From 73db56af5284a634692ef1d78dc1e700f1889e7a Mon Sep 17 00:00:00 2001
From: Tom DNetto <tom@tailscale.com>
Date: Tue, 4 Oct 2022 10:51:45 -0700
Subject: [PATCH] ipn/ipnlocal: filter peers with bad signatures when tka is
 enabled

Signed-off-by: Tom DNetto <tom@tailscale.com>
---
 envknob/envknob.go                |  5 +++
 ipn/ipnlocal/local.go             |  3 ++
 ipn/ipnlocal/network-lock.go      | 33 +++++++++++++++++
 ipn/ipnlocal/network-lock_test.go | 60 +++++++++++++++++++++++++++++++
 4 files changed, 101 insertions(+)

diff --git a/envknob/envknob.go b/envknob/envknob.go
index f8ff392bc..969cf7eac 100644
--- a/envknob/envknob.go
+++ b/envknob/envknob.go
@@ -277,6 +277,11 @@ func SSHPolicyFile() string { return String("TS_DEBUG_SSH_POLICY_FILE") }
 // SSHIgnoreTailnetPolicy is whether to ignore the Tailnet SSH policy for development.
 func SSHIgnoreTailnetPolicy() bool { return Bool("TS_DEBUG_SSH_IGNORE_TAILNET_POLICY") }
 
+
+// TKASkipSignatureCheck is whether to skip node-key signature checking for development.
+func TKASkipSignatureCheck() bool { return Bool("TS_UNSAFE_SKIP_NKS_VERIFICATION") }
+
+
 // NoLogsNoSupport reports whether the client's opted out of log uploads and
 // technical support.
 func NoLogsNoSupport() bool {
diff --git a/ipn/ipnlocal/local.go b/ipn/ipnlocal/local.go
index 51bbf32b8..12c4ca537 100644
--- a/ipn/ipnlocal/local.go
+++ b/ipn/ipnlocal/local.go
@@ -778,6 +778,9 @@ func (b *LocalBackend) setClientStatus(st controlclient.Status) {
 		if err := b.tkaSyncIfNeededLocked(st.NetMap); err != nil {
 			b.logf("[v1] TKA sync error: %v", err)
 		}
+		if !envknob.TKASkipSignatureCheck() {
+			b.tkaFilterNetmapLocked(st.NetMap)
+		}
 		if b.findExitNodeIDLocked(st.NetMap) {
 			prefsChanged = true
 		}
diff --git a/ipn/ipnlocal/network-lock.go b/ipn/ipnlocal/network-lock.go
index 1f1cc7ecb..6dcb92ba7 100644
--- a/ipn/ipnlocal/network-lock.go
+++ b/ipn/ipnlocal/network-lock.go
@@ -39,6 +39,39 @@ type tkaState struct {
 	storage   *tka.FS
 }
 
