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SubjectRE: [GIT PULL] x86/txt for v2.6.32
> From: Pavel Machek [mailto:pavel@ucw.cz]
> Sent: Tuesday, October 06, 2009 1:13 AM
>
> On Sat 2009-10-03 13:44:22, Roland Dreier wrote:
> >
> > > > > So I modify the RAM content so that BIOS does not think measured
> > > > > environment existed before suspend?
> >
> > > And it is ridiculously easy to pull off, too:
> > > http://www.engadget.com/2008/02/21/cold-boot-disk-encryption-attack-is-shockingly-
> effective/
> > >
> > > Shows the attack being used to read sensitive keys, but you can use it also
> > > to *modify* system running state (it will be more difficult, as you need to
> > > remove and replace the RAM while on S3 instead of S5, but it should be
> > > doable by someone who knows what he is doing).
> >
> > I believe the whole point of this TXT / S3 handling is that the resume
> > from S3 will then be able to detect that the contents of RAM have been
> > modified while the system was asleep.
>
> ...and you are able to read out any keys, etc. Maybe that's expected &
> ok, but Doc*/intel_txt.txt does not actually tell me what it protects
> against and is pretty much useless... making patches impossible to
> review.
>
> So... what does txt protect?

From Documentation/intel_txt.txt:
Intel TXT in Brief:
o Provides dynamic root of trust for measurement (DRTM)
o Data protection in case of improper shutdown
o Measurement and verification of launched environment

Intel TXT doesn't protect anything itself--it provides a foundation for software to provide protections and security. tboot and the associated Linux patches do this. The section of intel_txt.txt titled "Value Proposition for Linux or "Why should you care?"" tries to describe what is provided.

> Data integrity only?

Data integrity, yes, but not only. The code also provides for DRTM-based measurements, data protection in case of improper shutdown, etc.

> Data privacy, too?

No.

> Who is it designed to protect against?
>
> Remote attacker?

Yes.

> Local user trying to subvert it?

No.

> ...and has soldering iron he's willing to use?
>
> ...and has soldering iron, osciloscope and PCI analyzer he's willing to use?

No and no.

> > TXT simply produces a reasonably trustworthy measurement of system
> > state. If you modify RAM while the system is asleep, then you will not
> > be able to produce a measurement showing an unmodified system state.
>
> Well, actually I see some auditing to be done in proposed patches.

All comments are welcome.

Joe


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