git.stella-ops.org/docs/policy/dsl.md

Stella Policy DSL (stella-dsl@1)

Audience: Policy authors, reviewers, and tooling engineers building lint/compile flows for the Policy Engine v2 rollout (Sprint 20).

This document specifies the stella-dsl@1 grammar, semantics, and guardrails used by Stella Ops to transform SBOM facts, Concelier advisories, and Excititor VEX statements into effective findings. Use it with the Policy Engine Overview for architectural context and the upcoming lifecycle/run guides for operational workflows.

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1 · Design Goals

• Deterministic: Same policy + same inputs ⇒ identical findings on every machine.
• Declarative: No arbitrary loops, network calls, or clock access.
• Explainable: Every decision records the rule, inputs, and rationale in the explain trace.
• Lean authoring: Common precedence, severity, and suppression patterns are first-class.
• Offline-friendly: Grammar and built-ins avoid cloud dependencies, run the same in sealed deployments.

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2 · Document Structure

Policy packs ship one or more .stella files. Each file contains exactly one policy block:

policy "Default Org Policy" syntax "stella-dsl@1" {
  metadata {
    description = "Baseline severity + VEX precedence"
    tags = ["baseline","vex"]
  }

  profile severity {
    map vendor_weight {
      source "GHSA" => +0.5
      source "OSV"  => +0.0
      source "VendorX" => -0.2
    }
    env exposure_adjustments {
      if env.runtime == "serverless" then -0.5
      if env.exposure == "internal-only" then -1.0
    }
  }

  rule vex_precedence priority 10 {
    when vex.any(status in ["not_affected","fixed"])
      and vex.justification in ["component_not_present","vulnerable_code_not_present"]
    then status := vex.status
    because "Strong vendor justification prevails";
  }
}

High-level layout:

Section	Purpose
metadata	Optional descriptive fields surfaced in Console/CLI.
imports	Reserved for future reuse (not yet implemented in @1).
profile blocks	Declarative scoring modifiers (severity, trust, reachability).
rule blocks	When/then logic applied to each (component, advisory, vex[]) tuple.
settings	Optional evaluation toggles (sampling, default status overrides).

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3 · Lexical Rules

• Case sensitivity: Keywords are lowercase; identifiers are case-sensitive.
• Whitespace: Space, tab, newline act as separators. Indentation is cosmetic.
• Comments: // inline and /* block */ are ignored.
• Literals:
  • Strings use double quotes ("text"); escape with \", \n, \t.
  • Numbers are decimal; suffix % allowed for percentage weights (-2.5% becomes -0.025).
  • Booleans: true, false.
  • Lists: [1, 2, 3], ["a","b"].
• Identifiers: Start with letter or underscore, continue with letters, digits, _.
• Operators: =, ==, !=, <, <=, >, >=, in, not in, and, or, not, :=.

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4 · Grammar (EBNF)

policy      = "policy", string, "syntax", string, "{", policy-body, "}" ;
policy-body = { metadata | profile | settings | rule | helper } ;

metadata    = "metadata", "{", { meta-entry }, "}" ;
meta-entry  = identifier, "=", (string | list) ;

profile     = "profile", identifier, "{", { profile-item }, "}" ;
profile-item= map | env-map | scalar ;
map         = "map", identifier, "{", { "source", string, "=>", number, ";" }, "}" ;
env-map     = "env", identifier, "{", { "if", expression, "then", number, ";" }, "}" ;
scalar      = identifier, "=", (number | string | list), ";" ;

settings    = "settings", "{", { setting-entry }, "}" ;
setting-entry = identifier, "=", (number | string | boolean), ";" ;

rule        = "rule", identifier, [ "priority", integer ], "{",
                 "when", predicate,
                 { "and", predicate },
                 "then", { action },
                 [ "else", { action } ],
                 [ "because", string ],
             "}" ;

predicate   = expression ;
expression  = term, { ("and" | "or"), term } ;
term        = ["not"], factor ;
factor      = comparison | membership | function-call | literal | identifier | "(" expression ")" ;
comparison  = value, comparator, value ;
membership  = value, ("in" | "not in"), list ;
value       = identifier | literal | function-call | field-access ;
field-access= identifier, { ".", identifier | "[" literal "]" } ;
function-call = identifier, "(", [ arg-list ], ")" ;
arg-list    = expression, { ",", expression } ;
literal     = string | number | boolean | list ;

