eth, les, light: enforce CHT checkpoints on fast-sync too

6 files changed, 196 insertions, 103 deletions

diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 4db689f73..0ef833bb8 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -75,6 +75,7 @@ var (
 	errUnknownPeer             = errors.New("peer is unknown or unhealthy")
 	errBadPeer                 = errors.New("action from bad peer ignored")
 	errStallingPeer            = errors.New("peer is stalling")
+	errUnsyncedPeer            = errors.New("unsynced peer")
 	errNoPeers                 = errors.New("no peers to keep download active")
 	errTimeout                 = errors.New("timeout")
 	errEmptyHeaderSet          = errors.New("empty header set by peer")
@@ -99,10 +100,11 @@ type Downloader struct {
 	mode SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
 	mux  *event.TypeMux // Event multiplexer to announce sync operation events
 
-	genesis uint64   // Genesis block number to limit sync to (e.g. light client CHT)
-	queue   *queue   // Scheduler for selecting the hashes to download
-	peers   *peerSet // Set of active peers from which download can proceed
-	stateDB ethdb.Database
+	checkpoint uint64   // Checkpoint block number to enforce head against (e.g. fast sync)
+	genesis    uint64   // Genesis block number to limit sync to (e.g. light client CHT)
+	queue      *queue   // Scheduler for selecting the hashes to download
+	peers      *peerSet // Set of active peers from which download can proceed
+	stateDB    ethdb.Database
 
 	rttEstimate   uint64 // Round trip time to target for download requests
 	rttConfidence uint64 // Confidence in the estimated RTT (unit: millionths to allow atomic ops)
@@ -205,15 +207,15 @@ type BlockChain interface {
 }
 
 // New creates a new downloader to fetch hashes and blocks from remote peers.
-func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, chain BlockChain, lightchain LightChain, dropPeer peerDropFn) *Downloader {
+func New(mode SyncMode, checkpoint uint64, stateDb ethdb.Database, mux *event.TypeMux, chain BlockChain, lightchain LightChain, dropPeer peerDropFn) *Downloader {
 	if lightchain == nil {
 		lightchain = chain
 	}
-
 	dl := &Downloader{
 		mode:           mode,
 		stateDB:        stateDb,
 		mux:            mux,
+		checkpoint:     checkpoint,
 		queue:          newQueue(),
 		peers:          newPeerSet(),
 		rttEstimate:    uint64(rttMaxEstimate),
@@ -326,7 +328,7 @@ func (d *Downloader) Synchronise(id string, head common.Hash, td *big.Int, mode
 	case nil:
 	case errBusy:
 
-	case errTimeout, errBadPeer, errStallingPeer,
+	case errTimeout, errBadPeer, errStallingPeer, errUnsyncedPeer,
 		errEmptyHeaderSet, errPeersUnavailable, errTooOld,
 		errInvalidAncestor, errInvalidChain:
 		log.Warn("Synchronisation failed, dropping peer", "peer", id, "err", err)
@@ -577,6 +579,10 @@ func (d *Downloader) fetchHeight(p *peerConnection) (*types.Header, error) {
 				return nil, errBadPeer
 			}
 			head := headers[0]
+			if d.mode == FastSync && head.Number.Uint64() < d.checkpoint {
+				p.log.Warn("Remote head below checkpoint", "number", head.Number, "hash", head.Hash())
+				return nil, errUnsyncedPeer
+			}
 			p.log.Debug("Remote head header identified", "number", head.Number, "hash", head.Hash())
 			return head, nil
 
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index 1a42965d3..6db534219 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -26,7 +26,7 @@ import (
 	"testing"
 	"time"
 
-	ethereum "github.com/ethereum/go-ethereum"
+	"github.com/ethereum/go-ethereum"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/ethdb"
@@ -73,7 +73,8 @@ func newTester() *downloadTester {
 	}
 	tester.stateDb = ethdb.NewMemDatabase()
 	tester.stateDb.Put(testGenesis.Root().Bytes(), []byte{0x00})
-	tester.downloader = New(FullSync, tester.stateDb, new(event.TypeMux), tester, nil, tester.dropPeer)
+
+	tester.downloader = New(FullSync, 0, tester.stateDb, new(event.TypeMux), tester, nil, tester.dropPeer)
 	return tester
 }
 
