discovery: add banman for channel announcements

This commit introduces a ban manager that marks peers as banned if
they send too many invalid channel announcements to us. Expired
entries are purged after a certain period of time (currently 48 hours).

discovery/ban.go

@@ -0,0 +1,252 @@
+package discovery
+
+import (
+	"errors"
+	"sync"
+	"time"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/lightninglabs/neutrino/cache"
+	"github.com/lightninglabs/neutrino/cache/lru"
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+const (
+	// maxBannedPeers limits the maximum number of banned pubkeys that
+	// we'll store.
+	// TODO(eugene): tune.
+	maxBannedPeers = 10_000
+
+	// banThreshold is the point at which non-channel peers will be banned.
+	// TODO(eugene): tune.
+	banThreshold = 100
+
+	// banTime is the amount of time that the non-channel peer will be
+	// banned for. Channel announcements from channel peers will be dropped
+	// if it's not one of our channels.
+	// TODO(eugene): tune.
+	banTime = time.Hour * 48
+
+	// resetDelta is the time after a peer's last ban update that we'll
+	// reset its ban score.
+	// TODO(eugene): tune.
+	resetDelta = time.Hour * 48
+
+	// purgeInterval is how often we'll remove entries from the
+	// peerBanIndex and allow peers to be un-banned. This interval is also
+	// used to reset ban scores of peers that aren't banned.
+	purgeInterval = time.Minute * 10
+)
+
+var ErrPeerBanned = errors.New("peer has bypassed ban threshold - banning")
+
+// ClosedChannelTracker handles closed channels being gossiped to us.
+type ClosedChannelTracker interface {
+	// GraphCloser is used to mark channels as closed and to check whether
+	// certain channels are closed.
+	GraphCloser
+
+	// IsChannelPeer checks whether we have a channel with a peer.
+	IsChannelPeer(*btcec.PublicKey) (bool, error)
+}
+
+// GraphCloser handles tracking closed channels by their scid.
+type GraphCloser interface {
+	// PutClosedScid marks a channel as closed so that we won't validate
+	// channel announcements for it again.
+	PutClosedScid(lnwire.ShortChannelID) error
+
+	// IsClosedScid checks if a short channel id is closed.
+	IsClosedScid(lnwire.ShortChannelID) (bool, error)
+}
+
+// NodeInfoInquirier handles queries relating to specific nodes and channels
+// they may have with us.
+type NodeInfoInquirer interface {
+	// FetchOpenChannels returns the set of channels that we have with the
+	// peer identified by the passed-in public key.
+	FetchOpenChannels(*btcec.PublicKey) ([]*channeldb.OpenChannel, error)
+}
+
+// ScidCloserMan helps the gossiper handle closed channels that are in the
+// ChannelGraph.
+type ScidCloserMan struct {
+	graph     GraphCloser
+	channelDB NodeInfoInquirer
+}
+
+// NewScidCloserMan creates a new ScidCloserMan.
+func NewScidCloserMan(graph GraphCloser,
+	channelDB NodeInfoInquirer) *ScidCloserMan {
+
+	return &ScidCloserMan{
+		graph:     graph,
+		channelDB: channelDB,
+	}
+}
+
+// PutClosedScid marks scid as closed so the gossiper can ignore this channel
+// in the future.
+func (s *ScidCloserMan) PutClosedScid(scid lnwire.ShortChannelID) error {
+	return s.graph.PutClosedScid(scid)
+}
+
+// IsClosedScid checks whether scid is closed so that the gossiper can ignore
+// it.
+func (s *ScidCloserMan) IsClosedScid(scid lnwire.ShortChannelID) (bool,
+	error) {
+
+	return s.graph.IsClosedScid(scid)
+}
+
+// IsChannelPeer checks whether we have a channel with the peer.
+func (s *ScidCloserMan) IsChannelPeer(peerKey *btcec.PublicKey) (bool, error) {
+	chans, err := s.channelDB.FetchOpenChannels(peerKey)
+	if err != nil {
+		return false, err
+	}
+
+	return len(chans) > 0, nil
+}
+
+// A compile-time constraint to ensure ScidCloserMan implements
+// ClosedChannelTracker.
+var _ ClosedChannelTracker = (*ScidCloserMan)(nil)
+
+// cachedBanInfo is used to track a peer's ban score and if it is banned.
+type cachedBanInfo struct {
+	score      uint64
+	lastUpdate time.Time
+}
+
+// Size returns the "size" of an entry.
+func (c *cachedBanInfo) Size() (uint64, error) {
+	return 1, nil
+}
+
+// isBanned returns true if the ban score is greater than the ban threshold.
+func (c *cachedBanInfo) isBanned() bool {
+	return c.score >= banThreshold
+}
+
+// banman is responsible for banning peers that are misbehaving. The banman is
+// in-memory and will be reset upon restart of LND. If a node's pubkey is in
+// the peerBanIndex, it has a ban score. Ban scores start at 1 and are
+// incremented by 1 for each instance of misbehavior. It uses an LRU cache to
+// cut down on memory usage in case there are many banned peers and to protect
+// against DoS.
+type banman struct {
+	// peerBanIndex tracks our peers' ban scores and if they are banned and
+	// for how long. The ban score is incremented when our peer gives us
+	// gossip messages that are invalid.
+	peerBanIndex *lru.Cache[[33]byte, *cachedBanInfo]
+
+	wg   sync.WaitGroup
+	quit chan struct{}
+}
+
+// newBanman creates a new banman with the default maxBannedPeers.
+func newBanman() *banman {
+	return &banman{
+		peerBanIndex: lru.NewCache[[33]byte, *cachedBanInfo](
+			maxBannedPeers,
+		),
+		quit: make(chan struct{}),
+	}
+}
+
+// start kicks off the banman by calling purgeExpiredBans.
+func (b *banman) start() {
+	b.wg.Add(1)
+	go b.purgeExpiredBans()
+}
+
+// stop halts the banman.
+func (b *banman) stop() {
+	close(b.quit)
+	b.wg.Wait()
+}
+
+// purgeOldEntries removes ban entries if their ban has expired.
+func (b *banman) purgeExpiredBans() {
+	defer b.wg.Done()
+
+	purgeTicker := time.NewTicker(purgeInterval)
+	defer purgeTicker.Stop()
+
+	for {
+		select {
+		case <-purgeTicker.C:
+			b.purgeBanEntries()
+
+		case <-b.quit:
+			return
+		}
+	}
+}
+
+// purgeBanEntries does two things:
+// - removes peers from our ban list whose ban timer is up
+// - removes peers whose ban scores have expired.
+func (b *banman) purgeBanEntries() {
+	keysToRemove := make([][33]byte, 0)
+
+	sweepEntries := func(pubkey [33]byte, banInfo *cachedBanInfo) bool {
+		if banInfo.isBanned() {
+			// If the peer is banned, check if the ban timer has
+			// expired.
+			if banInfo.lastUpdate.Add(banTime).Before(time.Now()) {
+				keysToRemove = append(keysToRemove, pubkey)
+			}
+
+			return true
+		}
+
+		if banInfo.lastUpdate.Add(resetDelta).Before(time.Now()) {
+			// Remove non-banned peers whose ban scores have
+			// expired.
+			keysToRemove = append(keysToRemove, pubkey)
+		}
+
+		return true
+	}
+
+	b.peerBanIndex.Range(sweepEntries)
+
+	for _, key := range keysToRemove {
+		b.peerBanIndex.Delete(key)
+	}
+}
+
+// isBanned checks whether the peer identified by the pubkey is banned.
+func (b *banman) isBanned(pubkey [33]byte) bool {
+	banInfo, err := b.peerBanIndex.Get(pubkey)
+	switch {
+	case errors.Is(err, cache.ErrElementNotFound):
+		return false
+
+	default:
+		return banInfo.isBanned()
+	}
+}
+
+// incrementBanScore increments a peer's ban score.
+func (b *banman) incrementBanScore(pubkey [33]byte) {
+	banInfo, err := b.peerBanIndex.Get(pubkey)
+	switch {
+	case errors.Is(err, cache.ErrElementNotFound):
+		cachedInfo := &cachedBanInfo{
+			score:      1,
+			lastUpdate: time.Now(),
+		}
+		_, _ = b.peerBanIndex.Put(pubkey, cachedInfo)
+	default:
+		cachedInfo := &cachedBanInfo{
+			score:      banInfo.score + 1,
+			lastUpdate: time.Now(),
+		}
+
+		_, _ = b.peerBanIndex.Put(pubkey, cachedInfo)
+	}
+}

