ipn/ipnlocal,tailcfg: introduce capability to gate TKA init paths

Previously, `TAILSCALE_USE_WIP_CODE` was needed to hit a bunch of the TKA paths. With
this change:

 - Enablement codepaths (NetworkLockInit) and initialization codepaths (tkaBootstrapFromGenesisLocked via tkaSyncIfNeeded)
   require either the WIP envknob or CapabilityTailnetLockAlpha.
 - Normal operation codepaths (tkaSyncIfNeeded, tkaFilterNetmapLocked) require TKA to be initialized, or either-or the
   envknob / capability.
 - Auxillary commands (ie: changing tka keys) require TKA to be initialized.

The end result is that it shouldn't be possible to initialize TKA (or subsequently use any of its features) without being
sent the capability or setting the envknob on tailscaled yourself.

I've also pulled out a bunch of unnecessary checks for CanSupportNetworkLock().

Signed-off-by: Tom DNetto <tom@tailscale.com>

ipn/ipnlocal/local.go

@@ -162,6 +162,7 @@ type LocalBackend struct {
 	tka            *tkaState
 	state          ipn.State
 	capFileSharing bool // whether netMap contains the file sharing capability
+	capTailnetLock bool // whether netMap contains the tailnet lock capability
 	// hostinfo is mutated in-place while mu is held.
 	hostinfo *tailcfg.Hostinfo
 	// netMap is not mutated in-place once set.
@@ -869,6 +870,8 @@ func (b *LocalBackend) setClientStatus(st controlclient.Status) {
 	}
 	// Prefs will be written out; this is not safe unless locked or cloned.
 	if st.NetMap != nil {
+		b.capTailnetLock = hasCapability(st.NetMap, tailcfg.CapabilityTailnetLockAlpha)
+
 		b.mu.Unlock() // respect locking rules for tkaSyncIfNeeded
 		if err := b.tkaSyncIfNeeded(st.NetMap, prefs.View()); err != nil {
 			b.logf("[v1] TKA sync error: %v", err)

ipn/ipnlocal/network-lock.go

@@ -46,13 +46,21 @@ type tkaState struct {
 	filtered  []ipnstate.TKAFilteredPeer
 }
 
+// permitTKAInitLocked returns true if tailnet lock initialization may
+// occur.
+// b.mu must be held.
+func (b *LocalBackend) permitTKAInitLocked() bool {
+	return envknob.UseWIPCode() || b.capTailnetLock
+}
+
 // tkaFilterNetmapLocked checks the signatures on each node key, dropping
 // nodes from the netmap whose signature does not verify.
 //
 // b.mu must be held.
 func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
-	if !envknob.UseWIPCode() {
-		return // Feature-flag till network-lock is in Alpha.
+	// TODO(tom): Remove this guard for 1.35 and later.
+	if b.tka == nil && !b.permitTKAInitLocked() {
+		return
 	}
 	if b.tka == nil {
 		return // TKA not enabled.
@@ -61,7 +69,7 @@ func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
 	var toDelete map[int]bool // peer index => true
 	for i, p := range nm.Peers {
 		if p.UnsignedPeerAPIOnly {
-			// Not subject to TKA.
+			// Not subject to tailnet lock.
 			continue
 		}
 		if len(p.KeySignature) == 0 {
@@ -123,18 +131,18 @@ func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
 // tkaSyncIfNeeded immediately takes b.takeSyncLock which is held throughout,
 // and may take b.mu as required.
 func (b *LocalBackend) tkaSyncIfNeeded(nm *netmap.NetworkMap, prefs ipn.PrefsView) error {
-	if !envknob.UseWIPCode() {
-		// If the feature flag is not enabled, pretend we don't exist.
-		return nil
-	}
-
-	b.logf("tkaSyncIfNeeded: enabled=%v, head=%v", nm.TKAEnabled, nm.TKAHead)
-
 	b.tkaSyncLock.Lock() // take tkaSyncLock to make this function an exclusive section.
 	defer b.tkaSyncLock.Unlock()
 	b.mu.Lock() // take mu to protect access to synchronized fields.
 	defer b.mu.Unlock()
 
