Merge pull request #988 from karalabe/fix-downloader-vulnerabilities

Fix downloader vulnerabilities

4 files changed, 336 insertions, 122 deletions

diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 616971f73..f9bd5a635 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -2,7 +2,7 @@ package downloader
 
 import (
 	"errors"
-	"fmt"
+	"math/rand"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -15,29 +15,34 @@ import (
 )
 
 const (
-	maxBlockFetch    = 128              // Amount of max blocks to be fetched per chunk
+	maxHashFetch     = 512              // Amount of hashes to be fetched per chunk
+	maxBlockFetch    = 128              // Amount of blocks to be fetched per chunk
 	peerCountTimeout = 12 * time.Second // Amount of time it takes for the peer handler to ignore minDesiredPeerCount
-	hashTtl          = 20 * time.Second // The amount of time it takes for a hash request to time out
+	hashTTL          = 5 * time.Second  // Time it takes for a hash request to time out
 )
 
 var (
-	minDesiredPeerCount = 5                // Amount of peers desired to start syncing
-	blockTtl            = 20 * time.Second // The amount of time it takes for a block request to time out
-
-	errLowTd               = errors.New("peer's TD is too low")
-	ErrBusy                = errors.New("busy")
-	errUnknownPeer         = errors.New("peer's unknown or unhealthy")
-	errBadPeer             = errors.New("action from bad peer ignored")
-	errNoPeers             = errors.New("no peers to keep download active")
-	ErrPendingQueue        = errors.New("pending items in queue")
-	ErrTimeout             = errors.New("timeout")
-	errEmptyHashSet        = errors.New("empty hash set by peer")
-	errPeersUnavailable    = errors.New("no peers available or all peers tried for block download process")
-	errAlreadyInPool       = errors.New("hash already in pool")
-	errBlockNumberOverflow = errors.New("received block which overflows")
-	errCancelHashFetch     = errors.New("hash fetching cancelled (requested)")
-	errCancelBlockFetch    = errors.New("block downloading cancelled (requested)")
-	errNoSyncActive        = errors.New("no sync active")
+	blockTTL            = 5 * time.Second // Time it takes for a block request to time out
+	crossCheckCycle     = time.Second     // Period after which to check for expired cross checks
+	minDesiredPeerCount = 5               // Amount of peers desired to start syncing
+)
+
+var (
+	errLowTd            = errors.New("peer's TD is too low")
+	ErrBusy             = errors.New("busy")
+	errUnknownPeer      = errors.New("peer's unknown or unhealthy")
+	ErrBadPeer          = errors.New("action from bad peer ignored")
+	errNoPeers          = errors.New("no peers to keep download active")
+	ErrPendingQueue     = errors.New("pending items in queue")
+	ErrTimeout          = errors.New("timeout")
+	errEmptyHashSet     = errors.New("empty hash set by peer")
+	errPeersUnavailable = errors.New("no peers available or all peers tried for block download process")
+	errAlreadyInPool    = errors.New("hash already in pool")
+	ErrInvalidChain     = errors.New("retrieved hash chain is invalid")
+	ErrCrossCheckFailed = errors.New("block cross-check failed")
+	errCancelHashFetch  = errors.New("hash fetching cancelled (requested)")
+	errCancelBlockFetch = errors.New("block downloading cancelled (requested)")
+	errNoSyncActive     = errors.New("no sync active")
 )
 
 type hashCheckFn func(common.Hash) bool
@@ -58,9 +63,10 @@ type hashPack struct {
 type Downloader struct {
 	mux *event.TypeMux
 
-	mu    sync.RWMutex
-	queue *queue
-	peers *peerSet
+	mu     sync.RWMutex
+	queue  *queue                    // Scheduler for selecting the hashes to download
+	peers  *peerSet                  // Set of active peers from which download can proceed
+	checks map[common.Hash]time.Time // Pending cross checks to verify a hash chain
 
 	// Callbacks
 	hasBlock hashCheckFn
@@ -153,6 +159,7 @@ func (d *Downloader) Synchronise(id string, hash common.Hash) error {
 	// Reset the queue and peer set to clean any internal leftover state
 	d.queue.Reset()
 	d.peers.Reset()
+	d.checks = make(map[common.Hash]time.Time)
 
