turnstone/docs/openshell.md

OpenShell Sandbox Integration

Turnstone can run inside an OpenShell sandbox for kernel-enforced security boundaries around tool execution. OpenShell provides four layers of defense that Turnstone's application-level safety model does not cover:

Layer	Mechanism	What it prevents
Filesystem	Landlock	Writes to /etc, ~/.ssh, system paths
Network	Network namespace + seccomp + HTTP CONNECT proxy	Connections to unlisted hosts
Process	setuid drop + verification	Privilege escalation to root
Credentials	Proxy-level secret resolution	API keys in sandbox memory

Turnstone's own safety layers (human approval, intent judge, tool policies, output guard) remain active inside the sandbox and handle threats at the semantic level -- what the LLM means to do with its legitimate access.

See also: Security and Authentication, Intent Validation, Governance

---

Quick Start

# Run turnstone-server in an OpenShell sandbox
openshell sandbox run \
  --policy deploy/openshell/turnstone-policy.yaml \
  --workdir /path/to/project \
  -- python3 -m turnstone.server --host 0.0.0.0 --port 8080

With inference routing (API keys never enter the sandbox):

openshell sandbox run \
  --policy deploy/openshell/turnstone-policy.yaml \
  --inference-routes deploy/openshell/routes.yaml \
  --workdir /path/to/project \
  -- python3 -m turnstone.server --host 0.0.0.0 --port 8080 \
      --base-url https://inference.local

The inference.local hostname is intercepted by the OpenShell proxy before network policy evaluation -- no network policy entry is needed for it.

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Policy Files

deploy/openshell/turnstone-policy.yaml

The main sandbox policy. Covers filesystem, process, and network rules.

deploy/openshell/routes.yaml

Inference routing configuration. Maps inference.local to real LLM API backends. Uncomment and configure the provider(s) you use.

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Filesystem Policy

The policy uses Landlock (Linux 5.13+) for kernel-enforced filesystem access control. Paths are locked at sandbox creation and cannot be changed at runtime.

Path	Access	Purpose
--workdir	read-write	Project files (auto-added via include_workdir)
/tmp	read-write	Bash tool temp scripts, eval workdirs
/dev/null	read-write	Shell redirections (2>/dev/null)
/var/log	read-write	Log files
/usr, /lib, /lib64	read-only	Python runtime, installed packages
/etc	read-only	System config, SSL certificates
/proc, /dev/urandom	read-only	Process info, entropy
~/.config/turnstone	read-only	Config file (writes go to database)

Landlock runs in best_effort mode by default -- degrades gracefully on kernels without Landlock support. Set compatibility: hard_requirement for production hardened deployments.

---

Network Policy

Default-deny. Only explicitly listed host:port pairs are reachable. All child processes (MCP servers, bash commands, grep) inherit the network namespace and cannot bypass the proxy.

Included endpoints

Policy	Hosts	Purpose
openai_api	api.openai.com	OpenAI LLM API
anthropic_api	api.anthropic.com	Anthropic LLM API
tavily_api	api.tavily.com	Web search fallback
skills_registry	skills.sh	Skill discovery
github_api	api.github.com (read-only L7), raw.githubusercontent.com	Skill fetch, GitHub API
mcp_registry	registry.modelcontextprotocol.io (read-only L7)	MCP server discovery
web_fetch_common	readthedocs, python docs, GitHub Pages, PyPI, npm, Stack Overflow, Wikipedia	Curated web_fetch domains
bash_network_tools	Same as web_fetch_common	curl/wget from bash tool
package_registries	pypi.org, files.pythonhosted.org	pip/uv package installs
git_operations	github.com, gitlab.com (L7: clone/fetch only, no push)	Git read-only operations

L7 enforcement

Endpoints marked with protocol: rest and tls: terminate get HTTP-level inspection. The proxy TLS-terminates using an ephemeral per-sandbox CA, parses each request, and evaluates method + path against the rules.

The github_api, mcp_registry, and git_operations policies use L7 enforcement:

• GitHub API / MCP Registry: access: read-only -- only GET, HEAD, OPTIONS allowed
• Git operations: explicit rules allowing only info/refs (GET) and git-upload-pack (POST) -- clone and fetch work, push is blocked

Commented-out sections

The policy includes commented blocks for optional integrations. Uncomment and configure as needed:

• OIDC -- add your identity provider's hostname
• Discord -- discord.com, gateway.discord.gg, cdn.discordapp.com
• MCP HTTP servers -- any MCP servers using streamable-http transport

---

Customizing the Domain Allowlist

The web_fetch tool lets the LLM fetch arbitrary public URLs, but OpenShell cannot allow "all HTTPS" -- bare wildcard hosts are rejected by policy validation. Instead, the policy ships with a curated set of common reference domains.

