[SeaBIOS] [PATCH 1/3] Add 'measurement' code to the BIOS

Kevin O'Connor kevin at koconnor.net
Tue Jun 2 17:03:53 CEST 2015

On Tue, May 26, 2015 at 02:36:42PM -0400, Stefan Berger wrote:
> On 05/21/2015 06:32 PM, Kevin O'Connor wrote:
> >On Thu, May 21, 2015 at 06:09:23PM -0400, Stefan Berger wrote:
> >>On 05/21/2015 01:40 PM, Kevin O'Connor wrote:
> >>>On Fri, May 08, 2015 at 01:45:46PM -0400, Stefan Berger wrote:
> >>>>@@ -733,6 +743,8 @@ do_boot(int seq_nr)
> >>>>          break;
> >>>>      }
> >>>>+    tpm_returned_via_int18h();
> >>>I don't understand the name returned_via_int18h here as we're calling
> >>>int18 in this location, not returning from it.
> >>
> >>I did this assuming that we will eventually have to return from int18.
> >I don't understand - what does it mean to return from int18?
> Dropping this for now. Though the spec says:
> "If the IPL Code on an IPL device returns back to the BIOS through INT 18h
> or INT 19h, that event must be measured."
> http://www.trustedcomputinggroup.org/files/resource_files/CB0B2BFA-1A4B-B294-D0C3B9075B5AFF17/TCG_PCClientImplementation_1-21_1_00.pdf
> section 8.1

I interpret the above to mean "if the IPL code on an IPL device
re-invokes the BIOS by rasing an INT 18h or INT 19h, then that event
must be measured."

As I understand it, the INT 18/19 handlers are never supposed to
return (in the sense of the "iret" instruction) and the bootloader
code is never supposed to directly return to the bios (in the sense of
the "ret" instruction).


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