feat(guards): actions/runner-isolated — the scan, the two-condition rule, the exit codes (#58)

actions/runner-isolated/action.yml

@@ -0,0 +1,32 @@
+name: Runner isolated
+description: >-
+  Assert that no `pull_request`- (or `pull_request_target`-) triggered
+  workflow names a self-hosted runner (#58). A pull_request workflow runs
+  the PR branch's code — from a fork, unreviewed code — and a self-hosted
+  runner executes it on our own hardware, inside our own network; the
+  fork-PR write-token and secrets toggles protect credentials, not the
+  runner. The rule is file-level, deliberately: a file whose trigger block
+  names pull_request and which names self-hosted anywhere fails, even
+  across jobs — the fix is to split the workflow. Known gaps, so silence
+  is never read as coverage: workflow_call reachability is not followed
+  (a pull_request caller plus a self-hosted callee goes unseen), and
+  indirection is not resolved (runner groups, matrix or expression values
+  for runs-on). A missing workflows directory is a pass. The caller must
+  have checked out its own repository first: the guard reads the
+  consumer's tree at the workspace.
+inputs:
+  workflows-dir:
+    description: >-
+      Directory scanned for `*.yml`/`*.yaml` workflow files. A missing
+      directory passes — a guard that fails on absence is a guard nobody
+      adopts.
+    required: false
+    default: .github/workflows
+runs:
+  using: composite
+  steps:
+    - name: runner isolated
+      shell: bash
+      env:
+        WORKFLOWS_DIR: ${{ inputs.workflows-dir }}
+      run: bash "$GITHUB_ACTION_PATH/runner-isolated.sh"

