Re: [PATCH v11 1/3] x86/tdx: Add TDX Guest attestation interface driver

Bagas Sanjaya
2022-08-27T07:45:36Z

On 8/26/22 22:06, Kuppuswamy Sathyanarayanan wrote:
> Attestation is used to verify the TDX guest trustworthiness to other
> entities before provisioning secrets to the guest. For example, a key
> server may request for attestation before releasing the encryption keys
> to mount the encrypted rootfs or secondary drive.
>
> During the TDX guest launch, the initial contents (including the
> firmware image) and configuration of the guest are recorded by the
> Intel TDX module in build time measurement register (MRTD). After TDX
> guest is created, run-time measurement registers (RTMRs) can be used by
> the guest software to extend the measurements. TDX supports 4 RTMR
> registers, and TDG.MR.RTMR.EXTEND TDCALL is used to update the RTMR
> registers securely. RTMRs are mainly used to record measurements
> related to sections like the kernel image, command line parameters,
> initrd, ACPI tables, firmware data, configuration firmware volume (CFV)
> of TDVF, etc. For complete details, please refer to TDX Virtual
> Firmware design specification, sec titled "TD Measurement".
>
> At TDX guest runtime, the Intel TDX module reuses the Intel SGX
> attestation infrastructure to provide support for attesting to these
> measurements as described below.
>
> The attestation process consists of two steps: TDREPORT generation and
> Quote generation.
>
> TDREPORT (TDREPORT_STRUCT) is a fixed-size data structure generated by
> the TDX module which contains guest-specific information (such as build
> and boot measurements), platform security version, and the MAC to
> protect the integrity of the TDREPORT. The guest kernel uses
> TDCALL[TDG.MR.REPORT] to get the TDREPORT from the TDX module. A
> user-provided 64-Byte REPORTDATA is used as input and included in the
> TDREPORT. Typically it can be some nonce provided by attestation
> service so the TDREPORT can be verified uniquely. More details about
> the TDREPORT can be found in Intel TDX Module specification, section
> titled "TDG.MR.REPORT Leaf".
>
> TDREPORT by design can only be verified on the local platform as the
> MAC key is bound to the platform. To support remote verification of
> the TDREPORT, TDX leverages Intel SGX Quote Enclave (QE) to verify
> the TDREPORT locally and convert it to a remote verifiable Quote.
>
> After getting the TDREPORT, the second step of the attestation process
> is to send it to the QE to generate the Quote. TDX doesn't support SGX
> inside the guest, so the QE can be deployed in the host, or in another
> legacy VM with SGX support. QE checks the integrity of TDREPORT and if
> it is valid, a certified quote signing key is used to sign the Quote.
> How to send the TDREPORT to QE and receive the Quote is implementation
> and deployment specific.
>
> Implement a basic guest misc driver to allow userspace to get the
> TDREPORT. After getting TDREPORT, the userspace attestation software
> can choose whatever communication channel available (i.e. vsock or
> hypercall) to send the TDREPORT to QE and receive the Quote.
>
> Also note that explicit access permissions are not enforced in this
> driver because the quote and measurements are not a secret. However
> the access permissions of the device node can be used to set any
> desired access policy. The udev default is usually root access
> only.
>
> Operations like getting TDREPORT or Quote generation involves sending
> a blob of data as input and getting another blob of data as output. It
> was considered to use a sysfs interface for this, but it doesn't fit
> well into the standard sysfs model for configuring values. It would be
> possible to do read/write on files, but it would need multiple file
> descriptors, which would be somewhat messy. IOCTLs seems to be the best
> fitting and simplest model for this use case. This is similar to AMD
> SEV platform, which also uses IOCTL interface to support attestation.
>
> Any distribution enabling TDX is also expected to need attestation. So
> enable it by default with TDX guest support.
>

On what tree this patch series is based on?

And as this series is multi-patch, it's customary to have cover letter
(or [PATCH 0/?]).

Thanks.

--
An old man doll... just what I always wanted! - Clara