+	// TODO(tom): Remove this guard for 1.35 and later.
+	if b.tka == nil && !b.permitTKAInitLocked() {
+		return nil
+	}
+
+	b.logf("tkaSyncIfNeeded: enabled=%v, head=%v", nm.TKAEnabled, nm.TKAHead)
+
 	ourNodeKey := prefs.Persist().PublicNodeKey()
 
 	isEnabled := b.tka != nil
@@ -352,10 +360,6 @@ func (b *LocalBackend) tkaBootstrapFromGenesisLocked(g tkatype.MarshaledAUM, per
 // CanSupportNetworkLock returns nil if tailscaled is able to operate
 // a local tailnet key authority (and hence enforce network lock).
 func (b *LocalBackend) CanSupportNetworkLock() error {
-	if !envknob.UseWIPCode() {
-		return errors.New("this feature is not yet complete, a later release may support this functionality")
-	}
-
 	if b.tka != nil {
 		// If the TKA is being used, it is supported.
 		return nil
@@ -453,6 +457,13 @@ func (b *LocalBackend) NetworkLockInit(keys []tka.Key, disablementValues [][]byt
 	var ourNodeKey key.NodePublic
 	var nlPriv key.NLPrivate
 	b.mu.Lock()
+
+	// TODO(tom): Remove this guard for 1.35 and later.
+	if !b.permitTKAInitLocked() {
+		b.mu.Unlock()
+		return errors.New("this feature is not yet complete, a later release may support this functionality")
+	}
+
 	if p := b.pm.CurrentPrefs(); p.Valid() && p.Persist().Valid() && !p.Persist().PrivateNodeKey().IsZero() {
 		ourNodeKey = p.Persist().PublicNodeKey()
 		nlPriv = p.Persist().NetworkLockKey()
@@ -631,9 +642,6 @@ func (b *LocalBackend) NetworkLockModify(addKeys, removeKeys []tka.Key) (err err
 		return errors.New("no node-key: is tailscale logged in?")
 	}
 
-	if err := b.CanSupportNetworkLock(); err != nil {
-		return err
-	}
 	var nlPriv key.NLPrivate
 	if p := b.pm.CurrentPrefs(); p.Valid() && p.Persist().Valid() {
 		nlPriv = p.Persist().NetworkLockKey()
@@ -690,10 +698,6 @@ func (b *LocalBackend) NetworkLockModify(addKeys, removeKeys []tka.Key) (err err
 
 // NetworkLockDisable disables network-lock using the provided disablement secret.
 func (b *LocalBackend) NetworkLockDisable(secret []byte) error {
-	if err := b.CanSupportNetworkLock(); err != nil {
-		return err
-	}
-
 	var (
 		ourNodeKey key.NodePublic
 		head       tka.AUMHash

tailcfg/tailcfg.go

@@ -87,7 +87,8 @@ type CapabilityVersion int
 //   - 48: 2022-11-02: Node.UnsignedPeerAPIOnly
 //   - 49: 2022-11-03: Client understands EarlyNoise
 //   - 50: 2022-11-14: Client understands CapabilityIngress
-const CurrentCapabilityVersion CapabilityVersion = 50
+//   - 51: 2022-11-30: Client understands CapabilityTailnetLockAlpha
+const CurrentCapabilityVersion CapabilityVersion = 51
 
 type StableID string
 
@@ -1707,6 +1708,12 @@ const (
 	CapabilityDataPlaneAuditLogs = "https://tailscale.com/cap/data-plane-audit-logs" // feature enabled
 	CapabilityDebug              = "https://tailscale.com/cap/debug"                 // exposes debug endpoints over the PeerAPI
 
+	// CapabilityTailnetLockAlpha indicates the node is in the tailnet lock alpha,
+	// and initialization of tailnet lock may proceed.
+	//
+	// TODO(tom): Remove this for 1.35 and later.
+	CapabilityTailnetLockAlpha = "https://tailscale.com/cap/tailnet-lock-alpha"
+
 	// Inter-node capabilities as specified in the MapResponse.PacketFilter[].CapGrants.
 
 	// CapabilityFileSharingTarget grants the current node the ability to send