actions/runner-isolated/runner-isolated.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+# runner-isolated.sh [<workflows-dir>] — assert that no `pull_request`-
+# triggered workflow in the tree names a self-hosted runner (#58; epic #56
+# decision D5).
+#
+# The threat, stated once: a `pull_request` workflow runs code from the
+# PR's branch. When that branch comes from a fork and the repo's fork-PR
+# settings do not require approval, that code is UNREVIEWED. Point such a
+# job at a self-hosted runner and unreviewed code executes on our own
+# hardware, inside our own network. Nothing else in the fleet's setup
+# gates that path — the write-token and secrets toggles (correctly off,
+# #16's ruling) protect credentials, not the runner.
+#
+# Nothing was wrong the day this was written: incubator's deploy.yml is
+# push-triggered and self-hosted (legal), its pr-checks.yml is
+# PR-triggered and hosted, and the rule lived as a sentence in
+# pr-checks.yml's header, kept true by whoever remembered it. This guard
+# is that sentence moved into CI — the same move drill-recorded made
+# after three releases shipped through a documented-but-unenforced gate
+# (its header: "that is not a gate, it is luck with good manners").
+#
+# THE RULE IS FILE-LEVEL, DELIBERATELY. The precise rule — no JOB
+# reachable from a pull_request trigger runs self-hosted — needs a YAML
+# parser, and a second parser in bash is a new class of guard bug bought
+# in exchange for permitting a file shape we do not want. So: a file
+# FAILS when its trigger block names pull_request (which also matches
+# pull_request_target — intended) AND it names a self-hosted runner
+# anywhere, even in a different job. The false positive has a clean,
+# safer fix — SPLIT THE WORKFLOW; incubator already keeps pr-checks.yml
+# apart from deploy.yml, which is the shape this guard asks for. False
+# NEGATIVES are what a security guard must not have, and file-level
+# granularity has none for the modelled threat: it can only be stricter
+# than the precise rule, never laxer.
+#
+# Self-hosted detection covers two shapes, because the same-line rule
+# alone ("runs-on and self-hosted on one line") would pass the
+# block-sequence form — a false negative, the one defect this guard is
+# not allowed to have:
+#
+#     runs-on: [self-hosted, ci-runner]      # same line: caught
+#     runs-on:                               # block sequence: the bare
+#       - self-hosted                        #   key opens a window over
+#       - ci-runner                          #   its `- …` list items
+#
+# KNOWN LIMITS, named in action.yml's description too so a consumer
+# never reads silence as coverage:
+#   - workflow_call is not treated as PR-reachable in v1: a pull_request
+#     caller plus a self-hosted callee is a real path this guard does not
+#     see. Following `uses:` across files is the YAML-parsing problem
+#     again, and the family has no such caller today (ceremony's own
+#     release-exercise.yml is ubuntu-latest).
+#   - indirection is not resolved: a runner group (`runs-on: {group: …}`)
+#     or a matrix/expression value can reach self-hosted hardware without
+#     the string appearing on any line this guard reads.
+#
+# Comments are skipped on BOTH halves of the rule: a workflow that merely
+# mentions self-hosted in prose is not the bug (incubator's pr-checks.yml
+# header is exactly that prose), and a guard that cried wolf on comments
+# would be turned off within a week. A missing workflows directory is a
+# PASS, not an error — most repos in the family have one, but a guard
+# that fails on absence is a guard nobody adopts.
+#
+# A file of its own (not inlined in action.yml) so
+# test/runner-isolated.test.sh can drive it against constructed trees —
+# the same discipline as the four guards beside it.
+
+workflows_dir="${1:-${WORKFLOWS_DIR:-.github/workflows}}"
+
+if [ ! -d "$workflows_dir" ]; then
+  echo "runner-isolated: no workflows directory at '$workflows_dir' — nothing to scan"
+  exit 0
+fi
+
+shopt -s nullglob
+files=("$workflows_dir"/*.yml "$workflows_dir"/*.yaml)
+
+if [ "${#files[@]}" -eq 0 ]; then
+  echo "runner-isolated: 0 workflow files under '$workflows_dir' — nothing to scan"
+  exit 0
+fi
+
+offenders=0
+for file in "${files[@]}"; do
+  pr_triggered=0
+  in_on=0
+  in_runs_on=0
+  hits=()
+
+  lineno=0
+  while IFS= read -r line || [ -n "$line" ]; do
+    lineno=$((lineno + 1))
+
+    # A blank line ends nothing: it is not a top-level key, and a YAML
+    # sequence may legally continue past one.
+    case "$line" in
+      *[![:space:]]*) ;;
+      *) continue ;;
+    esac
+
+    stripped="${line#"${line%%[![:space:]]*}"}"
+
+    # Comment lines are invisible to both halves of the rule.
+    case "$stripped" in
+      '#'*) continue ;;
+    esac
+
+    # A line starting a top-level key opens or closes the trigger block.
+    # YAML 1.1 parses bare `on` as a boolean, so some repos quote the
+    # key — a guard that missed '"on":' would silently pass the file it
+    # most needs to read.
+    case "$line" in
+      [![:space:]]*)
+        in_runs_on=0
+        case "$line" in
+          'on:'*| '"on":'* | "'on':"*) in_on=1 ;;
+          *) in_on=0 ;;
+        esac
+        ;;
+    esac
+
+    # Half one: the trigger. Checked on the `on:` line itself (scalar and
+    # inline-list shapes) and on every line of its block.
+    if [ "$in_on" -eq 1 ]; then
+      case "$line" in
+        *pull_request*) pr_triggered=1 ;;
+      esac
+    fi
+
+    # Half two: the runner. The window a bare `runs-on:` key opened over
+    # its list items closes at the first line that is not a `- …` item.
+    if [ "$in_runs_on" -eq 1 ]; then
+      case "$stripped" in
+        '-'*)
+          case "$line" in
+            *self-hosted*) hits+=("$lineno: $line") ;;
+          esac
+          ;;
+        *) in_runs_on=0 ;;
+      esac
+    fi
+    case "$line" in
+      *runs-on*)
+        case "$line" in
+          *self-hosted*) hits+=("$lineno: $line") ;;
+        esac
+        case "$stripped" in
+          'runs-on:' | 'runs-on:'[[:space:]]*)
+            rest="${stripped#runs-on:}"
+            rest="${rest#"${rest%%[![:space:]]*}"}"
+            case "$rest" in
+              '' | '#'*) in_runs_on=1 ;;
+            esac
+            ;;
+        esac
+        ;;
+    esac
+  done <"$file"
+
+  if [ "$pr_triggered" -eq 1 ] && [ "${#hits[@]}" -gt 0 ]; then
+    offenders=$((offenders + 1))
+    {
+      echo "runner-isolated: $file is pull_request-triggered and names a self-hosted runner:"
+      printf '    %s\n' "${hits[@]}"
+    } >&2
+  fi
+done
+
+if [ "$offenders" -gt 0 ]; then
+  cat >&2 <<EOF
+runner-isolated: $offenders offending workflow file(s). A pull_request
+  workflow runs the PR branch's code; from a fork that code is unreviewed,
+  and a self-hosted runner would execute it on our own hardware, inside
+  our own network. The unblock is to SPLIT THE WORKFLOW: PR-triggered
+  checks in one file on hosted runners, self-hosted work behind
+  push/dispatch triggers in another — the shape incubator's pr-checks.yml
+  and deploy.yml already have. The rule is file-level on purpose (this
+  script's header): a file mixing the two is one editing mistake away
+  from being the real bug.
+EOF
+  exit 1
+fi
+
+echo "runner-isolated: ${#files[@]} workflow file(s) scanned under '$workflows_dir' — no pull_request-triggered work on a self-hosted runner"