types/persist: add AttestationKey (#17281)

Extend Persist with AttestationKey to record a hardware-backed
attestation key for the node's identity.

Add a flag to tailscaled to allow users to control the use of
hardware-backed keys to bind node identity to individual machines.

Updates tailscale/corp#31269


Change-Id: Idcf40d730a448d85f07f1bebf387f086d4c58be3

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

ipn/ipnlocal/hwattest.go

@@ -0,0 +1,48 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_tpm
+
+package ipnlocal
+
+import (
+	"errors"
+
+	"tailscale.com/feature"
+	"tailscale.com/types/key"
+	"tailscale.com/types/logger"
+	"tailscale.com/types/persist"
+)
+
+func init() {
+	feature.HookGenerateAttestationKeyIfEmpty.Set(generateAttestationKeyIfEmpty)
+}
+
+// generateAttestationKeyIfEmpty generates a new hardware attestation key if
+// none exists. It returns true if a new key was generated and stored in
+// p.AttestationKey.
+func generateAttestationKeyIfEmpty(p *persist.Persist, logf logger.Logf) (bool, error) {
+	// attempt to generate a new hardware attestation key if none exists
+	var ak key.HardwareAttestationKey
+	if p != nil {
+		ak = p.AttestationKey
+	}
+
+	if ak == nil || ak.IsZero() {
+		var err error
+		ak, err = key.NewHardwareAttestationKey()
+		if err != nil {
+			if !errors.Is(err, key.ErrUnsupported) {
+				logf("failed to create hardware attestation key: %v", err)
+			}
+		} else if ak != nil {
+			logf("using new hardware attestation key: %v", ak.Public())
+			if p == nil {
+				p = &persist.Persist{}
+			}
+			p.AttestationKey = ak
+			return true, nil
+		}
+	}
+	return false, nil
+}

ipn/ipnlocal/local.go

@@ -392,6 +392,23 @@ type LocalBackend struct {
 	//
 	// See tailscale/corp#29969.
 	overrideExitNodePolicy bool
+
+	// hardwareAttested is whether backend should use a hardware-backed key to
+	// bind the node identity to this device.
+	hardwareAttested atomic.Bool
+}
+
+// SetHardwareAttested enables hardware attestation key signatures in map
+// requests, if supported on this platform. SetHardwareAttested should be called
+// before Start.
+func (b *LocalBackend) SetHardwareAttested() {
+	b.hardwareAttested.Store(true)
+}
+
+// HardwareAttested reports whether hardware-backed attestation keys should be
+// used to bind the node's identity to this device.
+func (b *LocalBackend) HardwareAttested() bool {
+	return b.hardwareAttested.Load()
 }
 
 // HealthTracker returns the health tracker for the backend.
@@ -2455,10 +2472,23 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 	if b.reconcilePrefsLocked(newPrefs) {
 		prefsChanged = true
 	}
+
+	// neither UpdatePrefs or reconciliation should change Persist
+	newPrefs.Persist = b.pm.CurrentPrefs().Persist().AsStruct()
+
+	if buildfeatures.HasTPM {
+		if genKey, ok := feature.HookGenerateAttestationKeyIfEmpty.GetOk(); ok {
+			newKey, err := genKey(newPrefs.Persist, b.logf)
+			if err != nil {
+				b.logf("failed to populate attestation key from TPM: %v", err)
+			}
+			if newKey {
+				prefsChanged = true
+			}
+		}
+	}
+
 	if prefsChanged {
-		// Neither opts.UpdatePrefs nor prefs reconciliation
-		// is allowed to modify Persist; retain the old value.
-		newPrefs.Persist = b.pm.CurrentPrefs().Persist().AsStruct()
 		if err := b.pm.SetPrefs(newPrefs.View(), cn.NetworkProfile()); err != nil {
 			b.logf("failed to save updated and reconciled prefs: %v", err)
 		}
@@ -2491,8 +2521,6 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 
 	discoPublic := b.MagicConn().DiscoPublicKey()
 
