Wire capability-gated broker mediation into the process backend#277
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The process backend denied every guest `request(...)` call with a flat "not available for the process backend" error, so a process-backend guest had no path to mediated privileged operations -- even though the threat model lists "broker-mediated privileged operations" as a defended layer for that mode. Mirror the sub-interpreter backend's semantics: - child.py: `request(capability, action, payload)` is gated on the granted capability names. An ungranted capability raises PolicyError at the boundary; a granted one is framed to the supervisor as an "ev": "request" message. The guest never performs the privileged action itself. - process_backend.py: the granted capability names are sent in the bootstrap frame, and an incoming request frame is surfaced via recv() as a BrokerRequest -- the same object the sub-interpreter backend emits -- so the supervisor mediates the action. Only capability names cross the boundary, never secrets. - supervisor.py: thread `capabilities` through spawn -> _spawn_process -> ProcessSandbox (previously dropped for this backend). SECURITY.md and the threat model are updated; the latter's history notes this makes the broker-mediation layer real for the process backend. Full suite: 494 passed, 6 skipped. black/isort/flake8/mypy clean; pylint 9.16. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01PbbJc7Ntj159D9LNGevwC2
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Summary
The threat model lists "broker-mediated privileged operations" as a defended layer for
backend="process", but the implementation didn't match:child.py'srequest()denied every call with a flat "not available for the process backend" error, andcapabilitieswere dropped entirely on the way to the process backend. So a process-backend guest had no path to mediated privileged operations at all.This reconciles reality with the model by mirroring the sub-interpreter backend's semantics.
Changes
child.py—request(capability, action, payload)is now gated on the granted capability names. An ungranted capability raisesPolicyErrorat the boundary; a granted one is framed to the supervisor as an"ev": "request"message. The guest never performs the privileged action itself.process_backend.py— the granted capability names are sent in the bootstrap frame, and an incomingrequestframe is surfaced viarecv()as aBrokerRequest— the same object the sub-interpreter backend emits — so the supervisor mediates the action. Only capability names cross the boundary, never secret material.supervisor.py— threadscapabilitiesthroughspawn→_spawn_process→ProcessSandbox(previously dropped for this backend).Semantics (matches the thread backend)
request("network", "connect", {...})BrokerRequest(capability="network", action="connect", payload={...})viarecv()request("secret", "read", {})PolicyErrorraised at the boundaryDocs & tests
SECURITY.md(§3.1) and the threat model are updated; the threat model's history notes this makes the broker-mediation layer real for the process backend. New tests cover the granted path (payload round-trips as aBrokerRequest), the ungranted-with-no-capabilities path, and the ungranted-but-other-capabilities-present path.Full suite: 494 passed, 6 skipped. black/isort/flake8/mypy clean; pylint 9.16 (gate 8.0).
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