Re: [PATCH v2 2/2] efi/libstub: Add get_event_log() support for CC platforms

[Ilias Apalodimas]

Apologies for the late reply,


On Mon, 19 Feb 2024 at 09:34, Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> wrote:
>
> Hi Ilias,
>
> On 2/18/24 11:03 PM, Ilias Apalodimas wrote:
> > On Thu, 15 Feb 2024 at 05:02, Kuppuswamy Sathyanarayanan
> > <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> wrote:
> >> To allow event log info access after boot, EFI boot stub extracts
> >> the event log information and installs it in an EFI configuration
> >> table. Currently, EFI boot stub only supports installation of event
> >> log only for TPM 1.2 and TPM 2.0 protocols. Extend the same support
> >> for CC protocol. Since CC platform also uses TCG2 format, reuse TPM2
> >> support code as much as possible.
> >>
> >> Link: https://uefi.org/specs/UEFI/2.10/38_Confidential_Computing.html#efi-cc-measurement-protocol [1]
> >> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > [...]
> >
> >> +void efi_retrieve_eventlog(void)
> >> +{
> >> +       efi_physical_addr_t log_location = 0, log_last_entry = 0;
> >> +       efi_guid_t cc_guid = EFI_CC_MEASUREMENT_PROTOCOL_GUID;
> >> +       efi_guid_t tpm2_guid = EFI_TCG2_PROTOCOL_GUID;
> >> +       int version = EFI_TCG2_EVENT_LOG_FORMAT_TCG_2;
> >> +       efi_tcg2_protocol_t *tpm2 = NULL;
> >> +       efi_cc_protocol_t *cc = NULL;
> >> +       efi_bool_t truncated;
> >> +       efi_status_t status;
> >> +
> >> +       status = efi_bs_call(locate_protocol, &tpm2_guid, NULL, (void **)&tpm2);
> >> +       if (status == EFI_SUCCESS) {
> >> +               status = efi_call_proto(tpm2, get_event_log, version, &log_location,
> >> +                                       &log_last_entry, &truncated);
> >> +
> >> +               if (status != EFI_SUCCESS || !log_location) {
> >> +                       version = EFI_TCG2_EVENT_LOG_FORMAT_TCG_1_2;
> >> +                       status = efi_call_proto(tpm2, get_event_log, version,
> >> +                                               &log_location, &log_last_entry,
> >> +                                               &truncated);
> >> +                       if (status != EFI_SUCCESS || !log_location)
> >> +                               return;
> >> +               }
> >> +
> >> +               efi_retrieve_tcg2_eventlog(version, log_location, log_last_entry,
> >> +                                          truncated);
> >> +               return;
> >> +       }
> >> +
> >> +       status = efi_bs_call(locate_protocol, &cc_guid, NULL, (void **)&cc);
> >> +       if (status == EFI_SUCCESS) {
> >> +               version = EFI_CC_EVENT_LOG_FORMAT_TCG_2;
> >> +               status = efi_call_proto(cc, get_event_log, version, &log_location,
> >> +                                       &log_last_entry, &truncated);
> >> +               if (status != EFI_SUCCESS || !log_location)
> >> +                       return;
> >> +
> >> +               efi_retrieve_tcg2_eventlog(version, log_location, log_last_entry,
> >> +                                          truncated);
> >> +               return;
> >> +       }
> >> +}
> > [...]
> >
> > I haven't looked into CC measurements much, but do we always want to
> > prioritize the tcg2 protocol? IOW if you have firmware that implements
> > both, shouldn't we prefer the CC protocol for VMs?
>
> According the UEFI specification, sec "Conidential computing", if a firmware implements
> the TPM, then it should be used and CC interfaces should not be published. So I think
> we should check for TPM first, if it does not exist then try for CC.

Ok thanks, that makes sense. That document also says the services
should be implemented on a virtual firmware.
I am unsure at the moment though if it's worth checking that and
reporting an error otherwise. Thoughts?

Thanks
/Ilias
>
> https://uefi.org/specs/UEFI/2.10/38_Confidential_Computing.html#confidential-computing
>
> > Thanks
> > /Ilias
>
> --
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer
>