-	var err error
-
 	isNetstack := b.sys.IsNetstackRouter()
 	debugFlags := controlDebugFlags
 	if isNetstack {

ipn/ipnlocal/local_test.go

@@ -7030,6 +7030,27 @@ func TestDisplayMessageIPNBus(t *testing.T) {
 	}
 }
 
+func TestHardwareAttested(t *testing.T) {
+	b := new(LocalBackend)
+
+	// default false
+	if got := b.HardwareAttested(); got != false {
+		t.Errorf("HardwareAttested() = %v, want false", got)
+	}
+
+	// set true
+	b.SetHardwareAttested()
+	if got := b.HardwareAttested(); got != true {
+		t.Errorf("HardwareAttested() = %v, want true after SetHardwareAttested()", got)
+	}
+
+	// repeat calls are safe; still true
+	b.SetHardwareAttested()
+	if got := b.HardwareAttested(); got != true {
+		t.Errorf("HardwareAttested() = %v, want true after second SetHardwareAttested()", got)
+	}
+}
+
 func TestDeps(t *testing.T) {
 	deptest.DepChecker{
 		OnImport: func(pkg string) {

ipn/ipnlocal/profiles.go

@@ -19,7 +19,9 @@ import (
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnext"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/persist"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/eventbus"
 )
@@ -645,8 +647,8 @@ func (pm *profileManager) setProfileAsUserDefault(profile ipn.LoginProfileView)
 	return pm.WriteState(k, []byte(profile.Key()))
 }
 
-func (pm *profileManager) loadSavedPrefs(key ipn.StateKey) (ipn.PrefsView, error) {
-	bs, err := pm.store.ReadState(key)
+func (pm *profileManager) loadSavedPrefs(k ipn.StateKey) (ipn.PrefsView, error) {
+	bs, err := pm.store.ReadState(k)
 	if err == ipn.ErrStateNotExist || len(bs) == 0 {
 		return defaultPrefs, nil
 	}
@@ -654,10 +656,18 @@ func (pm *profileManager) loadSavedPrefs(key ipn.StateKey) (ipn.PrefsView, error
 		return ipn.PrefsView{}, err
 	}
 	savedPrefs := ipn.NewPrefs()
+
+	// if supported by the platform, create an empty hardware attestation key to use when deserializing
+	// to avoid type exceptions from json.Unmarshaling into an interface{}.
+	hw, _ := key.NewEmptyHardwareAttestationKey()
+	savedPrefs.Persist = &persist.Persist{
+		AttestationKey: hw,
+	}
+
 	if err := ipn.PrefsFromBytes(bs, savedPrefs); err != nil {
 		return ipn.PrefsView{}, fmt.Errorf("parsing saved prefs: %v", err)
 	}
-	pm.logf("using backend prefs for %q: %v", key, savedPrefs.Pretty())
+	pm.logf("using backend prefs for %q: %v", k, savedPrefs.Pretty())
 
 	// Ignore any old stored preferences for https://login.tailscale.com
 	// as the control server that would override the new default of

ipn/ipnlocal/profiles_test.go

@@ -151,6 +151,7 @@ func TestProfileDupe(t *testing.T) {
 				ID:        tailcfg.UserID(user),
 				LoginName: fmt.Sprintf("user%d@example.com", user),
 			},
+			AttestationKey: nil,
 		}
 	}
 	user1Node1 := newPersist(1, 1)

ipn/prefs.go

@@ -709,6 +709,7 @@ func NewPrefs() *Prefs {
 	// Provide default values for options which might be missing
 	// from the json data for any reason. The json can still
 	// override them to false.
+
 	p := &Prefs{
 		// ControlURL is explicitly not set to signal that
 		// it's not yet configured, which relaxes the CLI "up"

ipn/prefs_test.go

@@ -501,7 +501,7 @@ func TestPrefsPretty(t *testing.T) {
 				},
 			},
 			"linux",
-			`Prefs{ra=false dns=false want=false routes=[] nf=off update=off Persist{o=, n=[B1VKl] u=""}}`,
+			`Prefs{ra=false dns=false want=false routes=[] nf=off update=off Persist{o=, n=[B1VKl] u="" ak=-}}`,
 		},
 		{
 			Prefs{