To add domains your workloads need:

# In turnstone-policy.yaml, under web_fetch_common.endpoints:
      - host: docs.example.com
        port: 443

# Also add to bash_network_tools.endpoints if curl/wget should reach it:
      - host: docs.example.com
        port: 443

Wildcard patterns are supported:

• *.example.com -- matches one subdomain level (e.g. api.example.com)
• **.example.com -- matches any depth (e.g. deep.sub.example.com)

Unlisted domains return connection errors, which the LLM handles gracefully by telling the user it cannot reach that site.

---

Inference Routing

Inference routing keeps real API keys completely outside the sandbox. The sandbox process only sees opaque placeholder tokens in its environment (openshell:resolve:env:ANTHROPIC_API_KEY). The proxy rewrites these to real credentials on the wire before forwarding to the upstream API.

Setup

1. Edit deploy/openshell/routes.yaml -- uncomment your provider:

routes:
  # OpenAI
  - name: inference.local
    endpoint: https://api.openai.com/v1
    model: gpt-5
    provider_type: openai
    protocols:
      - openai_chat_completions
      - model_discovery
    api_key_env: OPENAI_API_KEY

  # Or Anthropic
  - name: inference.local
    endpoint: https://api.anthropic.com
    model: claude-sonnet-4-6
    provider_type: anthropic
    protocols:
      - anthropic_messages
    api_key_env: ANTHROPIC_API_KEY

2. Start with --inference-routes and point turnstone at inference.local:

openshell sandbox run \
  --inference-routes deploy/openshell/routes.yaml \
  --base-url https://inference.local \
  ...

3. When inference routing is active, the openai_api and anthropic_api network policies can be removed from the sandbox policy -- the proxy handles LLM traffic on a separate code path that bypasses OPA entirely.

Local model servers

For local servers (vLLM, llama.cpp) with no authentication, omit both api_key and api_key_env from the route config. No credential resolution is needed.

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MCP Server Subprocesses

MCP servers using stdio transport are spawned as child processes of turnstone. They automatically inherit all sandbox constraints:

• Network namespace -- kernel-level, cannot be bypassed
• Landlock filesystem -- kernel-level, cannot be relaxed
• Seccomp socket filter -- kernel-level, inherited on fork

No per-subprocess policy entries are needed for these constraints. However, if an MCP server makes outbound network requests (through the proxy), its binary must appear in a binaries[] entry for the relevant network policy. The proxy identifies the requesting process via /proc/<pid>/exe (not argv[0], which is spoofable).

Example for a Python-based MCP server that calls an external API:

  mcp_external_api:
    name: mcp-external
    endpoints:
      - host: api.example.com
        port: 443
    binaries:
      - path: /usr/bin/python3*
      - path: /usr/local/bin/python3*

MCP servers using streamable-http transport are remote -- they need a network policy entry for their host:port but no binary entry (the Python process making the HTTP call is already covered by the standard python3* binary entries).

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Security Model: Which Layer Enforces What

                      OpenShell (infrastructure)          Turnstone (application)
                     ─────────────────────────────     ──────────────────────────────
Filesystem access     Landlock kernel enforcement       (no enforcement)
Network egress        Netns + seccomp + proxy + OPA     SSRF check on web_fetch
Credentials           Placeholder injection + proxy     Output guard redaction
Privilege level       setuid drop + verification        (no enforcement)
Tool semantics        (no visibility)                   Heuristic + LLM judge
Tool policies         (no visibility)                   fnmatch admin policies
Prompt injection      (no visibility)                   Output guard detection
Human approval        (no visibility)                   Approval gate + "always"

OpenShell constrains what the process can physically reach. Turnstone constrains what the LLM does with its legitimate access. Neither layer is sufficient alone:

• Without OpenShell: a bash command can curl secrets to any endpoint, write to /etc/crontab, or read ~/.ssh/id_rsa -- all gated only by human approval
• Without Turnstone: the LLM can rm -rf the entire workdir, run destructive commands, or consume prompt injection payloads -- all within the sandbox's allowed scope

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Hardening Checklist

For production deployments:

• Set landlock.compatibility: hard_requirement
• Enable inference routing (removes API keys from sandbox)
• Remove openai_api/anthropic_api network policies when using inference routing (traffic goes through the router, not direct)
• Review and trim web_fetch_common domains to your actual needs
• Remove package_registries policy if pip/uv installs are not needed
• Add your OIDC provider endpoint if using SSO
• Consider removing bash_network_tools entirely if bash should not have network access