discovery/ban_test.go

@@ -0,0 +1,60 @@
+package discovery
+
+import (
+	"testing"
+	"time"
+
+	"github.com/lightninglabs/neutrino/cache"
+	"github.com/stretchr/testify/require"
+)
+
+// TestPurgeBanEntries tests that we properly purge ban entries on a timer.
+func TestPurgeBanEntries(t *testing.T) {
+	t.Parallel()
+
+	b := newBanman()
+
+	// Ban a peer by repeatedly incrementing its ban score.
+	peer1 := [33]byte{0x00}
+
+	for i := 0; i < banThreshold; i++ {
+		b.incrementBanScore(peer1)
+	}
+
+	// Assert that the peer is now banned.
+	require.True(t, b.isBanned(peer1))
+
+	// A call to purgeBanEntries should not remove the peer from the index.
+	b.purgeBanEntries()
+	require.True(t, b.isBanned(peer1))
+
+	// Now set the peer's last update time to two banTimes in the past so
+	// that we can assert that purgeBanEntries does remove it from the
+	// index.
+	banInfo, err := b.peerBanIndex.Get(peer1)
+	require.NoError(t, err)
+
+	banInfo.lastUpdate = time.Now().Add(-2 * banTime)
+
+	b.purgeBanEntries()
+	_, err = b.peerBanIndex.Get(peer1)
+	require.ErrorIs(t, err, cache.ErrElementNotFound)
+
+	// Increment the peer's ban score again but don't get it banned.
+	b.incrementBanScore(peer1)
+	require.False(t, b.isBanned(peer1))
+
+	// Assert that purgeBanEntries does nothing.
+	b.purgeBanEntries()
+	banInfo, err = b.peerBanIndex.Get(peer1)
+	require.Nil(t, err)
+
+	// Set its lastUpdate time to 2 resetDelta's in the past so that
+	// purgeBanEntries removes it.
+	banInfo.lastUpdate = time.Now().Add(-2 * resetDelta)
+
+	b.purgeBanEntries()
+
+	_, err = b.peerBanIndex.Get(peer1)
+	require.ErrorIs(t, err, cache.ErrElementNotFound)
+}