cmd/tailscaled: default --encrypt-state to true if TPM is available (#17376)

Whenever running on a platform that has a TPM (and tailscaled can access
it), default to encrypting the state. The user can still explicitly set
this flag to disable encryption.

Updates https://github.com/tailscale/corp/issues/32909

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

cmd/tailscaled/flag.go

@@ -0,0 +1,31 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import "strconv"
+
+// boolFlag is a flag.Value that tracks whether it was ever set.
+type boolFlag struct {
+	set bool
+	v   bool
+}
+
+func (b *boolFlag) String() string {
+	if b == nil || !b.set {
+		return "unset"
+	}
+	return strconv.FormatBool(b.v)
+}
+
+func (b *boolFlag) Set(s string) error {
+	v, err := strconv.ParseBool(s)
+	if err != nil {
+		return err
+	}
+	b.v = v
+	b.set = true
+	return nil
+}
+
+func (b *boolFlag) IsBoolFlag() bool { return true }

cmd/tailscaled/tailscaled.go

@@ -120,7 +120,7 @@ var args struct {
 	debug          string
 	port           uint16
 	statepath      string
-	encryptState   bool
+	encryptState   boolFlag
 	statedir       string
 	socketpath     string
 	birdSocketPath string
@@ -197,7 +197,7 @@ func main() {
 	flag.StringVar(&args.tunname, "tun", defaultTunName(), `tunnel interface name; use "userspace-networking" (beta) to not use TUN`)
 	flag.Var(flagtype.PortValue(&args.port, defaultPort()), "port", "UDP port to listen on for WireGuard and peer-to-peer traffic; 0 means automatically select")
 	flag.StringVar(&args.statepath, "state", "", "absolute path of state file; use 'kube:<secret-name>' to use Kubernetes secrets or 'arn:aws:ssm:...' to store in AWS SSM; use 'mem:' to not store state and register as an ephemeral node. If empty and --statedir is provided, the default is <statedir>/tailscaled.state. Default: "+paths.DefaultTailscaledStateFile())
-	flag.BoolVar(&args.encryptState, "encrypt-state", defaultEncryptState(), "encrypt the state file on disk; uses TPM on Linux and Windows, on all other platforms this flag is not supported")
+	flag.Var(&args.encryptState, "encrypt-state", `encrypt the state file on disk; when not set encryption will be enabled if supported on this platform; uses TPM on Linux and Windows, on all other platforms this flag is not supported`)
 	flag.StringVar(&args.statedir, "statedir", "", "path to directory for storage of config state, TLS certs, temporary incoming Taildrop files, etc. If empty, it's derived from --state when possible.")
 	flag.StringVar(&args.socketpath, "socket", paths.DefaultTailscaledSocket(), "path of the service unix socket")
 	flag.StringVar(&args.birdSocketPath, "bird-socket", "", "path of the bird unix socket")
@@ -275,7 +275,10 @@ func main() {
 		}
 	}
 
-	if args.encryptState {
+	if !args.encryptState.set {
+		args.encryptState.v = defaultEncryptState()
+	}
+	if args.encryptState.v {
 		if runtime.GOOS != "linux" && runtime.GOOS != "windows" {
 			log.SetFlags(0)
 			log.Fatalf("--encrypt-state is not supported on %s", runtime.GOOS)
@@ -351,7 +354,7 @@ func statePathOrDefault() string {
 	if path == "" && args.statedir != "" {
 		path = filepath.Join(args.statedir, "tailscaled.state")
 	}
-	if path != "" && !store.HasKnownProviderPrefix(path) && args.encryptState {
+	if path != "" && !store.HasKnownProviderPrefix(path) && args.encryptState.v {
 		path = store.TPMPrefix + path
 	}
 	return path
@@ -909,6 +912,6 @@ func defaultEncryptState() bool {
 		// (plan9/FreeBSD/etc).
 		return false
 	}
-	v, _ := policyclient.Get().GetBoolean(pkey.EncryptState, false)
+	v, _ := policyclient.Get().GetBoolean(pkey.EncryptState, feature.TPMAvailable())
 	return v
 }

feature/hooks.go

@@ -40,3 +40,15 @@ var HookProxySetSelfProxy Hook[func(...string)]
 // HookProxySetTransportGetProxyConnectHeader is a hook for feature/useproxy to register
 // [tshttpproxy.SetTransportGetProxyConnectHeader].
 var HookProxySetTransportGetProxyConnectHeader Hook[func(*http.Transport)]
+
+// HookTPMAvailable is a hook that reports whether a TPM device is supported
+// and available.
+var HookTPMAvailable Hook[func() bool]
+
+// TPMAvailable reports whether a TPM device is supported and available.
+func TPMAvailable() bool {
+	if f, ok := HookTPMAvailable.GetOk(); ok {
+		return f()
+	}
+	return false
+}

feature/tpm/tpm.go

@@ -39,6 +39,7 @@ var infoOnce = sync.OnceValue(info)
 
 func init() {
 	feature.Register("tpm")
+	feature.HookTPMAvailable.Set(tpmSupported)
 	hostinfo.RegisterHostinfoNewHook(func(hi *tailcfg.Hostinfo) {
 		hi.TPM = infoOnce()
 	})
@@ -51,6 +52,15 @@ func init() {
 	}
 }
 
+func tpmSupported() bool {
+	tpm, err := open()
+	if err != nil {
+		return false
+	}
+	tpm.Close()
+	return true
+}
+
 var verboseTPM = envknob.RegisterBool("TS_DEBUG_TPM")
 
 func info() *tailcfg.TPMInfo {

feature/tpm/tpm_test.go

@@ -277,15 +277,6 @@ func TestMigrateStateToTPM(t *testing.T) {
 	}
 }
 
-func tpmSupported() bool {
-	tpm, err := open()
-	if err != nil {
-		return false
-	}
-	tpm.Close()
-	return true
-}
-
 type mockTPMSealProvider struct {
 	path string
 	data map[ipn.StateKey][]byte

ipn/ipnlocal/local.go

@@ -7559,11 +7559,7 @@ func (b *LocalBackend) stateEncrypted() opt.Bool {
 		case version.IsMacAppStore():
 			return opt.NewBool(true)
 		case version.IsMacSysExt():
-			// MacSys still stores its state in plaintext on disk in addition to
+			sp, _ := b.polc.GetBoolean(pkey.EncryptState, true)
-			// 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, _ := b.polc.GetBoolean(pkey.EncryptState, false)
 			return opt.NewBool(sp)
 		default:
 			// Probably self-compiled tailscaled, we don't use the Keychain

util/syspolicy/pkey/pkey.go

@@ -136,7 +136,9 @@ const (
 	FlushDNSOnSessionUnlock Key = "FlushDNSOnSessionUnlock"
 
 	// EncryptState is a boolean setting that specifies whether to encrypt the
-	// tailscaled state file with a TPM device.
+	// tailscaled state file.
+	// Windows and Linux use a TPM device, Apple uses the Keychain.
+	// It's a noop on other platforms.
 	EncryptState Key = "EncryptState"
 
 	// PostureChecking indicates if posture checking is enabled and the client shall gather