jmgilman · Jun 28, 2026 · Jun 28, 2026
diff --git a/.github/workflows/attest.yml b/.github/workflows/attest.yml
@@ -0,0 +1,84 @@
+# Reusable workflow that generates artifact PROVENANCE in isolation from the build
+# jobs that call it. A reusable workflow runs in its own execution context with its
+# own OIDC identity — the caller can inject neither steps nor secrets — so the
+# signing material is unreachable by the build's user-defined steps. That isolation
+# is the SLSA Build L3 requirement; GitHub artifact attestations generated in-job
+# are only L2. Provenance is still written to GitHub's attestation API, so
+# `gh attestation verify` keeps working — but the signer-workflow is now this file:
+#
+#   gh attestation verify <artifact-or-oci-ref> --repo <repo> \
+#     --signer-workflow <repo>/.github/workflows/attest.yml --source-ref refs/tags/<tag>
+#
+# SLSA note: L3 makes the signed provenance unforgeable; it does not, by itself,
+# make the build hermetic. The build job still computes the checksums/digest it
+# passes in — L3's guarantee is that the signature cannot be tampered with because
+# the key lives only in this isolated workflow.
+
+name: Attest (reusable)
+
+on:
+  workflow_call:
+    inputs:
+      checksums-artifact:
+        description: Name of an uploaded artifact containing checksums.txt (binary provenance).
+        type: string
+        required: false
+        default: ''
+      subject-name:
+        description: Fully-qualified image/chart name without tag/digest (OCI provenance).
+        type: string
+        required: false
+        default: ''
+      subject-digest:
+        description: Image/chart digest in sha256:... form (OCI provenance).
+        type: string
+        required: false
+        default: ''
+      push-to-registry:
+        description: Attach the OCI provenance attestation to the registry.
+        type: boolean
+        required: false
+        default: false
+
+permissions: {}
+
+jobs:
+  attest:
+    name: Attest
+    runs-on: ubuntu-24.04
+    permissions:
+      id-token: write       # OIDC token minted HERE, isolated from the build job
+      attestations: write   # write provenance to GitHub's attestation API
+      contents: read
+      packages: write       # used only for the OCI attestation (push-to-registry)
+    steps:
+      - name: Download checksums
+        if: ${{ inputs.checksums-artifact != '' }}
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        with:
+          name: ${{ inputs.checksums-artifact }}
+
+      - name: Attest binary checksums
+        if: ${{ inputs.checksums-artifact != '' }}
+        uses: actions/attest@59d89421af93a897026c735860bf21b6eb4f7b26 # v4.1.0
+        with:
+          subject-checksums: checksums.txt
+
+      # The OCI provenance is attached to the registry as a referrer, so this
+      # isolated job needs its own GHCR login — the build job's docker login does
+      # not carry into the reusable workflow's separate runner.
+      - name: Log in to GitHub Container Registry
+        if: ${{ inputs.subject-digest != '' }}
+        uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121 # v4
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ github.token }}
+
+      - name: Attest OCI provenance
+        if: ${{ inputs.subject-digest != '' }}
+        uses: actions/attest-build-provenance@0f67c3f4856b2e3261c31976d6725780e5e4c373 # v4.1.1
+        with:
+          subject-name: ${{ inputs.subject-name }}
+          subject-digest: ${{ inputs.subject-digest }}
+          push-to-registry: ${{ inputs.push-to-registry }}
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
@@ -80,8 +80,6 @@ jobs:
       - resolve-release
     permissions:
       contents: write
-      id-token: write
-      attestations: write
     steps:
       - name: Check out repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
@@ -151,10 +149,31 @@ jobs:
           RELEASE_TAG: ${{ needs.resolve-release.outputs.tag }}
         run: gh release upload "$RELEASE_TAG" dist/release-assets/* --clobber
 
-      - name: Attest release checksums
-        uses: actions/attest@59d89421af93a897026c735860bf21b6eb4f7b26 # v4.1.0
+      - name: Upload checksums for isolated attestation
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          subject-checksums: dist/release-assets/checksums.txt
+          name: release-checksums
+          path: dist/release-assets/checksums.txt
+          if-no-files-found: error
+          retention-days: 1
+
+  # Generate binary provenance in an ISOLATED reusable workflow (SLSA L3): the
+  # signing OIDC token is minted in attest.yml, unreachable by the build steps.
+  attest-binaries:
+    needs:
+      - binary-release-assets
+    permissions:
+      id-token: write
+      attestations: write
+      contents: read
+      # attest.yml's shared job declares packages: write (for the OCI
+      # attestation's push-to-registry). A reusable workflow cannot request more
+      # permissions than its caller grants, so every caller must grant it — even
+      # the binary one, whose attestation never pushes to a registry.
+      packages: write
+    uses: ./.github/workflows/attest.yml
+    with:
+      checksums-artifact: release-checksums
 
