry.
> >
> > Fixes: fd5c78694f3f ("tpm: fix handling of the TPM 2.0 event logs")
> > Signed-off-by: Tyler Hicks <tyhicks@...ux.microsoft.com>
> > ---
> 
> I am all for stringent checks, but this could potentially break
> measured boot on systems that are working fine today, right?

There would not be any sane reason to implement a TPM chip like that.

/Jarkko