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Tiny AI — security model

ONNX models are data, not code, but that is not an excuse for a lazy runtime. libvmaf's DNN layer applies four layers of defence before any frame touches a graph.

Threat model

Class Example Mitigation
Hostile .onnx file Model with custom op that exfiltrates via network syscall Operator allowlist (next section).
Memory-exhaustion via huge model 10 GB .onnx passed to --tiny-model Size cap (VMAF_DNN_DEFAULT_MAX_BYTES, compile-time 50 MB).
Path-traversal via --tiny-model --tiny-model ../../etc/shadow vmaf_dnn_validate_onnx resolves symlinks, requires S_ISREG + readable, and can enforce the optional VMAF_TINY_MODEL_DIR directory jail.
Silent model substitution Attacker replaces signed model with a poisoned one Opt-in Sigstore (cosign) verification against the workflow identity.

Layer 1 — operator allowlist

core/src/dnn/op_allowlist.{h,c} holds a curated set of ONNX operator names. Before creating an ORT session the loader walks the graph and rejects any node whose op is not on the list.

The list includes the common building blocks of C1/C2/C3 architectures (Conv, Gemm, Relu, BatchNormalization, GlobalAveragePool, activations, pooling, arithmetic, reshape/transpose) plus the two control-flow ops Loop and If (added in ADR-0169 — required by MUSIQ / RAFT / small-VLM-class architectures) and the spatial sampler Resize (added in ADR-0258 — required by U-2-Net / mobilesal / BASNet / PiDiNet / FPN-style detectors). Scan remains rejected — its variant-typed input/output binding makes static bound-checking impractical for a wire-format scanner. Unknown op names (custom_op_xyz) are rejected, as is anything that could touch the filesystem or network.

Resize carries multiple mode settings — nearest, linear, cubic. Per ADR-0258 the wire scanner gates op-type only, not attributes; consumers shipping their own ONNX are expected to keep mode in ("nearest", "linear") (cubic is numerically less stable on quantised inputs and not exercised by any in-tree consumer).

For Loop / If, the wire-format scanner in onnx_scan.c recurses into the embedded subgraphs (Loop.body, If.then_branch, If.else_branch) and applies the same allowlist check at every depth (capped at 8 levels of nesting as a defence-in-depth bound). A forbidden op cannot hide inside a control-flow body.

Bounded-iteration guard (added in ADR-0171). Two layers mirror the doc-drift enforcement model:

  • Export-time (vmaf-train): vmaf_train.op_allowlist traces every Loop.M input back to a Constant int64 scalar and rejects when the producer is a graph input, a non-Constant op, or carries a value outside [0, MAX_LOOP_TRIP_COUNT] (default 1024, per-call overridable for legitimate longer pipelines). Recursion descends into nested subgraphs — a Loop inside a Loop.body must be statically bounded in its own scope.
  • Load-time (libvmaf wire scanner): a counter threaded through the scanner caps the total number of Loop nodes per model at VMAF_DNN_MAX_LOOP_NODES = 16 across the top-level graph and every embedded subgraph. The cap is coarser than the Python data-flow check by design — reproducing producer-map lookup in a wire-format scanner would violate the ADR D39 "bounded-auditable-scope" constraint that keeps the scanner from pulling in libprotobuf-c.

A model has to clear both layers to load. Models that bypass the trainer (HTTP-fetched, MCP-uploaded, third-party tiny-AI registries) still hit the load-time cap.

Extending the list is a conscious, reviewed act — changes to op_allowlist.c must be called out in the PR description and backed by a concrete model that needs the addition.

Layer 2 — resource bounds

  • Size cap. Loader refuses files larger than VMAF_DNN_DEFAULT_MAX_BYTES (50 MB, compile-time constant in core/src/dnn/model_loader.h). Applies before mapping the file. The historical VMAF_MAX_MODEL_BYTES env override was retired in T7-12 once two release cycles passed without a shipped model approaching the cap; callers that genuinely need a larger envelope must bump the constant and rebuild.

  • Path validation. vmaf_dnn_validate_onnx:

    • resolves symlinks,
    • asserts S_ISREG (no devices, pipes, directories),
    • returns -errno on any failure — caller must check.

    VMAF_TINY_MODEL_DIR is the optional deployment jail. When set to a directory, the loader canonicalises both the jail and the requested .onnx path, then requires the model to sit below that directory before any stat/read of the model file. Sibling-prefix escapes (/models vs /models-evil), symlink escapes, missing jail paths, and non-directory jail paths all fail closed with -EACCES. When the variable is unset or empty, the jail is a no-op and the normal symlink + regular-file + size + operator checks still apply. MCP callers also keep their separate path allowlist (mcp/index.md).

  • Shape sanity. The sidecar JSON declares input_name, output_name, and expected_output_range. Runtime values outside the range raise a warning to stderr; persistent violation aborts scoring for the frame.

Layer 3 — sandbox via ORT

ORT itself sandboxes graph execution — there is no interpreter, no shell-out, no arbitrary file I/O from inside a graph. Our layers 1/2 harden the envelope around ORT so that even a clever graph cannot consume unbounded memory or divert through a non-allowlisted op.

Layer 4 — signature verification (opt-in)

Release artifacts (including models shipped under model/tiny/) are signed by .github/workflows/supply-chain.yml using Sigstore's keyless flow. The workflow emits <artifact>.sig and <artifact>.pem beside each artifact. To verify locally before loading:

cosign verify-blob \
    --certificate-identity-regexp "https://github.com/VMAFx/vmafx/.github/workflows/supply-chain.yml@.*" \
    --certificate-oidc-issuer "https://token.actions.githubusercontent.com" \
    --bundle vmaf_tiny_fr_v1.onnx.bundle \
    vmaf_tiny_fr_v1.onnx

Now implemented (T6-9 / ADR-0211): the --tiny-model-verify flag invokes cosign verify-blob at load time via posix_spawnp(3p) and fails closed if the signature is missing or bad. Off by default for dev-friendliness; strongly recommended on for production deployments. The flag drives vmaf_dnn_verify_signature() in core/include/libvmaf/dnn.h, which looks up the model's sigstore_bundle field in model/tiny/registry.json — see model-registry.md for the full schema and CLI flow.

Reporting

If you believe a shipped model is hostile or find a way to bypass the allowlist, follow the disclosure process in SECURITY.md (90-day coordinated, PGP key listed).