@@ -1049,6 +1050,7 @@ func testBlockHeaderAttackerDropping(t *testing.T, protocol int) {
 		{errUnknownPeer, false},             // Peer is unknown, was already dropped, don't double drop
 		{errBadPeer, true},                  // Peer was deemed bad for some reason, drop it
 		{errStallingPeer, true},             // Peer was detected to be stalling, drop it
+		{errUnsyncedPeer, true},             // Peer was detected to be unsynced, drop it
 		{errNoPeers, false},                 // No peers to download from, soft race, no issue
 		{errTimeout, true},                  // No hashes received in due time, drop the peer
 		{errEmptyHeaderSet, true},           // No headers were returned as a response, drop as it's a dead end
@@ -1567,3 +1569,39 @@ func TestRemoteHeaderRequestSpan(t *testing.T) {
 		}
 	}
 }
+
+// Tests that peers below a pre-configured checkpoint block are prevented from
+// being fast-synced from, avoiding potential cheap eclipse attacks.
+func TestCheckpointEnforcement62(t *testing.T)      { testCheckpointEnforcement(t, 62, FullSync) }
+func TestCheckpointEnforcement63Full(t *testing.T)  { testCheckpointEnforcement(t, 63, FullSync) }
+func TestCheckpointEnforcement63Fast(t *testing.T)  { testCheckpointEnforcement(t, 63, FastSync) }
+func TestCheckpointEnforcement64Full(t *testing.T)  { testCheckpointEnforcement(t, 64, FullSync) }
+func TestCheckpointEnforcement64Fast(t *testing.T)  { testCheckpointEnforcement(t, 64, FastSync) }
+func TestCheckpointEnforcement64Light(t *testing.T) { testCheckpointEnforcement(t, 64, LightSync) }
+
+func testCheckpointEnforcement(t *testing.T, protocol int, mode SyncMode) {
+	t.Parallel()
+
+	// Create a new tester with a particular hard coded checkpoint block
+	tester := newTester()
+	defer tester.terminate()
+
+	tester.downloader.checkpoint = uint64(fsMinFullBlocks) + 256
+	chain := testChainBase.shorten(int(tester.downloader.checkpoint) - 1)
+
+	// Attempt to sync with the peer and validate the result
+	tester.newPeer("peer", protocol, chain)
+
+	var expect error
+	if mode == FastSync {
+		expect = errUnsyncedPeer
+	}
+	if err := tester.sync("peer", nil, mode); err != expect {
+		t.Fatalf("block sync error mismatch: have %v, want %v", err, expect)
+	}
+	if mode == FastSync {
+		assertOwnChain(t, tester, 1)
+	} else {
+		assertOwnChain(t, tester, chain.len())
+	}
+}
diff --git a/eth/handler.go b/eth/handler.go
index b42612a56..2c0ea1eb1 100644
--- a/eth/handler.go
+++ b/eth/handler.go
@@ -28,7 +28,6 @@ import (
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/consensus"
-	"github.com/ethereum/go-ethereum/consensus/misc"
 	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/eth/downloader"
@@ -55,7 +54,7 @@ const (
 )
 
 var (
-	daoChallengeTimeout = 15 * time.Second // Time allowance for a node to reply to the DAO handshake challenge
+	syncChallengeTimeout = 15 * time.Second // Time allowance for a node to reply to the sync progress challenge
 )
 
 // errIncompatibleConfig is returned if the requested protocols and configs are
@@ -72,6 +71,9 @@ type ProtocolManager struct {
 	fastSync  uint32 // Flag whether fast sync is enabled (gets disabled if we already have blocks)
 	acceptTxs uint32 // Flag whether we're considered synchronised (enables transaction processing)
 
