[SeaBIOS] [PATCH v8 0/8] Add TPM support to SeaBIOS

[Kevin O'Connor]

On Tue, Aug 26, 2014 at 10:42:31AM -0400, Stefan Berger wrote:
> >As we discussed in the past, the main concern I have is the addition
> >of the TPM boot menu.  The problem with the menu, is that I suspect
> >the number of people who will find utility in it is extremely small.
> >(Most people wont even know what a TPM is.)  However, many more users
> >are likely to see the prompt, click through it, and then get very
> >confused with the available options and the implications of choosing
> >them.  So, I think it is a poor trade off of complexity for gain.
> 
> Here's the justification for the menu: A TPM can have an owner who
> identifies himself via owner password. In the case that the owner forgets
> the password, there is no way for the owner to give up ownership of the TPM
> unless there is a BIOS menu that allows him to give up ownership under
> physical presence (1). Physical presence is only assumed while the BIOS is
> active and a user for example pressed a key when the machine initialized to
> indicate physical presence. (I would think pressing F11 to enter the BIOS
> menu would be enough for indicate physical presence). I don't know of
> another way of doing this.

If I understand the intent of the above, the goal is to prevent
malicious software on the guest from reprogramming the TPM without the
users' knowledge.  (The malicious software sees that the TPM is
password locked, so it unlocks the TPM, clears the password, and
continues.)

If this is the intent, can't we just pass a flag (via fw_cfg) from
QEMU command line to SeaBIOS to force a clear?  That is, the guest
software can't manipulate the QEMU command line (or its fw_cfg
entries) and so the ability to set a flag there is proof of physical
presence.  (Access to the virtual machine disk images and virtual
machine command line is as close to "physical" as one can get.)

On coreboot, a similar solution could be accomplished by setting a
flag in CBFS (the flash).  Granted, one doesn't need to be physically
present to reprogram the flash, but if one can reprogram the flash,
they could just as easily reprogram SeaBIOS anyway.

-Kevin