[edk2] Obtaining TCG final events on systems without TCG2 log support

Matthew Garrett mjg59 at srcf.ucam.org
Thu Dec 13 10:51:36 PST 2018


I don't see how that follows - regardless of whether or not we'd like to 
deprecate SHA1 support, people use it. There's little value in having an 
incomplete event log.

On Thu, Dec 13, 2018 at 01:23:35PM +0000, Yao, Jiewen wrote:
> Right.
> I think we are trying to deprecate the old SHA1 support, because SHA1 is considered as unsecure algorithm.
> We are moving to crypto agile. As such, we do not see the need to support old style event log.
> 
> Thank you
> Yao Jiewen
> 
> 
> > -----Original Message-----
> > From: Laszlo Ersek [mailto:lersek at redhat.com]
> > Sent: Thursday, December 13, 2018 8:36 PM
> > To: Matthew Garrett <mjg59 at srcf.ucam.org>
> > Cc: edk2-devel at lists.01.org; Yao, Jiewen <jiewen.yao at intel.com>;
> > Marc-André Lureau <marcandre.lureau at redhat.com>; Stefan Berger
> > <stefanb at linux.vnet.ibm.com>
> > Subject: Re: [edk2] Obtaining TCG final events on systems without TCG2 log
> > support
> > 
> > + Jiewen, Marc-André, Stefan
> > 
> > On 12/13/18 02:17, Matthew Garrett wrote:
> > > SetupEventLog() in Tcg2Dxe.c only installs the final event log
> > > configuration table if SupportedEventLogs includes the TCG2 log format.
> > > If the platform only supports the TCG1.2 log format then the final
> > > events table isn't installed. However, ExitBootServices() should
> > > generate an event even on systems that don't support the TCG2 log
> > > format. How is an OS supposed to obtain the log of the
> > > ExitBootServices() events in that case?
> > >
> > 
> > I don't think it can.
> > 
> > You probably refer to the code below the comment "No need to handle
> > EFI_TCG2_EVENT_LOG_FORMAT_TCG_1_2", in SetupEventLog(). This code
> > dates
> > back to commit fd46e831bc33 ("SecurityPkg: Update final event log
> > calculation.", 2016-01-18). And the commit message says, "... there is
> > no need to record TCG12 format log to final event log area ...".
> > 
> > Hence, the code is intentional. I even think the code is valid
> > (according to the spec [*]); I just think the commit message should have
> > said, "there is no *way* to record TCG12 format log to final event log
> > area". Because, IMO, the bug is in the spec.
> > 
> > [*] TCG EFI Protocol Specification
> >     Family “2.0”
> >     Level 00 Revision 00.13
> >     March 30, 2016
> > 
> > Here's why I think it's a spec bug:
> > 
> > 
> > (1) If EFI_TCG2_EVENT_LOG_FORMAT_TCG_2 is *clear* in
> > SupportedEventLogs,
> > then the platform advertizes GetEventLog() as unable to produce the
> > crypto agile log format.
> > 
> > In other words, the platform is unable to produce a log which consists
> > of TCG_PCR_EVENT2 entries, beyond the sole TCG_PCR_EVENT ("SHA1
> > format")
> > header entry.
> > 
> > Accordingly, GetEventLog() will fail with EFI_INVALID_PARAMETER, when
> > called with EventLogFormat=EFI_TCG2_EVENT_LOG_FORMAT_TCG_2. (BTW,
> > I
> > think EFI_UNSUPPORTED would have been better for this, but I digress.)
> > 
> > (2) EFI_TCG2_FINAL_EVENTS_TABLE is defined with TCG_PCR_EVENT2
> > entries
> > *only*. TCG_PCR_EVENT is not accommodated.
> > 
> > 
> > That's the contradiction. If a platform is unable to produce
> > TCG_PCR_EVENT2 entries in GetEventLog(), it is fairly certainly also
> > unable to produce them in the final events table.
> > 
> > And, while the first *instance* of the limitation is conformant, via
> > SupportedEventLogs, the second instance of the same limitation isn't.
> > 
> > Thanks,
> > Laszlo
-- 
Matthew Garrett | mjg59 at srcf.ucam.org


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