linux-kernel - Re: [PATCH v2 2/2] efi/libstub: Add get_event

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Message-ID: <7feb889f-f78e-4caa-a2f4-9d41acf6ca76@linux.intel.com>
Date: Sun, 18 Feb 2024 23:34:23 -0800
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Ilias Apalodimas <ilias.apalodimas@...aro.org>
Cc: Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
 linux-efi@...r.kernel.org
Subject: Re: [PATCH v2 2/2] efi/libstub: Add get_event_log() support for CC
 platforms

Hi Ilias,

On 2/18/24 11:03 PM, Ilias Apalodimas wrote:
> On Thu, 15 Feb 2024 at 05:02, Kuppuswamy Sathyanarayanan
> <sathyanarayanan.kuppuswamy@...ux.intel.com> 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@...ux.intel.com>
> [...]
>
>> +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.

https://uefi.org/specs/UEFI/2.10/38_Confidential_Computing.html#confidential-computing

> Thanks
> /Ilias

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer