Re: [GRUB PATCH RFC 00/18] i386: Intel TXT secure launcher

From: Andy Lutomirski
Date: Mon Jun 01 2020 - 12:51:40 EST

On Mon, Jun 1, 2020 at 8:33 AM Daniel P. Smith
<dpsmith@xxxxxxxxxxxxxxxxxxxx> wrote:
> On 5/7/20 7:06 AM, Daniel Kiper wrote:
> > Hi Åukasz,
> >
> > On Tue, May 05, 2020 at 04:38:02PM +0200, Lukasz Hawrylko wrote:
> >> On Tue, 2020-05-05 at 01:21 +0200, Daniel Kiper wrote:
> ...
> >> In OS-MLE table there is a buffer for TPM event log, however I see that
> >> you are not using it, but instead allocate space somewhere in the
> >
> > I think that this part requires more discussion. In my opinion we should
> > have this region dynamically allocated because it gives us more flexibility.
> > Of course there is a question about the size of this buffer too. I am
> > not sure about that because I have not checked yet how many log entries
> > are created by the SINIT ACM. Though probably it should not be large...
> >
> >> memory. I am just wondering if, from security perspective, it will be
> >> better to use memory from TXT heap for event log, like we do in TBOOT.
> >
> > Appendix F, TPM Event Log, has following sentence: There are no
> > requirements for event log to be in DMA protected memory â SINIT will
> > not enforce it.
> >
> > I was thinking about it and it seems to me that the TPM event log does
> > not require any special protections. Any changes in it can be quickly
> > detected by comparing hashes with the TPM PCRs. Does not it?
> >
> I think it would be beneficial to consider the following in deciding
> where the log is placed. There are two areas of attack/manipulation that
> need to be considered. The first area is the log contents itself, which
> as Daniel has pointed out, the log contents do not really need to be
> protected from tampering as that would/should be detected during
> verification by the attestor. The second area that we need to consider
> is the log descriptors themselves. If these are in an area that can be
> manipulated, it is an opportunity for an attacker to attempt to
> influence the ACM's execution. For a little perspective, the ACM
> executes from CRAM to take the most possible precaution to ensure that
> it cannot be tampered with during execution. This is very important
> since it runs a CPU mode (ACM Mode) that I would consider to be higher
> (or lower depending on how you view it) than SMM. As a result, the txt
> heap is also included in what is mapped into CRAM. If the event log is
> place in the heap, this ensures that the ACM is not using memory outside
> of CRAM during execution. Now as Daniel has pointed out, the down side
> to this is that it greatly restricts the log size and can only be
> managed by a combination of limiting the number of events and
> restricting what content is carried in the event data field.

Can you clarify what the actual flow of control is? If I had to guess, it's:

GRUB (or other bootloader) writes log.

GRUB transfers control to the ACM. At this point, GRUB is done
running and GRUB code will not run again.

ACM validates system configuration and updates TPM state using magic
privileged TPM access.

ACM transfers control to the shiny new Linux secure launch entry point

Maybe this is right, and maybe this is wrong. But I have some
questions about this whole setup. Is the ACM code going to inspect
this log at all? If so, why? Who supplies the ACM code? If the ACM
can be attacked by putting its inputs (e.g. this log) in the wrong
place in memory, why should this be considered anything other than a
bug in the ACM?

If GRUB is indeed done by the time anyone consumes the log, why does
GRUB care where the log ends up?

And finally, the log presumably has nonzero size, and it would be nice
not to pin some physical memory forever for the log. Could the kernel
copy it into tmpfs during boot so it's at least swappable and then
allow userspace to delete it when userspace is done with it?