action      = assignment | ignore | escalate | require | warn | defer | annotate ;
assignment  = target, ":=", expression, ";" ;
target      = identifier, { ".", identifier } ;
ignore      = "ignore", [ "until", expression ], [ "because", string ], ";" ;
escalate    = "escalate", [ "to", expression ], [ "when", expression ], ";" ;
require     = "requireVex", "{", require-fields, "}", ";" ;
warn        = "warn", [ "message", string ], ";" ;
defer       = "defer", [ "until", expression ], ";" ;
annotate    = "annotate", identifier, ":=", expression, ";" ;

Notes:

• helper is reserved for shared calculcations (not yet implemented in @1).
• else branch executes only if when predicates evaluate truthy and no prior rule earlier in priority handled the tuple.
• Semicolons inside rule bodies are optional when each clause is on its own line; the compiler emits canonical semicolons in IR.

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5 · Evaluation Context

Within predicates and actions you may reference the following namespaces:

Namespace	Fields	Description
sbom	purl, name, version, licenses, layerDigest, tags, usedByEntrypoint	Component metadata from Scanner.
advisory	id, source, aliases, severity, cvss, publishedAt, modifiedAt, content.raw	Canonical Concelier advisory view.
vex	status, justification, statementId, timestamp, scope	Current VEX statement when iterating; aggregator helpers available.
vex.any(...), vex.all(...), vex.count(...)	Functions operating over all matching statements.
run	policyId, policyVersion, tenant, timestamp	Metadata for explain annotations.
env	Arbitrary key/value pairs injected per run (e.g., environment, runtime).
telemetry	Optional reachability signals; missing fields evaluate to unknown.
profile.<name>	Values computed inside profile blocks (maps, scalars).

Missing fields evaluate to null, which is falsey in boolean context and propagates through comparisons unless explicitly checked.

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6 · Built-ins (v1)

Function / Property	Signature	Description
normalize_cvss(advisory)	Advisory → SeverityScalar	Parses advisory.content.raw for CVSS data; falls back to policy maps.
cvss(score, vector)	double × string → SeverityScalar	Constructs a severity object manually.
severity_band(value)	string → SeverityBand	Normalises strings like "critical", "medium".
risk_score(base, modifiers...)	Variadic	Multiplies numeric modifiers (severity × trust × reachability).
vex.any(predicate)	(Statement → bool) → bool	true if any statement satisfies predicate.
vex.all(predicate)	(Statement → bool) → bool	true if all statements satisfy predicate.
vex.latest()	→ Statement	Lexicographically newest statement.
advisory.has_tag(tag)	string → bool	Checks advisory metadata tags.
advisory.matches(pattern)	string → bool	Glob match against advisory identifiers.
sbom.has_tag(tag)	string → bool	Uses SBOM inventory tags (usage vs inventory).
exists(expression)	→ bool	true when value is non-null/empty.
coalesce(a, b, ...)	→ value	First non-null argument.
days_between(dateA, dateB)	→ int	Absolute day difference (UTC).
percent_of(part, whole)	→ double	Fractions for scoring adjustments.
lowercase(text)	string → string	Normalises casing deterministically (InvariantCulture).

All built-ins are pure; if inputs are null the result is null unless otherwise noted.

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7 · Rule Semantics

1. Ordering: Rules execute in ascending priority. When priorities tie, lexical order defines precedence.
2. Short-circuit: Once a rule sets status, subsequent rules only execute if they use combine. Use this sparingly to avoid ambiguity.
3. Actions:
   • status := <string> – Allowed values: affected, not_affected, fixed, suppressed, under_investigation, escalated.
   • severity := <SeverityScalar> – Either from normalize_cvss, cvss, or numeric map; ensures normalized and score.
   • ignore until <ISO-8601> – Temporarily treats finding as suppressed until timestamp; recorded in explain trace.
   • warn message "<text>" – Adds warn verdict and deducts warnPenalty.
   • escalate to severity_band("critical") when condition – Forces verdict severity upward when condition true.
   • requireVex { vendors = ["VendorX"], justifications = ["component_not_present"] } – Fails evaluation if matching VEX evidence absent.
   • annotate reason := "text" – Adds free-form key/value pairs to explain payload.
4. Because clause: Mandatory for actions changing status or severity; captured verbatim in explain traces.