+	checkpointNumber uint64      // Block number for the sync progress validator to cross reference
+	checkpointHash   common.Hash // Block hash for the sync progress validator to cross reference
+
 	txpool      txPool
 	blockchain  *core.BlockChain
 	chainconfig *params.ChainConfig
@@ -126,6 +128,11 @@ func NewProtocolManager(config *params.ChainConfig, mode downloader.SyncMode, ne
 	if mode == downloader.FastSync {
 		manager.fastSync = uint32(1)
 	}
+	// If we have trusted checkpoints, enforce them on the chain
+	if checkpoint, ok := params.TrustedCheckpoints[blockchain.Genesis().Hash()]; ok {
+		manager.checkpointNumber = (checkpoint.SectionIndex+1)*params.CHTFrequencyClient - 1
+		manager.checkpointHash = checkpoint.SectionHead
+	}
 	// Initiate a sub-protocol for every implemented version we can handle
 	manager.SubProtocols = make([]p2p.Protocol, 0, len(ProtocolVersions))
 	for i, version := range ProtocolVersions {
@@ -165,7 +172,7 @@ func NewProtocolManager(config *params.ChainConfig, mode downloader.SyncMode, ne
 		return nil, errIncompatibleConfig
 	}
 	// Construct the different synchronisation mechanisms
-	manager.downloader = downloader.New(mode, chaindb, manager.eventMux, blockchain, nil, manager.removePeer)
+	manager.downloader = downloader.New(mode, manager.checkpointNumber, chaindb, manager.eventMux, blockchain, nil, manager.removePeer)
 
 	validator := func(header *types.Header) error {
 		return engine.VerifyHeader(blockchain, header, true)
@@ -291,22 +298,22 @@ func (pm *ProtocolManager) handle(p *peer) error {
 	// after this will be sent via broadcasts.
 	pm.syncTransactions(p)
 
-	// If we're DAO hard-fork aware, validate any remote peer with regard to the hard-fork
-	if daoBlock := pm.chainconfig.DAOForkBlock; daoBlock != nil {
-		// Request the peer's DAO fork header for extra-data validation
-		if err := p.RequestHeadersByNumber(daoBlock.Uint64(), 1, 0, false); err != nil {
+	// If we have a trusted CHT, reject all peers below that (avoid fast sync eclipse)
+	if pm.checkpointHash != (common.Hash{}) {
+		// Request the peer's checkpoint header for chain height/weight validation
+		if err := p.RequestHeadersByNumber(pm.checkpointNumber, 1, 0, false); err != nil {
 			return err
 		}
 		// Start a timer to disconnect if the peer doesn't reply in time
-		p.forkDrop = time.AfterFunc(daoChallengeTimeout, func() {
-			p.Log().Debug("Timed out DAO fork-check, dropping")
+		p.syncDrop = time.AfterFunc(syncChallengeTimeout, func() {
+			p.Log().Warn("Checkpoint challenge timed out, dropping", "addr", p.RemoteAddr(), "type", p.Name())
 			pm.removePeer(p.id)
 		})
 		// Make sure it's cleaned up if the peer dies off
 		defer func() {
-			if p.forkDrop != nil {
-				p.forkDrop.Stop()
-				p.forkDrop = nil
+			if p.syncDrop != nil {
+				p.syncDrop.Stop()
+				p.syncDrop = nil
 			}
 		}()
 	}
@@ -438,41 +445,33 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 		if err := msg.Decode(&headers); err != nil {
 			return errResp(ErrDecode, "msg %v: %v", msg, err)
 		}
-		// If no headers were received, but we're expending a DAO fork check, maybe it's that
-		if len(headers) == 0 && p.forkDrop != nil {
-			// Possibly an empty reply to the fork header checks, sanity check TDs
-			verifyDAO := true
-
-			// If we already have a DAO header, we can check the peer's TD against it. If
-			// the peer's ahead of this, it too must have a reply to the DAO check
-			if daoHeader := pm.blockchain.GetHeaderByNumber(pm.chainconfig.DAOForkBlock.Uint64()); daoHeader != nil {
-				if _, td := p.Head(); td.Cmp(pm.blockchain.GetTd(daoHeader.Hash(), daoHeader.Number.Uint64())) >= 0 {
-					verifyDAO = false
-				}
-			}
-			// If we're seemingly on the same chain, disable the drop timer
-			if verifyDAO {
-				p.Log().Debug("Seems to be on the same side of the DAO fork")
-				p.forkDrop.Stop()
-				p.forkDrop = nil
-				return nil
+		// If no headers were received, but we're expencting a checkpoint header, consider it that
+		if len(headers) == 0 && p.syncDrop != nil {
+			// Stop the timer either way, decide later to drop or not
+			p.syncDrop.Stop()
+			p.syncDrop = nil
+
+			// If we're doing a fast sync, we must enforce the checkpoint block to avoid
+			// eclipse attacks. Unsynced nodes are welcome to connect after we're done
+			// joining the network
+			if atomic.LoadUint32(&pm.fastSync) == 1 {
+				p.Log().Warn("Dropping unsynced node during fast sync", "addr", p.RemoteAddr(), "type", p.Name())
+				return errors.New("unsynced node cannot serve fast sync")
 			}
 		}
 		// Filter out any explicitly requested headers, deliver the rest to the downloader
 		filter := len(headers) == 1
 		if filter {
-			// If it's a potential DAO fork check, validate against the rules
-			if p.forkDrop != nil && pm.chainconfig.DAOForkBlock.Cmp(headers[0].Number) == 0 {
-				// Disable the fork drop timer
-				p.forkDrop.Stop()
-				p.forkDrop = nil
+			// If it's a potential sync progress check, validate the content and advertised chain weight
+			if p.syncDrop != nil && headers[0].Number.Uint64() == pm.checkpointNumber {
+				// Disable the sync drop timer
+				p.syncDrop.Stop()
+				p.syncDrop = nil
 
 				// Validate the header and either drop the peer or continue
-				if err := misc.VerifyDAOHeaderExtraData(pm.chainconfig, headers[0]); err != nil {
-					p.Log().Debug("Verified to be on the other side of the DAO fork, dropping")
-					return err
+				if headers[0].Hash() != pm.checkpointHash {
+					return errors.New("checkpoint hash mismatch")
 				}
-				p.Log().Debug("Verified to be on the same side of the DAO fork")
 				return nil
 			}
 			// Otherwise if it's a whitelisted block, validate against the set
diff --git a/eth/handler_test.go b/eth/handler_test.go
index 9fffd9581..2b6b59165 100644
--- a/eth/handler_test.go
+++ b/eth/handler_test.go
@@ -449,79 +449,131 @@ func testGetReceipt(t *testing.T, protocol int) {
 	}
 }
 
-// Tests that post eth protocol handshake, DAO fork-enabled clients also execute
-// a DAO "challenge" verifying each others' DAO fork headers to ensure they're on
-// compatible chains.
-func TestDAOChallengeNoVsNo(t *testing.T)       { testDAOChallenge(t, false, false, false) }
-func TestDAOChallengeNoVsPro(t *testing.T)      { testDAOChallenge(t, false, true, false) }
-func TestDAOChallengeProVsNo(t *testing.T)      { testDAOChallenge(t, true, false, false) }
-func TestDAOChallengeProVsPro(t *testing.T)     { testDAOChallenge(t, true, true, false) }
-func TestDAOChallengeNoVsTimeout(t *testing.T)  { testDAOChallenge(t, false, false, true) }
-func TestDAOChallengeProVsTimeout(t *testing.T) { testDAOChallenge(t, true, true, true) }
-
-func testDAOChallenge(t *testing.T, localForked, remoteForked bool, timeout bool) {
-	// Reduce the DAO handshake challenge timeout
-	if timeout {
-		defer func(old time.Duration) { daoChallengeTimeout = old }(daoChallengeTimeout)
-		daoChallengeTimeout = 500 * time.Millisecond
-	}
-	// Create a DAO aware protocol manager
+// Tests that post eth protocol handshake, clients perform a mutual checkpoint
+// challenge to validate each other's chains. Hash mismatches, or missing ones
+// during a fast sync should lead to the peer getting dropped.
+func TestCheckpointChallenge(t *testing.T) {
+	tests := []struct {
+		syncmode   downloader.SyncMode
+		checkpoint bool
+		timeout    bool
+		empty      bool
+		match      bool
+		drop       bool
+	}{
+		// If checkpointing is not enabled locally, don't challenge and don't drop
+		{downloader.FullSync, false, false, false, false, false},
+		{downloader.FastSync, false, false, false, false, false},
+		{downloader.LightSync, false, false, false, false, false},
+
+		// If checkpointing is enabled locally and remote response is empty, only drop during fast sync
+		{downloader.FullSync, true, false, true, false, false},
+		{downloader.FastSync, true, false, true, false, true}, // Special case, fast sync, unsynced peer
+		{downloader.LightSync, true, false, true, false, false},
+
+		// If checkpointing is enabled locally and remote response mismatches, always drop
+		{downloader.FullSync, true, false, false, false, true},
+		{downloader.FastSync, true, false, false, false, true},
+		{downloader.LightSync, true, false, false, false, true},
