Re: [tboot-devel] Dynamic Root of Trust

[Cihula, J. <jos...@in...>]

> From: Atanas Filyanov [mailto:ata...@go...]
> Sent: Wednesday, March 25, 2009 1:52 PM
>
> Hi all,
>
> I'm currently doing some experiments with dynamic root of trust. From
> the tboot boot log I can see that the SENTER instruction is executed and
> the PCRs 17 and above are set to 0 and that PCRs 17 and 18 are extended.
> My question, if somebody could help me, is how to set PCR 17 or any
> other PCR to 0 from the running system and if I understand correctly the
> PCR value should change if I boot another XEN domain and should change
> back to the original value if I shut it down? Or am I mistaken?
> I'd appriciate any help.
>
> Best,
>   Atanas

The dynamic PCRs (16-23) are only resettable by the establishment of a hardware root of trust (e.g. GETSEC[SENTER]).  Xen uses TXT via the tboot module that performs SENTER at boot time.  The measurements for TXT are those of tboot, Xen, and dom0.  So non-dom0 domains are not measured as part of the current implementation.  Because the SENTER is performed at boot time, it will require a hard or soft reboot to re-execute tboot and the SENTER instruction.

Non- tboot or Xen uses of TXT could invoke SENTER multiple times within a single boot (after performing SEXIT) and the PCRs will be reset each time.

Joe