[PATCH 0/3] Allow guest to query AMD SEV(-ES) runtime attestation evidence

[Shirong Hao]

This patch series is provided to allow the guest application to query
AMD SEV(-ES) runtime attestation evidence by communicating with the host
service (Attestation Evidence Broker).

The following is the design document.

> Background

Compared with SEV-SNP and Intel TDX, the runtime attestation of
SEV(-ES) does NOT support guest-provided data included in the
attestation report [1,2]. In addition, SEV(-ES) also does NOT support
the dynamic measurement. During runtime, it can only generate static
attestation report with the constant launch digest reflecting the
initial measurement.

Although SEV(-ES) has above limitations, its runtime attestation is
still useful. When the SEV(-ES) guest is running, it can report the
attestation report with fixed launch digest as a heartbeat for trusted
healthy check.

SEV(-ES) runtime attestation includes two participants:
attester and verifier.

- The attester running in a SEV(-ES) guest is responsible for
  collecting attestation evidence.
- The verifier running in a trusted environment is responsible for
  verifying the attestation evidence provided by the attester. Note
  that the verifier can run on any platform even non-TEE.

> SEV(-ES) Attestation Evidence

Verifier uses the following SEV(-ES) certificate chains to verify the
signature of the attestation report generated by the `ATTESTATION`
command [2]:

1. certificate chain for device identity:
ARK -> ASK -> CEK -> PEK -> report
2. certificate chain for platform owner identity:
OCA -> PEK -> report

- `foo -> bar` indicates using the public key of `foo` to verify the
  signature of `bar`.
- ARK is the root of trust of AMD and OCA is the root of trust for
  platform owners. OCA has two ways:
    1. Self-owned: The OCA Key Pair and self-signed OCA certificate
       are automatically generated by the SEV(-ES) firmware.
    2. Externally-owned: External users use OCA Key Pair to generate
       self-signed OCA certificates in a trusted environment.

Verifier needs to verify the attestation report with the certificate chain.
ARK and ASK can be obtained directly from the AMD KDS server. CEK, PEK,
OCA, and attestation report are related to the specific SEV(-ES) platform,
therefore SEV(-ES) Attestation Evidence collected by attester should
include attestation report (with the constant launch digest), PEK, CEK,
and OCA certificate.

| Contents of SEV(-ES) Attestation Evidence 	| SEV(-ES) firmware command	|
| :-: | :-: |
| attestation report				| ATTESTATION			|
| CEK						| GET_ID			|
| OCA,PEK					| PDH_CERT_EXPORT		|

> Query SEV(-ES) Attestation Evidence

According to the official feedback[3], SEV(-ES) firmware APIs don't support
query attestation report in SEV(-ES) guest and there is no plan to support
it in the future. Instead, this capability will be available in SEV-SNP.

In some scenarios, the guest application needs to query the attestation
report to establish an attested channel with the remote peer. There are
two approaches for a guest application to query an attestation evidence:

- Hypercall approach
- VSOCK approach

Considering time and cost, we only need to implement one of them.

- Hypercall approach

SEV(-ES) guest exits to VMM using `hypercall` and then interacts with SEV
firmware to query the components composing an attestation evidence,
including attestation report, PEK, CEK, OCA certificate. To build an
attestation evidence, the steps include:

1. The guest application requests a shared memory page, initiates a
   hypercall, and switches from the guest mode to the host mode.
2. In the host mode, KVM sends the `GET_ID, PDH_CERT_EXPORT, ATTESTATION`
   command requests to SEV firmware.
3. The shared memory page is filled with the data returned by the
   SEV firmware.
4. The guest application can obtain attestation evidence by reading the
   data in the shared memory.

Although this method can meet our requirements, it requires a lot of
modifications to the guest kernel and KVM.

- VSOCK approach

In the current implementation, QEMU provides the QMP interface
"query-sev-attestation-report" to query the attestation report in the host.
 However, QEMU is not the only VMM. In order to support various VMM in
different scenarios, it is necessary to design a general host service, such
as attestation evidence Broker (AEB) to query attestation evidence from the
host.

The workflow of AEB is as followed:

1. The user-level application in the guest sends a request
   (including guest firmware handle) to AEB through VSOCK.
2. AEB requests to query attestation report, PEK, CEK, OCA certificate by
   calling multiple SEV firmware APIs (refer to the table above for
   specific API commands) and assembles these information into the
   attestation evidence.
3. AEB returns the attestation evidence to the application in the guest.

To query the attestation report in host with AEB, we provides three patches
to achieve the following two goals.

1. It is necessary to add a `SEV_GET_REPORT` interface in ccp driver so
   that AEB can execute `ioctl` on the `/dev/sev` device and then call
   the `SEV_GET_REPORT` interface to send the `ATTESTATION` command to
   the SEV firmware to query attestation report.

2. In order to obtain the guest handle required by the `ATTESTATION`
   command to the SEV firmware, a new hypercall needs to be added to the
   KVM. The application in the guest obtains the guest handle through this
   newly added hypercal, and then sends it to the AEB service through
   VSOCK. The AEB service uses the guest handle as the input parameter
   of the `ATTESTATION` command to interact with the SEV firmware.

Note that hypercall is not the only way to obtain the guest handle.
Actually the qmp interface `query-sev` can query the guest handle as well.
However, as mentioned previously, qemu is not the only VMM.

> Communication protocol

Below is the communication protocol between the guest application and AEB.

```protobuf
syntax = "proto3";
...
message RetrieveAttestationEvidenceSizeRequest{
    uint32 guest_handle = 1;
}
message RetrieveAttestationEvidenceRequest{
    uint32 guest_handle = 1;
    uint32 evidence_size = 2;
}
message RetrieveAttestationEvidenceSizeResponse{
    uint32 error_code = 1;
    uint32 evidence_size = 2;
}
message RetrieveAttestationEvidenceResponse{
    uint32 error_code = 1;
    uint32 evidence_size = 2;
    bytes evidence = 3;
}
service AEBService {
    rpc RetrieveAttestationEvidenceSize
         (RetrieveAttestationEvidenceSizeRequest)
         returns (RetrieveAttestationEvidenceSizeResponse);
    rpc RetrieveAttestationEvidence(RetrieveAttestationEvidenceRequest)
         returns (RetrieveAttestationEvidenceResponse);
}
```

> Reference

[1] https://www.amd.com/system/files/TechDocs/
55766_SEV-KM_API_Specification.pdf
[2] https://www.amd.com/system/files/TechDocs/56860.pdf
[3] https://github.com/AMDESE/AMDSEV/issues/71#issuecomment-926118314

Shirong Hao (3):
  KVM: X86: Introduce KVM_HC_VM_HANDLE hypercall
  KVM/SVM: move the implementation of sev_get_attestation_report to ccp
    driver
  crypto: ccp: Implement SEV_GET_REPORT ioctl command

 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/svm/sev.c          | 49 +++-------------------
 arch/x86/kvm/svm/svm.c          | 11 +++++
 arch/x86/kvm/x86.c              |  7 +++-
 drivers/crypto/ccp/sev-dev.c    | 74 +++++++++++++++++++++++++++++++++
 include/linux/psp-sev.h         |  7 ++++
 include/uapi/linux/kvm_para.h   |  1 +
 include/uapi/linux/psp-sev.h    | 17 ++++++++
 8 files changed, 123 insertions(+), 44 deletions(-)

-- 
2.27.0