+// tkaFilterNetmapLocked checks the signatures on each node key, dropping
+// nodes from the netmap who's signature does not verify.
+func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
+	if !envknob.UseWIPCode() {
+		return // Feature-flag till network-lock is in Alpha.
+	}
+	if b.tka == nil {
+		return // TKA not enabled.
+	}
+
+	toDelete := make(map[int]struct{}, len(nm.Peers))
+	for i, p := range nm.Peers {
+		if len(p.KeySignature) == 0 {
+			b.logf("Network lock is dropping peer %v(%v) due to missing signature", p.ID, p.StableID)
+			toDelete[i] = struct{}{}
+		} else {
+			if err := b.tka.authority.NodeKeyAuthorized(p.Key, p.KeySignature); err != nil {
+				b.logf("Network lock is dropping peer %v(%v) due to failed signature check: %v", p.ID, p.StableID, err)
+				toDelete[i] = struct{}{}
+			}
+		}
+	}
+
+	// nm.Peers is ordered, so deletion must be order-preserving.
+	peers := make([]*tailcfg.Node, 0, len(nm.Peers))
+	for i, p := range nm.Peers {
+		if _, delete := toDelete[i]; !delete {
+			peers = append(peers, p)
+		}
+	}
+	nm.Peers = peers
+}
+
 // tkaSyncIfNeededLocked examines TKA info reported from the control plane,
 // performing the steps necessary to synchronize local tka state.
 //
diff --git a/ipn/ipnlocal/network-lock_test.go b/ipn/ipnlocal/network-lock_test.go
index 8f151f654..3933d2e33 100644
--- a/ipn/ipnlocal/network-lock_test.go
+++ b/ipn/ipnlocal/network-lock_test.go
@@ -15,7 +15,9 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"tailscale.com/control/controlclient"
+	"tailscale.com/envknob"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
 	"tailscale.com/tailcfg"
@@ -484,3 +486,61 @@ func TestTKASync(t *testing.T) {
 		})
 	}
 }
+
+func TestTKAFilterNetmap(t *testing.T) {
+	envknob.Setenv("TAILSCALE_USE_WIP_CODE", "1")
+
+	nlPriv := key.NewNLPrivate()
+	nlKey := tka.Key{Kind: tka.Key25519, Public: nlPriv.Public().Verifier(), Votes: 2}
+	storage := &tka.Mem{}
+	authority, _, err := tka.Create(storage, tka.State{
+		Keys:               []tka.Key{nlKey},
+		DisablementSecrets: [][]byte{bytes.Repeat([]byte{0xa5}, 32)},
+	}, nlPriv)
+	if err != nil {
+		t.Fatalf("tka.Create() failed: %v", err)
+	}
+
+	n1, n2, n3, n4, n5 := key.NewNode(), key.NewNode(), key.NewNode(), key.NewNode(), key.NewNode()
+	n1GoodSig, err := signNodeKey(tailcfg.TKASignInfo{NodePublic: n1.Public()}, nlPriv)
+	if err != nil {
+		t.Fatal(err)
+	}
+	n4Sig, err := signNodeKey(tailcfg.TKASignInfo{NodePublic: n4.Public()}, nlPriv)
+	if err != nil {
+		t.Fatal(err)
+	}
+	n4Sig.Signature[3] = 42 // mess up the signature
+	n4Sig.Signature[4] = 42 // mess up the signature
+	n5GoodSig, err := signNodeKey(tailcfg.TKASignInfo{NodePublic: n5.Public()}, nlPriv)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	nm := netmap.NetworkMap{
+		Peers: []*tailcfg.Node{
+			{ID: 1, Key: n1.Public(), KeySignature: n1GoodSig.Serialize()},
+			{ID: 2, Key: n2.Public(), KeySignature: nil},                   // missing sig
+			{ID: 3, Key: n3.Public(), KeySignature: n1GoodSig.Serialize()}, // someone elses sig
+			{ID: 4, Key: n4.Public(), KeySignature: n4Sig.Serialize()},     // messed-up signature
+			{ID: 5, Key: n5.Public(), KeySignature: n5GoodSig.Serialize()},
+		},
+	}
+
+	b := &LocalBackend{
+		logf: t.Logf,
+		tka:  &tkaState{authority: authority},
+	}
+	b.tkaFilterNetmapLocked(&nm)
+
+	want := []*tailcfg.Node{
+		{ID: 1, Key: n1.Public(), KeySignature: n1GoodSig.Serialize()},
+		{ID: 5, Key: n5.Public(), KeySignature: n5GoodSig.Serialize()},
+	}
+	nodePubComparer := cmp.Comparer(func(x, y key.NodePublic) bool {
+		return x.Raw32() == y.Raw32()
+	})
+	if diff := cmp.Diff(nm.Peers, want, nodePubComparer); diff != "" {
+		t.Errorf("filtered netmap differs (-want, +got):\n%s", diff)
+	}
+}