GPU TEE attestation: verify GPU confidential mode and bind it to the CVM (currently unverified)

Inizia da docs/gpu-attestation-design.md, poi esamina verifier/src/verification.rs, vmm/src/app/qemu.rs:840, dstack-attest/src/attestation.rs:176 e guest-agent/src/rpc_service.rs:327. Traccia il confine esistente degli eventi misurati e i servizi di avvio della GPU prima di scegliere un filone di lavoro. Per considerare il lavoro completato, è necessaria una verifica e un binding della GPU fail-closed, con le modifiche a verifier, KMS, RPC, service-gating e alla documentazione coperte ove applicabile.

Problem

dstack attaches NVIDIA GPUs to CVMs but never verifies them. The CPU-side stack has no GPU/NRAS logic, and the guest OS flips the GPU to "ready" unconditionally at boot, before the workload runs. Consequences:

• A non-confidential GPU (CC off in host BIOS, or a normal GPU) is exposed plaintext to the workload with no error that stops it — no attacker required.
• Even a genuine CC GPU is not bound to the CVM: nothing lets a relying party (or the KMS) trust that a specific attested CVM is backed by a genuine GPU in confidential mode.

"Dual attestation" in docs/security/security-model.md is a documented expectation, not something enforced anywhere.

Full design write-up: docs/gpu-attestation-design.md (branch gpu-tee-nras-verification).

Current gaps (evidence)

• No CPU-side GPU verification — AttestationQuote is CPU-TEE only; VerificationDetails has no GPU fields (verifier/src/verification.rs).
• GPU attached as plain vfio-pci, no CC-mode/attestation (vmm/src/app/qemu.rs:840).
• nvidia-smi conf-compute -srs 1 run unconditionally at boot, no cc_mode check (meta-dstack nvidia-persistenced.service).
• app-compose.service has no dependency on GPU bring-up — the workload starts even if it fails.
• No app-facing RPC to query/enable GPU confidentiality.

Threat model (what this closes / doesn't)

Workstreams

1. Offline local verifier (NVAT)

• Adopt NVIDIA C++ Attestation SDK (libnvat, NVIDIA/attestation-sdk); Python nvtrust is EOL 2026-09-15.
• Bake libnvat + pre-provisioned filesystem RIM store into the NVIDIA image (re-provision on driver/VBIOS upgrade).
• Handle OCSP (the one online dep, ocsp.ndis.nvidia.com): in-CVM caching/replay proxy at --ocsp-url, and/or a Rego policy tolerating x-nvidia-cert-ocsp-status.

2. Binding via measured append-only state (before the app boundary)

• dstack-util emits a gpu-attestation event committing H(nvat_eat‖cert_chain‖claims) before system-ready.
• Verifier trusts it via the existing find_event boundary (breaks at system-ready, dstack-attest/src/attestation.rs:176) → RTMR3-bound on TDX.
• report_data stays for freshness + RA-TLS key binding only — not the GPU verdict.

3. Fail-fast enforcement & app gate

• dstack-gpu-attest.service: verify every configured GPU (num_gpus>0 is measured), require cc_mode==ON (reject OFF and DEVTOOLS), set -srs 1 only on pass.
• app-compose.service Requires=+After=dstack-gpu-attest.service; fail closed (no system-ready, no workload).
• Guest-agent RPC GetGpuAttestation() / EnsureGpuReady() so apps can confirm/enable before use.

4. KMS gating, verifier surfacing, docs

• KMS gates key release on the gpu-attestation event (like compose-hash).
• Verifier parses NVAT EAT/claims into VerificationDetails.
• security-model.md: state the guarantee (measured-guest-vouches + channel-bound) and the co-location residual.

Caveats

• SEV-SNP is blocked: no runtime measurement register (tpm_runtime_pcr()=None, has_tdx()=false); decode_app_info_sev_snp reads identity from launch-time HOST_DATA/MrConfigV3 and ignores the runtime log. SNP needs an SVSM/coconut-vTPM (PCR channel) before this works — until then SNP GPU attestation is strictly weaker than TDX. (See #713.)
• DEVTOOLS CC mode enables CC APIs without memory encryption — must be rejected.
• Co-location residual stands until Blackwell TEE-IO/TDISP.

Hardware validation checklist (unconfirmed in docs)

• NVAT behavior on OCSP connection failure (fail-open vs closed); can Rego neutralize revocation?
• OCSP nextUpdate window; warm-cache ride-through.
• Exact libnvat env-var spellings; trust-root store overridability.
• Does -srs 1 error or no-op on a CC-off GPU?
• Does the driver expose SPDM KEY_EXCHANGE key (optional session-binding stretch)?
• SNP SVSM vTPM feasibility in meta-dstack.