[SeaBIOS] Questions on SeaBIOS TPM
kevin at koconnor.net
Fri Nov 20 18:26:08 CET 2015
On Fri, Nov 20, 2015 at 05:09:07PM +0000, Stefan Berger wrote:
> "Kevin O'Connor" <kevin at koconnor.net> wrote on 11/20/2015 11:12:53 AM:
> > I don't understand the error handling in tpm_extend_acpi_log() and
> > tpm_extend(). Why does a log overflow in tpm_extend_acpi_log()
> > shutdown the tpm chip (via tpm_set_failure() )? In particular,
> > tpm_extend_acpi_log can be called from clients via the 16bit BIOS
> > interface, and it's the only way a client could cause the tpm chip to
> > shutdown. Why does tpm_extend() call reset_acpi_log() on failure? It
> > seems odd that a failure in communication with the TPM chip would
> > result in an ACPI log reset - no other TPM chip failure does that.
> The intention here was to invalidate the log that is supposed to be
> written along with PCR extensions, resulting in attestation not being
> possible due to a failure in the extend.
Okay - so there isn't a specific need to handle a failure in
tpm_extend_acpi_log() differently from a failure in tpm_extend()?
That is, the goal is just to prevent attestation after a failed
> > Is it expected that the tcpa ACPI table could move or be modified at
> > runtime? The code rescans for the table twice on every call to
> > tpm_extend_acpi_log() - if it can't move or be modified then I think
> > it would be simpler to cache the values.
> I will have a look whether we can cache that. I guess for as long as it's
> scanned for 'early' we are not write-protected, yet. You may have seen I
> am also rescanning the log every time something is appended to it. Here
> the reason was that we are running in ROM mode and I cannot write to
> static variables anymore that would allow to set the pointer to the last
> entry. So back when I wrote this I handled the ACPI table in the same way
> - rescanning.
Oh, so the code could assume that the tcpa table is not moved nor
modified _and_ that the BIOS is also the only writer to the log?
BTW, to make a global variable read/writable at runtime, one can add
the VARLOW attribute to the variable.
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