 	// Retrieve the origin peer and initiate the downloading process
 	p := d.peers.Peer(id)
@@ -177,7 +184,7 @@ func (d *Downloader) syncWithPeer(p *peer, hash common.Hash) (err error) {
 	defer func() {
 		// reset on error
 		if err != nil {
-			d.queue.Reset()
+			d.Cancel()
 			d.mux.Post(FailedEvent{err})
 		} else {
 			d.mux.Post(DoneEvent{})
@@ -221,66 +228,98 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 
 	start := time.Now()
 
-	// Add the hash to the queue first
+	// Add the hash to the queue first, and start hash retrieval
 	d.queue.Insert([]common.Hash{h})
-
-	// Get the first batch of hashes
 	p.getHashes(h)
 
 	var (
-		failureResponseTimer = time.NewTimer(hashTtl)
-		attemptedPeers       = make(map[string]bool) // attempted peers will help with retries
-		activePeer           = p                     // active peer will help determine the current active peer
-		hash                 common.Hash             // common and last hash
+		active = p             // active peer will help determine the current active peer
+		head   = common.Hash{} // common and last hash
+
+		timeout     = time.NewTimer(hashTTL)          // timer to dump a non-responsive active peer
+		attempted   = make(map[string]bool)           // attempted peers will help with retries
+		crossTicker = time.NewTicker(crossCheckCycle) // ticker to periodically check expired cross checks
 	)
-	attemptedPeers[p.id] = true
+	defer crossTicker.Stop()
 
-out:
-	for {
+	attempted[p.id] = true
+	for finished := false; !finished; {
 		select {
 		case <-d.cancelCh:
 			return errCancelHashFetch
+
 		case hashPack := <-d.hashCh:
 			// Make sure the active peer is giving us the hashes
-			if hashPack.peerId != activePeer.id {
+			if hashPack.peerId != active.id {
 				glog.V(logger.Debug).Infof("Received hashes from incorrect peer(%s)\n", hashPack.peerId)
 				break
 			}
-
-			failureResponseTimer.Reset(hashTtl)
+			timeout.Reset(hashTTL)
 
 			// Make sure the peer actually gave something valid
 			if len(hashPack.hashes) == 0 {
-				glog.V(logger.Debug).Infof("Peer (%s) responded with empty hash set\n", activePeer.id)
-				d.queue.Reset()
-
+				glog.V(logger.Debug).Infof("Peer (%s) responded with empty hash set\n", active.id)
 				return errEmptyHashSet
 			}
 			// Determine if we're done fetching hashes (queue up all pending), and continue if not done
 			done, index := false, 0
-			for index, hash = range hashPack.hashes {
-				if d.hasBlock(hash) || d.queue.GetBlock(hash) != nil {
-					glog.V(logger.Debug).Infof("Found common hash %x\n", hash[:4])
+			for index, head = range hashPack.hashes {
+				if d.hasBlock(head) || d.queue.GetBlock(head) != nil {
+					glog.V(logger.Debug).Infof("Found common hash %x\n", head[:4])
 					hashPack.hashes = hashPack.hashes[:index]
 					done = true
 					break
 				}
 			}
-			d.queue.Insert(hashPack.hashes)
-
+			// Insert all the new hashes, but only continue if got something useful
+			inserts := d.queue.Insert(hashPack.hashes)
+			if len(inserts) == 0 && !done {
+				glog.V(logger.Debug).Infof("Peer (%s) responded with stale hashes\n", active.id)
+				return ErrBadPeer
+			}
 			if !done {
-				activePeer.getHashes(hash)
+				// Try and fetch a random block to verify the hash batch
+				cross := inserts[rand.Intn(len(inserts))]
+				glog.V(logger.Detail).Infof("Cross checking (%s) with %x", active.id, cross)
+
+				d.checks[cross] = time.Now().Add(blockTTL)
+				active.getBlocks([]common.Hash{cross})
+
+				// Also fetch a fresh
+				active.getHashes(head)
 				continue
 			}
 			// We're done, allocate the download cache and proceed pulling the blocks
 			offset := 0
-			if block := d.getBlock(hash); block != nil {
+			if block := d.getBlock(head); block != nil {
 				offset = int(block.NumberU64() + 1)
 			}
 			d.queue.Alloc(offset)
-			break out
+			finished = true
 
