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.