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8 · Scoping Helpers

• Maps: Use profile severity { map vendor_weight { ... } } to declare additive factors. Retrieve with profile.severity.vendor_weight["GHSA"].
• Environment overrides: env profiles allow conditional adjustments based on runtime metadata.
• Tenancy: run.tenant ensures policies remain tenant-aware; avoid hardcoding single-tenant IDs.
• Default values: Use settings { default_status = "affected"; } to override built-in defaults.

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9 · Examples

9.1 Baseline Severity Normalisation

rule advisory_normalization {
  when advisory.source in ["GHSA","OSV"]
  then severity := normalize_cvss(advisory)
  because "Align vendor severity to CVSS baseline";
}

9.2 VEX Override with Quiet Mode

rule vex_strong_claim priority 5 {
  when vex.any(status == "not_affected")
       and vex.justification in ["component_not_present","vulnerable_code_not_present"]
  then status := vex.status
       annotate winning_statement := vex.latest().statementId
       warn message "VEX override applied"
  because "Strong VEX justification";
}

9.3 Environment-Specific Escalation

rule internet_exposed_guard {
  when env.exposure == "internet"
       and severity.normalized >= "High"
  then escalate to severity_band("Critical")
  because "Internet-exposed assets require critical posture";
}

9.4 Anti-pattern (flagged by linter)

rule catch_all {
  when true
  then status := "suppressed"
  because "Suppress everything"  // ❌ Fails lint: unbounded suppression
}

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10 · Validation & Tooling

• stella policy lint ensures:
  • Grammar compliance and canonical formatting.
  • Static determinism guard (no forbidden namespaces).
  • Anti-pattern detection (e.g., unconditional suppression, missing because).
• stella policy compile emits IR (.stella.ir.json) and SHA-256 digest used in policy_runs.
• CI pipelines (see DEVOPS-POLICY-20-001) compile sample packs and fail on lint violations.
• Simulation harnesses (stella policy simulate) highlight provided/queried fields so policy authors affirm assumptions before promotion.

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11 · Anti-patterns & Mitigations

Anti-pattern	Risk	Mitigation
Catch-all suppress/ignore without scope	Masks all findings	Linter blocks rules with when true unless priority > 1000 and justification includes remediation plan.
Comparing strings with inconsistent casing	Missed matches	Wrap comparisons in lowercase(value) to align casing or normalise metadata during ingest.
Referencing telemetry without fallback	Null propagation	Wrap access in exists(telemetry.reachability).
Hardcoding tenant IDs	Breaks multi-tenant	Prefer env.tenantTag or metadata-sourced predicates.
Duplicated rule names	Explain trace ambiguity	Compiler enforces unique rule identifiers within a policy.

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12 · Versioning & Compatibility

• syntax "stella-dsl@1" is mandatory.
• Future revisions (@2, …) will be additive; existing packs continue to compile with their declared version.
• The compiler canonicalises documents (sorted keys, normalised whitespace) before hashing to ensure reproducibility.

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13 · Compliance Checklist

• Grammar validated: Policy compiles with stella policy lint and matches syntax "stella-dsl@1".
• Deterministic constructs only: No use of forbidden namespaces (DateTime.Now, Guid.NewGuid, external services).
• Rationales present: Every status/severity change includes a because clause or annotate entry.
• Scoped suppressions: Rules that ignore/suppress findings reference explicit components, vendors, or VEX justifications.
• Explain fields verified: annotate keys align with Console/CLI expectations (documented in upcoming lifecycle guide).
• Offline parity tested: Policy pack simulated in sealed mode (--sealed) to confirm absence of network dependencies.

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Last updated: 2025-10-26 (Sprint 20).