-		case <-failureResponseTimer.C:
+		case blockPack := <-d.blockCh:
+			// Cross check the block with the random verifications
+			if blockPack.peerId != active.id || len(blockPack.blocks) != 1 {
+				continue
+			}
+			block := blockPack.blocks[0]
+			if _, ok := d.checks[block.Hash()]; ok {
+				if !d.queue.Has(block.ParentHash()) {
+					return ErrCrossCheckFailed
+				}
+				delete(d.checks, block.Hash())
+			}
+
+		case <-crossTicker.C:
+			// Iterate over all the cross checks and fail the hash chain if they're not verified
+			for hash, deadline := range d.checks {
+				if time.Now().After(deadline) {
+					glog.V(logger.Debug).Infof("Cross check timeout for %x", hash)
+					return ErrCrossCheckFailed
+				}
+			}
+
+		case <-timeout.C:
 			glog.V(logger.Debug).Infof("Peer (%s) didn't respond in time for hash request\n", p.id)
 
 			var p *peer // p will be set if a peer can be found
@@ -288,21 +327,20 @@ out:
 			// already fetched hash list. This can't guarantee 100% correctness but does
 			// a fair job. This is always either correct or false incorrect.
 			for _, peer := range d.peers.AllPeers() {
-				if d.queue.Has(peer.head) && !attemptedPeers[peer.id] {
+				if d.queue.Has(peer.head) && !attempted[peer.id] {
 					p = peer
 					break
 				}
 			}
 			// if all peers have been tried, abort the process entirely or if the hash is
 			// the zero hash.
-			if p == nil || (hash == common.Hash{}) {
-				d.queue.Reset()
+			if p == nil || (head == common.Hash{}) {
 				return ErrTimeout
 			}
 			// set p to the active peer. this will invalidate any hashes that may be returned
 			// by our previous (delayed) peer.
-			activePeer = p
-			p.getHashes(hash)
+			active = p
+			p.getHashes(head)
 			glog.V(logger.Debug).Infof("Hash fetching switched to new peer(%s)\n", p.id)
 		}
 	}
@@ -325,12 +363,26 @@ out:
 		select {
 		case <-d.cancelCh:
 			return errCancelBlockFetch
+
 		case blockPack := <-d.blockCh:
+			// Short circuit if it's a stale cross check
+			if len(blockPack.blocks) == 1 {
+				block := blockPack.blocks[0]
+				if _, ok := d.checks[block.Hash()]; ok {
+					delete(d.checks, block.Hash())
+					continue
+				}
+			}
 			// If the peer was previously banned and failed to deliver it's pack
 			// in a reasonable time frame, ignore it's message.
 			if peer := d.peers.Peer(blockPack.peerId); peer != nil {
-				// Deliver the received chunk of blocks, but drop the peer if invalid
+				// Deliver the received chunk of blocks
 				if err := d.queue.Deliver(blockPack.peerId, blockPack.blocks); err != nil {
+					if err == ErrInvalidChain {
+						// The hash chain is invalid (blocks are not ordered properly), abort
+						return err
+					}
+					// Peer did deliver, but some blocks were off, penalize
 					glog.V(logger.Debug).Infof("Failed delivery for peer %s: %v\n", blockPack.peerId, err)
 					peer.Demote()
 					break
@@ -348,7 +400,7 @@ out:
 			// that badly or poorly behave are removed from the peer set (not banned).
 			// Bad peers are excluded from the available peer set and therefor won't be
 			// reused. XXX We could re-introduce peers after X time.
-			badPeers := d.queue.Expire(blockTtl)
+			badPeers := d.queue.Expire(blockTTL)
 			for _, pid := range badPeers {
 				// XXX We could make use of a reputation system here ranking peers
 				// in their performance
@@ -361,7 +413,6 @@ out:
 			}
 			// After removing bad peers make sure we actually have sufficient peer left to keep downloading
 			if d.peers.Len() == 0 {
-				d.queue.Reset()
 				return errNoPeers
 			}
 			// If there are unrequested hashes left start fetching
@@ -395,9 +446,7 @@ out:
 				// Make sure that we have peers available for fetching. If all peers have been tried
 				// and all failed throw an error
 				if d.queue.InFlight() == 0 {
-					d.queue.Reset()
-
-					return fmt.Errorf("%v peers available = %d. total peers = %d. hashes needed = %d", errPeersUnavailable, len(idlePeers), d.peers.Len(), d.queue.Pending())
+					return errPeersUnavailable
 				}
 
