Documenting the proposal for TPM 2.0 security in the face of bus interposer attacks

[James Bottomley]

On Wed, 2018-11-21 at 01:13 +0200, Jarkko Sakkinen wrote:
> On Tue, Nov 20, 2018 at 09:25:43AM -0800, James Bottomley wrote:
> > On Tue, 2018-11-20 at 14:41 +0200, Jarkko Sakkinen wrote:
> > > On Tue, Nov 20, 2018 at 01:10:49PM +0200, Jarkko Sakkinen wrote:
> > > > This is basically rewrite of TPM genie paper with extras. Maybe
> > > > just shorten it to include the proposed architecture and point
> > > > to the TPM Genie paper (which is not in the references at all
> > > > ATM).
> > > > 
> > > > The way I see it the data validation is way more important than
> > > > protecting against physical interposer to be frank.
> > > > 
> > > > The attack scenario would require to open the damn device. For
> > > > laptop that would leave physical marks (i.e. evil maid). In a
> > > > data center with armed guards I would wish you good luck
> > > > accomplishing it. It is not anything like sticking a USB stick
> > > > and run.
> > > > 
> > > > We can take a fix into Linux with a clean implementation but it
> > > > needs to be an opt-in feature because not all users will want
> > > > to use it.
> > > 
> > > Someone (might have been either Mimi or David Howells but cannot
> > > recall) correctly pointed out at LSS 2018 that you could just as
> > > easily spy and corrupt RAM if you have a time window to perform
> > > this type of attack.
> > 
> > Not using the simple plug in on the TPM bus, you can't.  The point
> > is basically the difference in the technology: the interposer is a
> > simple, easy to construct, plugin; a RAM spy is a huge JTAG thing
> > that would be hard even to fit into a modern thin laptop, let alone
> > extremely difficult to build.
> 
> Why you wouldn't use DMA to spy the RAM?

You mean from a plugin on the TPM bus? most of the buses the TPM is on
don't get DMA access.  Some of them barely get interrupts, which is why
we waste a lot of time polling in TPM drivers.

James