[PATCH] efi_loader: explicitly return EFI_UNSUPPORTED for TCG 1.0 compatibility

[Stuart Yoder]

On 5/30/23 1:39 AM, Ilias Apalodimas wrote:
> In the EFI TCG spec EFI_TCG2_BOOT_SERVICE_CAPABILITY struct is
> versioned -- there are 1.0 and 1.1 versions of that struct.
> The spec [0] describes:
> "Version of the EFI_TCG2_BOOT_SERVICE_CAPABILITY
> structure itself. For this version of the protocol, the Major version
> SHALL be set to 1 and the Minor version SHALL be set to 1."
> which is what we currently support.
> 
> The SCT tests perfromed By Arms SIE(Security interface extensions) [1]
> perform a check for clients supporting the older 1.0 version of the
> spec (Test 30.1.1.4). Given than this spec is 7 years old,  there should
> be no need for the older 1.0 version support.  Instead of returning
> EFI_BUFFER_TOO_SMALLL switch to EFI_UNSUPPORTED which is more
> appropriate.  It's worth noting that the spec doesn't explicitly
> describe the return value at the moment.
> 
> [0] https://trustedcomputinggroup.org/wp-content/uploads/EFI-Protocol-Specification-rev13-160330final.pdf
> [1] https://github.com/stuyod01/edk2-test/blob/master/uefi-sct/Doc/UEFI-SCT-Case-Spec/30_Protocols_TCG2_Test.md
> 
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> ---
> 
> Heinrich,  Stuart is investigating the chance of the spec getting updated
> adding EFI_UNSUPPORTED.  In any case I think the patch should be aplied since
> the new return code makes more sense.  If for some reason the spec change is
> rejected, I can go back and add support for 1.0 structure versions.
> 
>   lib/efi_loader/efi_tcg2.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/efi_loader/efi_tcg2.c b/lib/efi_loader/efi_tcg2.c
> index a83ae7a46cf3..220c442bdf93 100644
> --- a/lib/efi_loader/efi_tcg2.c
> +++ b/lib/efi_loader/efi_tcg2.c
> @@ -750,7 +750,7 @@ efi_tcg2_get_capability(struct efi_tcg2_protocol *this,
> 
>   	if (capability->size < sizeof(*capability)) {
>   		capability->size = sizeof(*capability);
> -		efi_ret = EFI_BUFFER_TOO_SMALL;
> +		efi_ret = EFI_UNSUPPORTED;
>   		goto out;
>   	}

Hi Ilias,

I don't think this is the right fix.

The struct looks like this:

struct tdEFI_TCG2_BOOT_SERVICE_CAPABILITY {
   UINT8 Size;
   EFI_TCG2_VERSION StructureVersion;
   EFI_TCG2_VERSION ProtocolVersion;
   EFI_TCG2_EVENT_ALGORITHM_BITMAP HashAlgorithmBitmap
   EFI_TCG2_EVENT_LOG_BITMAP SupportedEventLogs
   BOOLEAN TPMPresentFlag
   UINT16 MaxCommandSize
   UINT16 MaxResponseSize
   UINT32 ManufacturerID
   UINT32 NumberOfPcrBanks
   EFI_TCG2_EVENT_ALGORITHM_BITMAP ActivePcrBanks
};

The intent in the TCG spec is for the caller to be able to
determine the size of the struct by passing in a small
buffer (e.g. 1 byte buffer), and then the firmware
should return sizeof(EFI_TCG2_BOOT_SERVICE_CAPABILITY) in the
1-byte "Size" field.  And the return value should be
EFI_BUFFER_TOO_SMALL as per the spec.

In order to detect a 1.0 client, which you don't want to support,
I think you need a _new_ check that is something like this:

// detect 1.0 client
if (capability->size < sizeof(*capability) &&
     capability->size >= offsetof(EFI_TCG2_BOOT_SERVICE_CAPABILITY, 
NumberOfPcrBanks)) {
     efi_ret = EFI_UNSUPPORTED;
     goto out;
}

The last field in the 1.0 struct was ManufacturerID, so you should
be able to detect 1.0 clients that expect that based on the size they
pass in.

Thanks,
Stuart