+
+		// If checkpointing is enabled locally and remote response matches, never drop
+		{downloader.FullSync, true, false, false, true, false},
+		{downloader.FastSync, true, false, false, true, false},
+		{downloader.LightSync, true, false, false, true, false},
+
+		// If checkpointing is enabled locally and remote times out, always drop
+		{downloader.FullSync, true, true, false, true, true},
+		{downloader.FastSync, true, true, false, true, true},
+		{downloader.LightSync, true, true, false, true, true},
+	}
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf("sync %v checkpoint %v timeout %v empty %v match %v", tt.syncmode, tt.checkpoint, tt.timeout, tt.empty, tt.match), func(t *testing.T) {
+			testCheckpointChallenge(t, tt.syncmode, tt.checkpoint, tt.timeout, tt.empty, tt.match, tt.drop)
+		})
+	}
+}
+
+func testCheckpointChallenge(t *testing.T, syncmode downloader.SyncMode, checkpoint bool, timeout bool, empty bool, match bool, drop bool) {
+	// Reduce the checkpoint handshake challenge timeout
+	defer func(old time.Duration) { syncChallengeTimeout = old }(syncChallengeTimeout)
+	syncChallengeTimeout = 250 * time.Millisecond
+
+	// Initialize a chain and generate a fake CHT if checkpointing is enabled
 	var (
-		evmux   = new(event.TypeMux)
-		pow     = ethash.NewFaker()
 		db      = ethdb.NewMemDatabase()
-		config  = &params.ChainConfig{DAOForkBlock: big.NewInt(1), DAOForkSupport: localForked}
-		gspec   = &core.Genesis{Config: config}
-		genesis = gspec.MustCommit(db)
+		config  = new(params.ChainConfig)
+		genesis = (&core.Genesis{Config: config}).MustCommit(db)
 	)
-	blockchain, err := core.NewBlockChain(db, nil, config, pow, vm.Config{}, nil)
+	// If checkpointing is enabled, create and inject a fake CHT and the corresponding
+	// chllenge response.
+	var response *types.Header
+	if checkpoint {
+		index := uint64(rand.Intn(500))
+		number := (index+1)*params.CHTFrequencyClient - 1
+		response = &types.Header{Number: big.NewInt(int64(number)), Extra: []byte("valid")}
+
+		cht := &params.TrustedCheckpoint{
+			SectionIndex: index,
+			SectionHead:  response.Hash(),
+		}
+		params.TrustedCheckpoints[genesis.Hash()] = cht
+		defer delete(params.TrustedCheckpoints, genesis.Hash())
+	}
+	// Create a checkpoint aware protocol manager
+	blockchain, err := core.NewBlockChain(db, nil, config, ethash.NewFaker(), vm.Config{}, nil)
 	if err != nil {
 		t.Fatalf("failed to create new blockchain: %v", err)
 	}
-	pm, err := NewProtocolManager(config, downloader.FullSync, DefaultConfig.NetworkId, evmux, new(testTxPool), pow, blockchain, db, nil)
+	pm, err := NewProtocolManager(config, syncmode, DefaultConfig.NetworkId, new(event.TypeMux), new(testTxPool), ethash.NewFaker(), blockchain, db, nil)
 	if err != nil {
 		t.Fatalf("failed to start test protocol manager: %v", err)
 	}
 	pm.Start(1000)
 	defer pm.Stop()
 
-	// Connect a new peer and check that we receive the DAO challenge
+	// Connect a new peer and check that we receive the checkpoint challenge
 	peer, _ := newTestPeer("peer", eth63, pm, true)
 	defer peer.close()
 