   # Build the per-arch signed apk on a native runner (no QEMU). Each runner mints
   # its own ephemeral melange key (distinct filename) and uploads its apk plus its
@@ -359,15 +378,22 @@ jobs:
           sbom-path: image.spdx.json
           push-to-registry: true
 
-      # SLSA provenance (GitHub artifact attestation). This is the attestation the
-      # chart's optional Kyverno policy verifies; the signer stays release.yml here.
-      # Moving it into an isolated reusable workflow (SLSA L3) is a later change.
-      - name: Attest container image
-        uses: actions/attest@59d89421af93a897026c735860bf21b6eb4f7b26 # v4.1.0
-        with:
-          subject-name: ${{ steps.manifest.outputs.name }}
-          subject-digest: ${{ steps.manifest.outputs.digest }}
-          push-to-registry: true
+  # Generate container provenance in the ISOLATED reusable workflow (SLSA L3). This
+  # is the attestation the chart's optional Kyverno policy verifies — its signer is
+  # now attest.yml, which the chart's Kyverno defaults trust.
+  attest-image:
+    needs:
+      - container-image-release
+    permissions:
+      id-token: write
+      attestations: write
+      packages: write
+      contents: read
+    uses: ./.github/workflows/attest.yml
+    with:
+      subject-name: ${{ needs.container-image-release.outputs.image-name }}
+      subject-digest: ${{ needs.container-image-release.outputs.image-digest }}
+      push-to-registry: true
 
   helm-chart-release:
     name: Helm Chart Release
@@ -378,10 +404,8 @@ jobs:
     permissions:
       contents: read
       packages: write
-      id-token: write
-      attestations: write
-      artifact-metadata: write
     outputs:
+      chart-name: ${{ steps.push-chart.outputs.name }}
       chart-digest: ${{ steps.push-chart.outputs.digest }}
     steps:
       - name: Check out repository
@@ -486,14 +510,23 @@ jobs:
             exit "$push_rc"
           fi
 
+          echo "name=$CHART_NAME" >> "$GITHUB_OUTPUT"
           echo "digest=$digest" >> "$GITHUB_OUTPUT"
 
-      - name: Attest Helm chart
-        uses: actions/attest@59d89421af93a897026c735860bf21b6eb4f7b26 # v4.1.0
-        with:
-          subject-name: ${{ env.CHART_NAME }}
-          subject-digest: ${{ steps.push-chart.outputs.digest }}
-          push-to-registry: true
+  # Generate Helm chart provenance in the ISOLATED reusable workflow (SLSA L3).
+  attest-chart:
+    needs:
+      - helm-chart-release
+    permissions:
+      id-token: write
+      attestations: write
+      packages: write
+      contents: read
+    uses: ./.github/workflows/attest.yml
+    with:
+      subject-name: ${{ needs.helm-chart-release.outputs.chart-name }}
+      subject-digest: ${{ needs.helm-chart-release.outputs.chart-digest }}
+      push-to-registry: true
 
   release-inspection-summary:
     name: Release Inspection Summary
@@ -503,6 +536,9 @@ jobs:
       - binary-release-assets
       - container-image-release
       - helm-chart-release
+      - attest-binaries
+      - attest-image
+      - attest-chart
     permissions: {}
     steps:
       - name: Write inspection summary
@@ -523,7 +559,7 @@ jobs:
             echo '```sh'
             echo "gh release view $RELEASE_TAG --repo $GITHUB_REPOSITORY --json isDraft,assets"
             echo "asset=\"template-k8s_${RELEASE_VERSION}_\$(go env GOOS)_\$(go env GOARCH)\""
-            echo "gh attestation verify \"dist/release-assets/\${asset}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/release.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
+            echo "gh attestation verify \"dist/release-assets/\${asset}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/attest.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
             echo '```'
             echo
             echo "Container verification commands:"
@@ -532,7 +568,8 @@ jobs:
             echo "docker login ghcr.io"
             echo "docker pull \"${IMAGE_NAME}:${RELEASE_TAG}\""
             echo "docker run --rm \"${IMAGE_NAME}:${RELEASE_TAG}\" --help"
-            echo "gh attestation verify \"oci://${IMAGE_NAME}@${IMAGE_DIGEST}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/release.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
+            echo "gh attestation verify \"oci://${IMAGE_NAME}@${IMAGE_DIGEST}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/attest.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
+            echo "cosign verify \"${IMAGE_NAME}@${IMAGE_DIGEST}\" --certificate-identity-regexp \"^https://github.com/${GITHUB_REPOSITORY}/.github/workflows/release.yml@.*\" --certificate-oidc-issuer https://token.actions.githubusercontent.com"
             echo '```'
             echo
             echo "Helm chart verification commands:"
@@ -541,7 +578,7 @@ jobs:
             echo "helm show chart \"${CHART_REF}\" --version \"${RELEASE_VERSION}\""
             echo "helm pull \"${CHART_REF}\" --version \"${RELEASE_VERSION}\""
             echo "helm install template-k8s \"${CHART_REF}\" --version \"${RELEASE_VERSION}\" --namespace template-k8s-system --create-namespace"
-            echo "gh attestation verify \"${CHART_REF}@${CHART_DIGEST}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/release.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
+            echo "gh attestation verify \"${CHART_REF}@${CHART_DIGEST}\" --repo \"$GITHUB_REPOSITORY\" --signer-workflow \"$GITHUB_REPOSITORY/.github/workflows/attest.yml\" --source-ref \"refs/tags/$RELEASE_TAG\" --deny-self-hosted-runners"
             echo '```'
             echo
             echo "Publish or reject the draft release manually after inspection. The container image and Helm chart are already available in GHCR because GHCR does not have a draft release state."

