fix/batches: discard extracted Git metadata before Git initialization (#1370)

Author: cbrnrd

internal/batches/workspace/bind_workspace.go

@@ -43,6 +43,13 @@ func (wc *dockerBindWorkspaceCreator) Create(ctx context.Context, repo *graphql.
 }
 
 func (*dockerBindWorkspaceCreator) prepareGitRepo(ctx context.Context, w *dockerBindWorkspace) error {
+	// Windows can normalize archive entry names such as ".git." to ".git".
+	// Always discard extracted Git metadata before running Git so repository
+	// contents cannot configure commands that execute on the host.
+	if err := os.RemoveAll(filepath.Join(w.dir, ".git")); err != nil {
+		return errors.Wrap(err, "removing extracted git metadata")
+	}
+
 	if _, err := runGitCmd(ctx, w.dir, "init"); err != nil {
 		return errors.Wrap(err, "git init failed")
 	}

internal/batches/workspace/bind_workspace_test.go

@@ -143,6 +143,47 @@ func TestDockerBindWorkspaceCreator_Create(t *testing.T) {
 	})
 }
 
+func TestPrepareGitRepoRemovesUntrustedGitMetadata(t *testing.T) {
+	dir := t.TempDir()
+	dotGit := filepath.Join(dir, ".git")
+	if err := os.Mkdir(dotGit, 0755); err != nil {
+		t.Fatal(err)
+	}
+	maliciousConfig := "[core]\n\trepositoryformatversion = 0\n\tbare = false\n\tfsmonitor = ./fsmonitor\n"
+	if err := os.WriteFile(filepath.Join(dotGit, "config"), []byte(maliciousConfig), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dotGit, "attacker-controlled"), nil, 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "fsmonitor"), []byte("#!/bin/sh\ntouch fsmonitor-ran\n"), 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "tracked.txt"), []byte("tracked\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	creator := &dockerBindWorkspaceCreator{}
+	workspace := &dockerBindWorkspace{dir: dir}
+	if err := creator.prepareGitRepo(context.Background(), workspace); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := os.Stat(filepath.Join(dir, "fsmonitor-ran")); !os.IsNotExist(err) {
+		t.Fatalf("fsmonitor command ran on the host: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(dotGit, "attacker-controlled")); !os.IsNotExist(err) {
+		t.Fatalf("untrusted Git metadata was preserved: %v", err)
+	}
+	config, err := os.ReadFile(filepath.Join(dotGit, "config"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(string(config), "fsmonitor") {
+		t.Fatalf("untrusted Git config was preserved:\n%s", config)
+	}
+}
+
 func TestDockerBindWorkspace_DiffRestoresTrustedGitConfig(t *testing.T) {
 	archivePath := zipUpFiles(t, t.TempDir(), map[string]string{
 		"tracked.txt": "before\n",