Tighten legacy instance config disk permissions on startup

Author: cursoragent

lib/instances/manager.go

@@ -311,6 +311,9 @@ func NewManagerWithConfigE(p *paths.Paths, imageManager images.Manager, systemMa
 	// Restrict permissions on metadata written by older versions (may be 0644
 	// and contains env values / credential bindings).
 	m.tightenMetadataPermissions()
+	// Restrict permissions on guest config disks written by older versions
+	// (may be 0644 and embed config.json with env values).
+	m.tightenConfigDiskPermissions()
 
 	return m, nil
 }

lib/instances/metadata_permissions_test.go

@@ -86,6 +86,26 @@ func TestManagerTightensLegacyMetadataPermissions(t *testing.T) {
 		"legacy 0644 metadata must be tightened to 0600 at startup")
 }
 
+// TestManagerTightensLegacyConfigDiskPermissions proves the startup sweep
+// upgrades config disks written by older versions (mode 0644) to 0600.
+func TestManagerTightensLegacyConfigDiskPermissions(t *testing.T) {
+	t.Parallel()
+	dataDir := t.TempDir()
+	p := paths.New(dataDir)
+	id := "inst-config-disk-legacy"
+
+	// Simulate a legacy config disk written with world-readable permissions.
+	require.NoError(t, os.MkdirAll(p.InstanceDir(id), 0755))
+	require.NoError(t, os.WriteFile(p.InstanceConfigDisk(id), []byte("ext4-bytes-placeholder"), 0644))
+
+	newPermTestManager(t, dataDir)
+
+	info, err := os.Stat(p.InstanceConfigDisk(id))
+	require.NoError(t, err)
+	require.Equal(t, os.FileMode(0600), info.Mode().Perm(),
+		"legacy 0644 config disks must be tightened to 0600 at startup")
+}
+
 // TestMergeEnvUpdateSkipsRedactionSentinel proves a redacted read response
 // round-tripped into an env update cannot clobber real secret values.
 func TestMergeEnvUpdateSkipsRedactionSentinel(t *testing.T) {

lib/instances/storage.go

@@ -154,6 +154,33 @@ func (m *manager) tightenMetadataPermissions() {
 	}
 }
 
+// tightenConfigDiskPermissions restricts existing guest config disk files to
+// owner-only access. Disks written before restrictive permissions were
+// introduced may be mode 0644; they embed config.json with environment values.
+// Best-effort: individual failures are logged, not fatal.
+func (m *manager) tightenConfigDiskPermissions() {
+	log := logger.FromContext(context.Background())
+	entries, err := os.ReadDir(m.paths.GuestsDir())
+	if err != nil {
+		return // no guests directory yet
+	}
+	for _, entry := range entries {
+		if !entry.IsDir() {
+			continue
+		}
+		configDiskPath := m.paths.InstanceConfigDisk(entry.Name())
+		info, err := os.Stat(configDiskPath)
+		if err != nil {
+			continue
+		}
+		if info.Mode().Perm() != 0600 {
+			if err := os.Chmod(configDiskPath, 0600); err != nil {
+				log.Warn("failed to tighten instance config disk permissions", "path", configDiskPath, "error", err)
+			}
+		}
+	}
+}
+
 // createOverlayDisk creates a sparse overlay disk for the instance
 func (m *manager) createOverlayDisk(id string, sizeBytes int64) error {
 	overlayPath := m.paths.InstanceOverlay(id)