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

Laszlo Ersek lersek at redhat.com
Thu Dec 13 04:36:09 PST 2018


+ 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


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