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Yichen Jiang 7606a99813 feat(sandbox): deny confined executions read access to the credential document

The credential store is 0600 under a 0700 directory, which stops other OS
users but not the model: tool processes run as the same user, so under
the shipped danger-full-access default they read it like any other file.

SandboxExecutionPolicy grows readDenyPaths, and sandbox-policy defaults
it to $DSH_HOME/.env — the exact file rather than the harness home, so
the model keeps its documented access to its own session log. Seatbelt
appends a trailing deny (last matching rule wins) and bwrap maps
/dev/null over each path after any workspace bind; Landlock grants are a
pure allow-list that cannot subtract from its own / read grant, so
confine() reports partial enforcement there instead of claiming a
boundary the process does not have.

A real-kernel Seatbelt e2e proves the shape: the same read succeeds
unconfined and fails under the denial, while a sibling file in the same
directory stays readable. Both READMEs state the residual boundary
plainly — no confining mode means no boundary — and record the OS
keychain provider as the real answer.

2026-07-30 16:02:13 +08:00

5.0 KiB

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dsh-sandbox-policy — the sandbox policy home (`ctx.sandboxPolicy`)

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The single owner of sandbox-policy resolution: the deployment's default SandboxMode and fallback root, plus each session's durable mode override and immutable workspace root. Every enforcing capability family receives one resolved mode-and-root policy per call.

Why a shared home

Two families enforce the same mode vocabulary: the sandboxed bash executor (@deepseek-ai/dsh-bash-sandbox) and the sandboxed filesystem provider (@deepseek-ai/dsh-fs-sandbox). If each resolved its own mode + workspaceRoot, the two could drift into a split world — bash confined to one root while fs fences another, exactly what the sandbox RFC warns against. Both tool layers resolve policy through ctx.sandboxPolicy, and both enforcing backends consume that complete per-call result. The cross-family fs sandbox RFC records the shared-policy decision.

Config

mode — the deployment default SandboxMode (read-only / workspace-write / danger-full-access), validated at load. Default read-only (fail-safe).
workspaceRoot — the fallback directory workspace-write may write under for agentless calls or sessions without a cwd. Default process.cwd(), resolved to its absolute filesystem identity either way. A normal agent call uses its session header's immutable cwd instead.

Read denials

readDenyPaths names absolute paths a confined execution must not read, whatever its mode otherwise permits. Omitted (or empty) denies the harness credential document $DSH_HOME/.env; a non-empty list replaces that default. Denials name exact paths rather than roots on purpose: denying the whole harness home would also take away the model's documented access to its own session log.

Enforcement is backend-shaped. Seatbelt appends a trailing deny file-read* file-write* (last matching rule wins) and bwrap maps /dev/null over each path after any workspace bind; Landlock grants are a pure allow-list, so a read grant on / cannot be subtracted from and confine() reports partial enforcement rather than pretending the boundary exists. danger-full-access confines nothing at all, so no denial applies there — the credential document is then protected only by its file mode, which does not stop a same-UID tool process.

Surface

ctx.sandboxPolicy.resolve({ session?, mode? }) — resolves one complete per-call policy. An explicit approved mode outranks the session's last sandbox/mode event, which outranks defaultMode; the session's immutable cwd is canonicalized with filesystem semantics before becoming workspaceRoot, otherwise the configured fallback applies. Canonicalization precedes lexical normalization so symlink/.. agrees with process working-directory resolution.
ctx.sandboxPolicy.defaultMode / ctx.sandboxPolicy.workspaceRoot — the deployment default and fallback root used by resolve().
effectiveSandboxMode(events) — the pure fold of a session's sandbox/mode events (the last switch wins, or undefined), used inside resolve().
setSandboxMode(session, mode) — THE write path for a per-session override: appends exactly one sandbox/mode event. The switch IS its event; nothing mutates the mode out of band.
SANDBOX_MODES — every mode, for option advertisement and runtime validation.

The optional ./invariant companion rejects a forged durable sandbox/mode event whose value falls outside that closed vocabulary; Session and its companion own the surrounding storage and core execution-enclosure rules.

The per-session store

A runtime switch is one log-only sandbox/mode event on the session it applies to. effective = explicit grant ?? fold(events) ?? deployment default, so an override survives restart by replay and two sessions never see each other's state. Workspace identity does not need another event: the immutable SessionHeader.cwd recorded at creation is the root for every call in that session. The event is log-only (the approval/* precedent): the model learns the mode from the enforcing tools' denial markers, never from the event.

Model Experience

Indirectly, through dsh-tool-bash and dsh-tool-fs, which render the effective mode this service holds in their [sandbox: …] denial markers and escalation prompts; the sandbox/mode event itself never reaches the model.

KV Cache effect

No direct invalidation; the named consumers own any request-prefix changes, and the mode is deliberately absent from the prompt.

Known Limitations and Deferred Work

One primary workspace root per session — policy resolves SessionHeader.cwd; extra writable roots are not part of SandboxExecutionPolicy.
File-effect modes only — SandboxMode governs file effects; network and process policy are outside its vocabulary, so no knob here restricts them.

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dsh-sandbox-policy — the sandbox policy home (ctx.sandboxPolicy)