fix: confine Docker copy dependency sources

pkg/skaffold/docker/dockertest.go

@@ -106,6 +106,14 @@ func (f *fakeImageFetcher) fetch(_ context.Context, image string, _ Config) (*v1
 				},
 			},
 		}, nil
+	case "parentpath:onbuild":
+		return &v1.ConfigFile{
+			Config: v1.Config{
+				OnBuild: []string{
+					"COPY ../secret.txt /secret.txt",
+				},
+			},
+		}, nil
 	case "library/ruby:2.3.0":
 		return nil, fmt.Errorf("retrieving image \"library/ruby:2.3.0\": unsupported MediaType: \"application/vnd.docker.distribution.manifest.v1+prettyjws\", see https://github.com/google/go-containerregistry/issues/377")
 	}

pkg/skaffold/docker/parse.go

@@ -212,9 +212,13 @@ func expandSrcGlobPatterns(workspace string, cpCmds []*copyCommand) ([]FromTo, e
 		matchesOne := false
 
 		for _, p := range cpCmd.srcs {
-			path := filepath.Join(workspace, p)
+			path, relPath, err := resolveCopySourceInWorkspace(workspace, p)
+			if err != nil {
+				return nil, err
+			}
+
 			if _, err := os.Stat(path); err == nil {
-				fts = append(fts, FromTo{From: filepath.Clean(p), To: cpCmd.dest, ToIsDir: cpCmd.destIsDir, StartLine: cpCmd.startLine, EndLine: cpCmd.endLine})
+				fts = append(fts, FromTo{From: relPath, To: cpCmd.dest, ToIsDir: cpCmd.destIsDir, StartLine: cpCmd.startLine, EndLine: cpCmd.endLine})
 				matchesOne = true
 				continue
 			}
@@ -232,6 +236,9 @@ func expandSrcGlobPatterns(workspace string, cpCmds []*copyCommand) ([]FromTo, e
 				if err != nil {
 					return nil, fmt.Errorf("getting relative path of %s", f)
 				}
+				if !isRelativePathInWorkspace(rel) {
+					return nil, fmt.Errorf("copy source %q resolves outside build context %q", p, workspace)
+				}
 
 				fts = append(fts, FromTo{From: rel, To: cpCmd.dest, ToIsDir: cpCmd.destIsDir, StartLine: cpCmd.startLine, EndLine: cpCmd.endLine})
 			}
@@ -248,6 +255,37 @@ func expandSrcGlobPatterns(workspace string, cpCmds []*copyCommand) ([]FromTo, e
 	return fts, nil
 }
 
+func resolveCopySourceInWorkspace(workspace, src string) (string, string, error) {
+	src = filepath.Clean(src)
+	if filepath.IsAbs(src) {
+		return "", "", fmt.Errorf("copy source %q resolves outside build context %q", src, workspace)
+	}
+
+	workspaceAbs, err := filepath.Abs(workspace)
+	if err != nil {
+		return "", "", fmt.Errorf("resolving build context %q: %w", workspace, err)
+	}
+
+	sourceAbs, err := filepath.Abs(filepath.Join(workspace, src))
+	if err != nil {
+		return "", "", fmt.Errorf("resolving copy source %q: %w", src, err)
+	}
+
+	rel, err := filepath.Rel(workspaceAbs, sourceAbs)
+	if err != nil {
+		return "", "", fmt.Errorf("getting relative path of %s", sourceAbs)
+	}
+	if !isRelativePathInWorkspace(rel) {
+		return "", "", fmt.Errorf("copy source %q resolves outside build context %q", src, workspace)
+	}
+
+	return filepath.Join(workspace, src), rel, nil
+}
+
+func isRelativePathInWorkspace(rel string) bool {
+	return rel != ".." && !strings.HasPrefix(rel, ".."+string(filepath.Separator)) && !filepath.IsAbs(rel)
+}
+
 func extractCopyCommands(ctx context.Context, nodes []*parser.Node, onlyLastImage bool, cfg Config) ([]*copyCommand, error) {
 	stages := map[string]bool{
 		"scratch": true,

pkg/skaffold/docker/parse_test.go

@@ -116,6 +116,43 @@ func TestReadCopyCmdsFromDockerfile(t *testing.T) {
 	}
 }
 
+func TestReadCopyCmdsFromDockerfileRejectsSourcesOutsideWorkspace(t *testing.T) {
+	tests := []struct {
+		description string
+		dockerfile  string
+	}{
+		{
+			description: "direct copy parent path",
+			dockerfile:  "FROM nginx\nCOPY ../secret.txt /secret.txt",
+		},
+		{
+			description: "onbuild copy parent path",
+			dockerfile:  "FROM parentpath:onbuild",
+		},
+	}
+
+	for _, test := range tests {
+		testutil.Run(t, test.description, func(t *testutil.T) {
+			imageFetcher := fakeImageFetcher{}
+			t.Override(&RetrieveImage, imageFetcher.fetch)
+
+			tmp := t.NewTempDir()
+			tmp.Mkdir("workspace/project")
+			tmp.Write("workspace/secret.txt", "secret")
+
+			dockerfilePath := tmp.Path("workspace/project/Dockerfile")
+			err := os.WriteFile(dockerfilePath, []byte(test.dockerfile), 0644)
+			if err != nil {
+				t.Error(err)
+			}
+
+			cfg := mockConfig{mode: config.RunModes.Build}
+			_, err = ReadCopyCmdsFromDockerfile(context.Background(), false, dockerfilePath, tmp.Path("workspace/project"), make(map[string]*string), cfg)
+			t.CheckErrorContains("resolves outside build context", err)
+		})
+	}
+}
+
 func TestRemoveExtraBuildArgs(t *testing.T) {
 	tests := []struct {
 		description string