tailcfg: report StateEncrypted in Hostinfo (#16434)

Report whether the client is configured with state encryption (which
varies by platform and can be optional on some). Wire it up to
`--encrypt-state` in tailscaled, which is set for Linux/Windows, and set
defaults for other platforms. Macsys will also report this if full
Keychain migration is done.

Updates #15830

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

feature/tpm/tpm.go

@@ -159,6 +159,8 @@ func newStore(logf logger.Logf, path string) (ipn.StateStore, error) {
 // tpmStore is an ipn.StateStore that stores the state in a secretbox-encrypted
 // file using a TPM-sealed symmetric key.
 type tpmStore struct {
+	ipn.EncryptedStateStore
+
 	logf logger.Logf
 	path string
 	key  [32]byte

ipn/ipnlocal/local.go

@@ -2244,6 +2244,7 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 	hostinfo.Userspace.Set(b.sys.IsNetstack())
 	hostinfo.UserspaceRouter.Set(b.sys.IsNetstackRouter())
 	hostinfo.AppConnector.Set(b.appConnector != nil)
+	hostinfo.StateEncrypted = b.stateEncrypted()
 	b.logf.JSON(1, "Hostinfo", hostinfo)
 
 	// TODO(apenwarr): avoid the need to reinit controlclient.
@@ -7801,3 +7802,29 @@ func (b *LocalBackend) vipServicesFromPrefsLocked(prefs ipn.PrefsView) []*tailcf
 var (
 	metricCurrentWatchIPNBus = clientmetric.NewGauge("localbackend_current_watch_ipn_bus")
 )
+
+func (b *LocalBackend) stateEncrypted() opt.Bool {
+	switch runtime.GOOS {
+	case "android", "ios":
+		return opt.NewBool(true)
+	case "darwin":
+		switch {
+		case version.IsMacAppStore():
+			return opt.NewBool(true)
+		case version.IsMacSysExt():
+			// MacSys still stores its state in plaintext on disk in addition to
+			// the Keychain. A future release will clean up the on-disk state
+			// files.
+			// TODO(#15830): always return true here once MacSys is fully migrated.
+			sp, _ := syspolicy.GetBoolean(syspolicy.EncryptState, false)
+			return opt.NewBool(sp)
+		default:
+			// Probably self-compiled tailscaled, we don't use the Keychain
+			// there.
+			return opt.NewBool(false)
+		}
+	default:
+		_, ok := b.store.(ipn.EncryptedStateStore)
+		return opt.NewBool(ok)
+	}
+}

ipn/store.go

@@ -113,3 +113,9 @@ func ReadStoreInt(store StateStore, id StateKey) (int64, error) {
 func PutStoreInt(store StateStore, id StateKey, val int64) error {
 	return WriteState(store, id, fmt.Appendf(nil, "%d", val))
 }
+
+// EncryptedStateStore is a marker interface implemented by StateStores that
+// encrypt data at rest.
+type EncryptedStateStore interface {
+	stateStoreIsEncrypted()
+}

tailcfg/tailcfg.go

@@ -162,7 +162,8 @@ type CapabilityVersion int
 //   - 115: 2025-03-07: Client understands DERPRegion.NoMeasureNoHome.
 //   - 116: 2025-05-05: Client serves MagicDNS "AAAA" if NodeAttrMagicDNSPeerAAAA set on self node
 //   - 117: 2025-05-28: Client understands DisplayMessages (structured health messages), but not necessarily PrimaryAction.
-const CurrentCapabilityVersion CapabilityVersion = 117
+//   - 118: 2025-07-01: Client sends Hostinfo.StateEncrypted to report whether the state file is encrypted at rest (#15830)
+const CurrentCapabilityVersion CapabilityVersion = 118
 
 // ID is an integer ID for a user, node, or login allocated by the
 // control plane.
@@ -878,6 +879,12 @@ type Hostinfo struct {
 	Location *Location `json:",omitempty"`
 
 	TPM *TPMInfo `json:",omitempty"` // TPM device metadata, if available
+	// StateEncrypted reports whether the node state is stored encrypted on
+	// disk. The actual mechanism is platform-specific:
+	//   * Apple nodes use the Keychain
+	//   * Linux and Windows nodes use the TPM
+	//   * Android apps use EncryptedSharedPreferences
+	StateEncrypted opt.Bool `json:",omitempty"`
 
 	// NOTE: any new fields containing pointers in this type
 	//       require changes to Hostinfo.Equal.

tailcfg/tailcfg_clone.go

@@ -188,6 +188,7 @@ var _HostinfoCloneNeedsRegeneration = Hostinfo(struct {
 	ServicesHash    string
 	Location        *Location
 	TPM             *TPMInfo
+	StateEncrypted  opt.Bool
 }{})
 
 // Clone makes a deep copy of NetInfo.

tailcfg/tailcfg_test.go

@@ -69,6 +69,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"ServicesHash",
 		"Location",
 		"TPM",
+		"StateEncrypted",
 	}
 	if have := fieldsOf(reflect.TypeFor[Hostinfo]()); !reflect.DeepEqual(have, hiHandles) {
 		t.Errorf("Hostinfo.Equal check might be out of sync\nfields: %q\nhandled: %q\n",

tailcfg/tailcfg_view.go

@@ -303,6 +303,7 @@ func (v HostinfoView) ServicesHash() string                   { return v.ж.Serv
 func (v HostinfoView) Location() LocationView                 { return v.ж.Location.View() }
 func (v HostinfoView) TPM() views.ValuePointer[TPMInfo]       { return views.ValuePointerOf(v.ж.TPM) }
 
+func (v HostinfoView) StateEncrypted() opt.Bool   { return v.ж.StateEncrypted }
 func (v HostinfoView) Equal(v2 HostinfoView) bool { return v.ж.Equal(v2.ж) }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
@@ -346,6 +347,7 @@ var _HostinfoViewNeedsRegeneration = Hostinfo(struct {
 	ServicesHash    string
 	Location        *Location
 	TPM             *TPMInfo
+	StateEncrypted  opt.Bool
 }{})
 
 // View returns a read-only view of NetInfo.