control/keyfallback: add baked-in fallback for control key

Similar to how we bake in the DERPMap to ensure that we can reach the
DERP servers if DNS isn't working, also bake in the control key for the
default control server that we use if the control server is down.

Updates #13890

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I18ef0381e266bd3db10063685993bc3cb76b2f42

cmd/k8s-operator/depaware.txt

@@ -659,6 +659,7 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
         tailscale.com/control/controlclient                          from tailscale.com/ipn/ipnlocal+
         tailscale.com/control/controlhttp                            from tailscale.com/control/controlclient
         tailscale.com/control/controlknobs                           from tailscale.com/control/controlclient+
+        tailscale.com/control/keyfallback                            from tailscale.com/control/controlclient
         tailscale.com/derp                                           from tailscale.com/derp/derphttp+
         tailscale.com/derp/derphttp                                  from tailscale.com/ipn/localapi+
         tailscale.com/disco                                          from tailscale.com/derp+

cmd/tailscaled/depaware.txt

@@ -250,6 +250,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/control/controlclient                          from tailscale.com/cmd/tailscaled+
         tailscale.com/control/controlhttp                            from tailscale.com/control/controlclient
         tailscale.com/control/controlknobs                           from tailscale.com/control/controlclient+
+        tailscale.com/control/keyfallback                            from tailscale.com/control/controlclient
         tailscale.com/derp                                           from tailscale.com/derp/derphttp+
         tailscale.com/derp/derphttp                                  from tailscale.com/cmd/tailscaled+
         tailscale.com/disco                                          from tailscale.com/derp+

control/controlclient/direct.go

@@ -29,9 +29,11 @@
 
 	"go4.org/mem"
 	"tailscale.com/control/controlknobs"
+	"tailscale.com/control/keyfallback"
 	"tailscale.com/envknob"
 	"tailscale.com/health"
 	"tailscale.com/hostinfo"
+	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/logtail"
 	"tailscale.com/net/dnscache"
@@ -90,6 +92,7 @@ type Direct struct {
 	mu                   sync.Mutex        // mutex guards the following fields
 	serverLegacyKey      key.MachinePublic // original ("legacy") nacl crypto_box-based public key; only used for signRegisterRequest on Windows now
 	serverNoiseKey       key.MachinePublic
+	usedFallbackNoiseKey bool // true if we used the baked-in fallback key
 
 	sfGroup     singleflight.Group[struct{}, *NoiseClient] // protects noiseClient creation.
 	noiseClient *NoiseClient
@@ -498,6 +501,7 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 	tryingNewKey := c.tryingNewKey
 	serverKey := c.serverLegacyKey
 	serverNoiseKey := c.serverNoiseKey
+	usedFallback := c.usedFallbackNoiseKey
 	authKey, isWrapped, wrappedSig, wrappedKey := tka.DecodeWrappedAuthkey(c.authKey, c.logf)
 	hi := c.hostInfoLocked()
 	backendLogID := hi.BackendLogID
@@ -528,7 +532,7 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 	}
 
 	c.logf("doLogin(regen=%v, hasUrl=%v)", regen, opt.URL != "")
-	if serverKey.IsZero() {
+	if serverKey.IsZero() || usedFallback {
 		keys, err := loadServerPubKeys(ctx, c.httpc, c.serverURL)
 		if err != nil && c.interceptedDial != nil && c.interceptedDial.Load() {
 			c.health.SetUnhealthy(macOSScreenTime, nil)
@@ -536,13 +540,21 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 			c.health.SetHealthy(macOSScreenTime)
 		}
 		if err != nil {
+			if k2, err := c.getFallbackServerPubKeys(); err == nil {
+				keys = k2
+				usedFallback = true
+			} else {
 				return regen, opt.URL, nil, err
 			}
-		c.logf("control server key from %s: ts2021=%s, legacy=%v", c.serverURL, keys.PublicKey.ShortString(), keys.LegacyPublicKey.ShortString())
+		} else {
+			usedFallback = false
+			c.logf("control server key from %s: ts2021=%s", c.serverURL, keys.PublicKey.ShortString())
+		}
 
 		c.mu.Lock()
 		c.serverLegacyKey = keys.LegacyPublicKey
 		c.serverNoiseKey = keys.PublicKey
+		c.usedFallbackNoiseKey = usedFallback
 		c.mu.Unlock()
 		serverKey = keys.LegacyPublicKey
 		serverNoiseKey = keys.PublicKey
@@ -751,6 +763,22 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 	return false, resp.AuthURL, nil, nil
 }
 