 			} else if d.queue.InFlight() == 0 {
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index 50fe00d42..d55664314 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -23,25 +23,26 @@ func createHashes(start, amount int) (hashes []common.Hash) {
 	for i := range hashes[:len(hashes)-1] {
 		binary.BigEndian.PutUint64(hashes[i][:8], uint64(i+2))
 	}
-
 	return
 }
 
-func createBlock(i int, prevHash, hash common.Hash) *types.Block {
+func createBlock(i int, parent, hash common.Hash) *types.Block {
 	header := &types.Header{Number: big.NewInt(int64(i))}
 	block := types.NewBlockWithHeader(header)
 	block.HeaderHash = hash
-	block.ParentHeaderHash = prevHash
+	block.ParentHeaderHash = parent
 	return block
 }
 
 func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
 	blocks := make(map[common.Hash]*types.Block)
-
-	for i, hash := range hashes {
-		blocks[hash] = createBlock(len(hashes)-i, knownHash, hash)
+	for i := 0; i < len(hashes); i++ {
+		parent := knownHash
+		if i < len(hashes)-1 {
+			parent = hashes[i+1]
+		}
+		blocks[hashes[i]] = createBlock(len(hashes)-i, parent, hashes[i])
 	}
-
 	return blocks
 }
 
@@ -75,9 +76,40 @@ func newTester(t *testing.T, hashes []common.Hash, blocks map[common.Hash]*types
 	return tester
 }
 
-func (dl *downloadTester) sync(peerId string, hash common.Hash) error {
+// sync is a simple wrapper around the downloader to start synchronisation and
+// block until it returns
+func (dl *downloadTester) sync(peerId string, head common.Hash) error {
 	dl.activePeerId = peerId
-	return dl.downloader.Synchronise(peerId, hash)
+	return dl.downloader.Synchronise(peerId, head)
+}
+
+// syncTake is starts synchronising with a remote peer, but concurrently it also
+// starts fetching blocks that the downloader retrieved. IT blocks until both go
+// routines terminate.
+func (dl *downloadTester) syncTake(peerId string, head common.Hash) (types.Blocks, error) {
+	// Start a block collector to take blocks as they become available
+	done := make(chan struct{})
+	took := []*types.Block{}
+	go func() {
+		for running := true; running; {
+			select {
+			case <-done:
+				running = false
+			default:
+				time.Sleep(time.Millisecond)
+			}
+			// Take a batch of blocks and accumulate
+			took = append(took, dl.downloader.TakeBlocks()...)
+		}
+		done <- struct{}{}
+	}()
+	// Start the downloading, sync the taker and return
+	err := dl.sync(peerId, head)
+
+	done <- struct{}{}
+	<-done
+
+	return took, err
 }
 
 func (dl *downloadTester) insertBlocks(blocks types.Blocks) {
@@ -99,18 +131,37 @@ func (dl *downloadTester) getBlock(hash common.Hash) *types.Block {
 	return dl.blocks[knownHash]
 }
 
-func (dl *downloadTester) getHashes(hash common.Hash) error {
-	dl.downloader.DeliverHashes(dl.activePeerId, dl.hashes)
+// getHashes retrieves a batch of hashes for reconstructing the chain.
+func (dl *downloadTester) getHashes(head common.Hash) error {
+	// Gather the next batch of hashes
+	hashes := make([]common.Hash, 0, maxHashFetch)
+	for i, hash := range dl.hashes {
+		if hash == head {
+			i++
+			for len(hashes) < cap(hashes) && i < len(dl.hashes) {
+				hashes = append(hashes, dl.hashes[i])
+				i++
+			}
+			break
+		}
+	}
+	// Delay delivery a bit to allow attacks to unfold
+	id := dl.activePeerId
+	go func() {
+		time.Sleep(time.Millisecond)
+		dl.downloader.DeliverHashes(id, hashes)
+	}()
 	return nil
 }
 
 func (dl *downloadTester) getBlocks(id string) func([]common.Hash) error {
 	return func(hashes []common.Hash) error {
-		blocks := make([]*types.Block, len(hashes))
-		for i, hash := range hashes {
-			blocks[i] = dl.blocks[hash]
+		blocks := make([]*types.Block, 0, len(hashes))
+		for _, hash := range hashes {
+			if block, ok := dl.blocks[hash]; ok {
+				blocks = append(blocks, block)
+			}
 		}
-
 		go dl.downloader.DeliverBlocks(id, blocks)
 