-	challenge := &getBlockHeadersData{
-		Origin:  hashOrNumber{Number: config.DAOForkBlock.Uint64()},
-		Amount:  1,
-		Skip:    0,
-		Reverse: false,
-	}
-	if err := p2p.ExpectMsg(peer.app, GetBlockHeadersMsg, challenge); err != nil {
-		t.Fatalf("challenge mismatch: %v", err)
-	}
-	// Create a block to reply to the challenge if no timeout is simulated
-	if !timeout {
-		blocks, _ := core.GenerateChain(&params.ChainConfig{}, genesis, ethash.NewFaker(), db, 1, func(i int, block *core.BlockGen) {
-			if remoteForked {
-				block.SetExtra(params.DAOForkBlockExtra)
+	if checkpoint {
+		challenge := &getBlockHeadersData{
+			Origin:  hashOrNumber{Number: response.Number.Uint64()},
+			Amount:  1,
+			Skip:    0,
+			Reverse: false,
+		}
+		if err := p2p.ExpectMsg(peer.app, GetBlockHeadersMsg, challenge); err != nil {
+			t.Fatalf("challenge mismatch: %v", err)
+		}
+		// Create a block to reply to the challenge if no timeout is simulated
+		if !timeout {
+			if empty {
+				if err := p2p.Send(peer.app, BlockHeadersMsg, []*types.Header{}); err != nil {
+					t.Fatalf("failed to answer challenge: %v", err)
+				}
+			} else if match {
+				if err := p2p.Send(peer.app, BlockHeadersMsg, []*types.Header{response}); err != nil {
+					t.Fatalf("failed to answer challenge: %v", err)
+				}
+			} else {
+				if err := p2p.Send(peer.app, BlockHeadersMsg, []*types.Header{{Number: response.Number}}); err != nil {
+					t.Fatalf("failed to answer challenge: %v", err)
+				}
 			}
-		})
-		if err := p2p.Send(peer.app, BlockHeadersMsg, []*types.Header{blocks[0].Header()}); err != nil {
-			t.Fatalf("failed to answer challenge: %v", err)
 		}
-		time.Sleep(100 * time.Millisecond) // Sleep to avoid the verification racing with the drops
-	} else {
-		// Otherwise wait until the test timeout passes
-		time.Sleep(daoChallengeTimeout + 500*time.Millisecond)
 	}
-	// Verify that depending on fork side, the remote peer is maintained or dropped
-	if localForked == remoteForked && !timeout {
-		if peers := pm.peers.Len(); peers != 1 {
-			t.Fatalf("peer count mismatch: have %d, want %d", peers, 1)
-		}
-	} else {
+	// Wait until the test timeout passes to ensure proper cleanup
+	time.Sleep(syncChallengeTimeout + 100*time.Millisecond)
+
+	// Verify that the remote peer is maintained or dropped
+	if drop {
 		if peers := pm.peers.Len(); peers != 0 {
 			t.Fatalf("peer count mismatch: have %d, want %d", peers, 0)
 		}
+	} else {
+		if peers := pm.peers.Len(); peers != 1 {
+			t.Fatalf("peer count mismatch: have %d, want %d", peers, 1)
+		}
 	}
 }
 
diff --git a/eth/peer.go b/eth/peer.go
index b5f450855..54c0ea0d5 100644
--- a/eth/peer.go
+++ b/eth/peer.go
@@ -79,7 +79,7 @@ type peer struct {
 	rw p2p.MsgReadWriter
 
 	version  int         // Protocol version negotiated
-	forkDrop *time.Timer // Timed connection dropper if forks aren't validated in time
+	syncDrop *time.Timer // Timed connection dropper if sync progress isn't validated in time
 
 	head common.Hash
 	td   *big.Int
diff --git a/eth/sync.go b/eth/sync.go
index bfcfb6716..3e23c5378 100644
--- a/eth/sync.go
+++ b/eth/sync.go
@@ -188,14 +188,12 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 		atomic.StoreUint32(&pm.fastSync, 1)
 		mode = downloader.FastSync
 	}
-
 	if mode == downloader.FastSync {
 		// Make sure the peer's total difficulty we are synchronizing is higher.
 		if pm.blockchain.GetTdByHash(pm.blockchain.CurrentFastBlock().Hash()).Cmp(pTd) >= 0 {
 			return
 		}
 	}
-
 	// Run the sync cycle, and disable fast sync if we've went past the pivot block
 	if err := pm.downloader.Synchronise(peer.id, pHead, pTd, mode); err != nil {
 		return