+func (c *Direct) getFallbackServerPubKeys() (*tailcfg.OverTLSPublicKeyResponse, error) {
+	// If we saw an error, try to use the fallback key if
+	// we're dialing the default control server.
+	if ipn.IsLoginServerSynonym(c.serverURL) {
+		return nil, errors.New("not using default control server")
+	}
+
+	kf, err := keyfallback.Get()
+	if err != nil {
+		return nil, err
+	}
+
+	c.logf("using fallback server key: ts2021=%s", kf.PublicKey.ShortString())
+	return kf, nil
+}
+
 // newEndpoints acquires c.mu and sets the local port and endpoints and reports
 // whether they've changed.
 //

control/keyfallback/control-key.json

@@ -0,0 +1,4 @@
+{
+	"legacyPublicKey": "mkey:9e5156a4c65121306dd2d8ed8f92cb8d738e2533011344b522c5d28409bc4970",
+	"publicKey": "mkey:7d2792f9c98d753d2042471536801949104c247f95eac770f8fb321595e2173b"
+}

control/keyfallback/keyfallback.go

@@ -0,0 +1,32 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package keyfallback contains a fallback mechanism for starting up Tailscale
+// when the control server cannot be reached to obtain the primary Noise key.
+//
+// The data is backed by a JSON file `control-key.json` that is updated by
+// `update.go`:
+//
+//	(cd control/keyfallback; go run update.go)
+package keyfallback
+
+import (
+	_ "embed"
+	"encoding/json"
+
+	"tailscale.com/tailcfg"
+)
+
+// Get returns the fallback control server public key that was baked into the
+// binary at compile time. It is only valid for the main Tailscale control
+// server instance.
+func Get() (*tailcfg.OverTLSPublicKeyResponse, error) {
+	out := &tailcfg.OverTLSPublicKeyResponse{}
+	if err := json.Unmarshal(controlKeyJSON, out); err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+//go:embed control-key.json
+var controlKeyJSON []byte

control/keyfallback/keyfallback_test.go

@@ -0,0 +1,77 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package keyfallback
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"reflect"
+	"testing"
+	"time"
+
+	"tailscale.com/ipn"
+	"tailscale.com/tailcfg"
+	"tailscale.com/tstest/nettest"
+	"tailscale.com/util/must"
+)
+
+func TestHasValidControlKey(t *testing.T) {
+	t.Parallel()
+	keys, err := Get()
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	if keys.PublicKey.IsZero() {
+		t.Fatalf("zero key")
+	}
+}
+
+// TestKeyIsUpToDate fetches the control key from the control server and
+// compares it to the baked-in key, to verify that it's up-to-date. If the
+// control server is unreachable, the test is skipped.
+func TestKeyIsUpToDate(t *testing.T) {
+	nettest.SkipIfNoNetwork(t)
+
+	// Optimistically fetch the control key and check if it's up to date,
+	// but ignore if we don't have network access (e.g. running tests on an
+	// airplane).
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	keyURL := fmt.Sprintf("%v/key?v=%d", ipn.DefaultControlURL, tailcfg.CurrentCapabilityVersion)
+	req := must.Get(http.NewRequestWithContext(ctx, "GET", keyURL, nil))
+	res, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Logf("fetch control key: %v", err)
+		return
+	}
+	defer res.Body.Close()
+
+	if res.StatusCode != 200 {
+		t.Fatalf("fetch control key: bad status; got %v, want 200", res.Status)
+	}
+	b, err := io.ReadAll(res.Body)
+	if err != nil {
+		t.Fatalf("read control key: %v", err)
+	}
+
+	// Verify that the key is up to date and matches the baked-in key.
+	out := &tailcfg.OverTLSPublicKeyResponse{}
+	if err := json.Unmarshal(b, out); err != nil {
+		t.Fatalf("unmarshal control key: %v", err)
+	}
+
+	keys, err := Get()
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+
+	if !reflect.DeepEqual(keys, out) {
+		t.Errorf("control key is out of date")
+		t.Logf("old key: %v", keys)
+		t.Logf("new key: %v", out)
+	}
+}

control/keyfallback/update.go

@@ -0,0 +1,47 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ignore
+
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+
+	"tailscale.com/ipn"
+	"tailscale.com/tailcfg"
+)
+
+func main() {
+	keyURL := fmt.Sprintf("%v/key?v=%d", ipn.DefaultControlURL, tailcfg.CurrentCapabilityVersion)
+	res, err := http.Get(keyURL)
+	if err != nil {
+		log.Fatalf("fetch control key: %v", err)
+	}
+	defer res.Body.Close()
+	b, err := io.ReadAll(io.LimitReader(res.Body, 64<<10))
+	if err != nil {
+		log.Fatalf("read control key: %v", err)
+	}
+	if res.StatusCode != 200 {
+		log.Fatalf("fetch control key: bad status; got %v, want 200", res.Status)
+	}
+
+	// Unmarshal to make sure it's valid.
+	var out tailcfg.OverTLSPublicKeyResponse
+	if err := json.Unmarshal(b, &out); err != nil {
+		log.Fatalf("unmarshal control key: %v", err)
+	}
+	if out.PublicKey.IsZero() {
+		log.Fatalf("control key is zero")
+	}
+
+	if err := os.WriteFile("control-key.json", b, 0644); err != nil {
+		log.Fatalf("write control key: %v", err)
+	}
+}