Re: [PATCH] tpm: Require that all digests are present in TCG_PCR_EVENT2 structures

Ard Biesheuvel
2020-06-30T19:10:28Z

On Tue, 30 Jun 2020 at 20:53, Peter Jones <pjones@redhat.com> wrote:
>
> 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@linux.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@linux.microsoft.com>
> > > ---
> >
> > 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.
> Same with the zero-supported-hashes case. It would be nice to at log it
> and have a way for them to opt-in to allowing the old measurement to go
> through, so they can recover their data, though I don't know what that
> method would be if the measured command line is one of their
> dependencies.
>

Maybe use a EFI variable?