Bound the manifest layer index in ociLayoutPackage.GetLayer

pkg/buildpack/oci_layout_package.go

@@ -145,6 +145,9 @@ func (o *ociLayoutPackage) GetLayer(diffID string) (io.ReadCloser, error) {
 	if index == -1 {
 		return nil, errors.Errorf("layer %s not found in rootfs", style.Symbol(diffID))
 	}
+	if index >= len(o.manifest.Layers) {
+		return nil, errors.Errorf("layer %s is referenced by the config rootfs but the manifest only has %d layer(s)", style.Symbol(diffID), len(o.manifest.Layers))
+	}
 
 	layerDescriptor := o.manifest.Layers[index]
 	layerPath := paths.CanonicalTarPath(pathFromDescriptor(layerDescriptor))

pkg/buildpack/oci_layout_package_internal_test.go

@@ -0,0 +1,57 @@
+package buildpack
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/heroku/color"
+	"github.com/opencontainers/go-digest"
+	v1 "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/sclevine/spec"
+	"github.com/sclevine/spec/report"
+)
+
+func TestOCILayoutPackageInternal(t *testing.T) {
+	color.Disable(true)
+	defer color.Disable(false)
+	spec.Run(t, "GetLayer", testOCILayoutPackageGetLayer, spec.Parallel(), spec.Report(report.Terminal{}))
+}
+
+// This is an internal (whitebox) test because GetLayer is an unexported method on
+// the unexported ociLayoutPackage type; the external buildpack_test package cannot
+// construct one directly.
+func testOCILayoutPackageGetLayer(t *testing.T, when spec.G, it spec.S) {
+	when("#GetLayer", func() {
+		when("the config rootfs references a layer index beyond the manifest layers", func() {
+			it("returns an error instead of panicking", func() {
+				// The config rootfs lists two diffIDs but the manifest only carries a
+				// single layer, so resolving the second diffID yields index 1 which is
+				// out of range for manifest.Layers. Before the bounds check this
+				// panicked with index out of range; it should now return an error.
+				layer0 := digest.FromString("layer-0")
+				outOfRangeDiffID := digest.FromString("layer-with-no-manifest-entry")
+
+				pkg := &ociLayoutPackage{
+					imageInfo: v1.Image{
+						RootFS: v1.RootFS{
+							DiffIDs: []digest.Digest{layer0, outOfRangeDiffID},
+						},
+					},
+					manifest: v1.Manifest{
+						Layers: []v1.Descriptor{
+							{Digest: layer0},
+						},
+					},
+				}
+
+				_, err := pkg.GetLayer(outOfRangeDiffID.String())
+				if err == nil {
+					t.Fatal("expected an error but got nil")
+				}
+				if want := "the manifest only has 1 layer(s)"; !strings.Contains(err.Error(), want) {
+					t.Fatalf("expected error to contain %q, got %q", want, err.Error())
+				}
+			})
+		})
+	})
+}