diff --git a/DELETE_ME.md b/DELETE_ME.md
@@ -220,8 +220,8 @@ that shape, trim the release files before the first release.
 - `charts/template-k8s/values.yaml`
   - Update `image.repository`.
   - Update `kyverno.imageVerification.attestor.subjectRegExp` so optional
-    Kyverno image verification trusts the generated repository's release
-    workflow.
+    Kyverno image verification trusts the generated repository's provenance
+    signer, the reusable `.github/workflows/attest.yml` (not `release.yml`).
   - Add, remove, or rename values for real controller runtime options.
   - Keep fixed image tags or digests; do not default to `latest`.
 
@@ -243,7 +243,7 @@ that shape, trim the release files before the first release.
 
 - `charts/template-k8s/templates/kyverno-image-policy.yaml`
   - Update the policy name helper and default attestor subject if the chart or
-    release workflow identity changes.
+    the provenance signer workflow (`attest.yml`) identity changes.
   - Keep it optional unless Kyverno is a hard prerequisite for the generated
     repository.
 
@@ -290,6 +290,12 @@ that shape, trim the release files before the first release.
   - Update Helm chart paths, rendered-output assertions, install examples, and
     release inspection summary commands.
 
+- `.github/workflows/attest.yml`
+  - The reusable workflow that signs binary/image/chart provenance in isolation
+    (SLSA Build L3). It has no project-specific identifiers, but it IS the signer
+    identity the Kyverno policy and the release inspection summary trust — keep it
+    in sync with `kyverno.imageVerification.attestor.subjectRegExp`.
+
 - `.github/workflows/release-dry-run.yml`
   - Update image and chart refs.
   - Update binary validation names, dry-run image names, OCI archive names,

diff --git a/charts/template-k8s/values.yaml b/charts/template-k8s/values.yaml
@@ -23,7 +23,7 @@ kyverno:
     attestor:
       issuer: https://token.actions.githubusercontent.com
       subject: ""
-      subjectRegExp: ^https://github\.com/meigma/template-k8s/\.github/workflows/release\.yml@refs/tags/v[0-9]+\.[0-9]+\.[0-9]+(-[0-9A-Za-z][0-9A-Za-z.-]*)?$
+      subjectRegExp: ^https://github\.com/meigma/template-k8s/\.github/workflows/attest\.yml@refs/tags/v[0-9]+\.[0-9]+\.[0-9]+(-[0-9A-Za-z][0-9A-Za-z.-]*)?$
       rekor:
         url: https://rekor.sigstore.dev
     attestation:

diff --git a/test/chart/rbac_test.go b/test/chart/rbac_test.go
@@ -101,7 +101,7 @@ func TestKyvernoImageVerificationPolicyRendersGitHubAttestationPolicy(t *testing
 	requireNestedString(
 		t,
 		keyless,
-		"^https://github\\.com/meigma/template-k8s/\\.github/workflows/release\\.yml@refs/tags/"+
+		"^https://github\\.com/meigma/template-k8s/\\.github/workflows/attest\\.yml@refs/tags/"+
 			"v[0-9]+\\.[0-9]+\\.[0-9]+(-[0-9A-Za-z][0-9A-Za-z.-]*)?$",
 		"subjectRegExp",
 	)