[SeaBIOS] [PATCH 00/10] Some TPM simplifications

Kevin O'Connor kevin at koconnor.net
Tue Jan 5 21:35:35 CET 2016


On Tue, Jan 05, 2016 at 02:05:55PM -0500, Stefan Berger wrote:
> "Kevin O'Connor" <kevin at koconnor.net> wrote on 01/05/2016 12:16:18 PM:
> > On Tue, Dec 29, 2015 at 07:17:40PM -0500, Kevin O'Connor wrote:
> > > The following series involves some code reorganization in the TPM code
> > > that I found useful in understanding the code.
> > 
> > FYI, I committed patches 1-9 (after some bug fixes).
> 
> The result of the 2nd patch set also looks good.

Thanks - I pushed most of that series as well.

I've been looking further at error recovery in the TPM code.  I'm not
sure I fully understand what needs to be done on an error.

If I understand it correctly, the SeaBIOS TPM code has three major
requirements:

- Pass through commands from the 16bit bios interface to the TPM.
  This is useful for bootloaders/oproms that don't have a TPM driver.

  - I think the only requirement for error recovery here is that we
    return an appropriate error code to the caller of the 16bit BIOS
    interface.

- Take "measurements" during the boot process so that later on users
  can verify if some low-level code has changed (and thus attempt to
  identify if malicious code may have been inserted into the
  firmware).

  - The major requirement here seems to be that if we can't take a
    measurement that we either "cap" measurements or shutdown the TPM.
    If we don't do this, it opens the possibility of a malicious
    program forging measurements.

  - It's also useful to skip taking measurements if the TPM device is
    not present so that we don't waste CPU time taking measurements
    that will never be used.

- Implement physical presence capability so that critical settings in
  the TPM can only be changed by someone that can prove they are
  physically present at the hardware (and thus attempt to prevent
  malicious code that temporarily obtains escalated privileges from
  altering these important settings).

  - The major requirement here seems to be that the "physical
    presence" lock always be set prior to starting the boot loader.
    And, of course, we need to implement the ability to change the
    critical settings (via the tpm menu).

Does the above seem correct?

-Kevin



More information about the SeaBIOS mailing list