 		return nil
@@ -134,7 +185,7 @@ func (dl *downloadTester) badBlocksPeer(id string, td *big.Int, hash common.Hash
 
 func TestDownload(t *testing.T) {
 	minDesiredPeerCount = 4
-	blockTtl = 1 * time.Second
+	blockTTL = 1 * time.Second
 
 	targetBlocks := 1000
 	hashes := createHashes(0, targetBlocks)
@@ -183,7 +234,7 @@ func TestMissing(t *testing.T) {
 
 func TestTaking(t *testing.T) {
 	minDesiredPeerCount = 4
-	blockTtl = 1 * time.Second
+	blockTTL = 1 * time.Second
 
 	targetBlocks := 1000
 	hashes := createHashes(0, targetBlocks)
@@ -224,7 +275,7 @@ func TestInactiveDownloader(t *testing.T) {
 
 func TestCancel(t *testing.T) {
 	minDesiredPeerCount = 4
-	blockTtl = 1 * time.Second
+	blockTTL = 1 * time.Second
 
 	targetBlocks := 1000
 	hashes := createHashes(0, targetBlocks)
@@ -250,7 +301,7 @@ func TestCancel(t *testing.T) {
 
 func TestThrottling(t *testing.T) {
 	minDesiredPeerCount = 4
-	blockTtl = 1 * time.Second
+	blockTTL = 1 * time.Second
 
 	targetBlocks := 16 * blockCacheLimit
 	hashes := createHashes(0, targetBlocks)
@@ -263,32 +314,7 @@ func TestThrottling(t *testing.T) {
 	tester.badBlocksPeer("peer4", big.NewInt(0), common.Hash{})
 
 	// Concurrently download and take the blocks
-	errc := make(chan error, 1)
-	go func() {
-		errc <- tester.sync("peer1", hashes[0])
-	}()
-
-	done := make(chan struct{})
-	took := []*types.Block{}
-	go func() {
-		for running := true; running; {
-			select {
-			case <-done:
-				running = false
-			default:
-				time.Sleep(time.Millisecond)
-			}
-			// Take a batch of blocks and accumulate
-			took = append(took, tester.downloader.TakeBlocks()...)
-		}
-		done <- struct{}{}
-	}()
-
-	// Synchronise the two threads and verify
-	err := <-errc
-	done <- struct{}{}
-	<-done
-
+	took, err := tester.syncTake("peer1", hashes[0])
 	if err != nil {
 		t.Fatalf("failed to synchronise blocks: %v", err)
 	}
@@ -336,3 +362,137 @@ func TestNonExistingParentAttack(t *testing.T) {
 		t.Fatalf("tester doesn't know about the origin hash")
 	}
 }
+
+// Tests that if a malicious peers keeps sending us repeating hashes, we don't
+// loop indefinitely.
+func TestRepeatingHashAttack(t *testing.T) {
+	// Create a valid chain, but drop the last link
+	hashes := createHashes(0, blockCacheLimit)
+	blocks := createBlocksFromHashes(hashes)
+	forged := hashes[:len(hashes)-1]
+
+	// Try and sync with the malicious node
+	tester := newTester(t, forged, blocks)
+	tester.newPeer("attack", big.NewInt(10000), forged[0])
+
+	errc := make(chan error)
+	go func() {
+		errc <- tester.sync("attack", hashes[0])
+	}()
+
+	// Make sure that syncing returns and does so with a failure
+	select {
+	case <-time.After(100 * time.Millisecond):
+		t.Fatalf("synchronisation blocked")
+	case err := <-errc:
+		if err == nil {
+			t.Fatalf("synchronisation succeeded")
+		}
+	}
+	// Ensure that a valid chain can still pass sync
+	tester.hashes = hashes
+	tester.newPeer("valid", big.NewInt(20000), hashes[0])
+	if err := tester.sync("valid", hashes[0]); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+}
+
+// Tests that if a malicious peers returns a non-existent block hash, it should
+// eventually time out and the sync reattempted.
+func TestNonExistingBlockAttack(t *testing.T) {
+	// Create a valid chain, but forge the last link
+	hashes := createHashes(0, blockCacheLimit)
+	blocks := createBlocksFromHashes(hashes)
+	origin := hashes[len(hashes)/2]
+
+	hashes[len(hashes)/2] = unknownHash
+
+	// Try and sync with the malicious node and check that it fails
+	tester := newTester(t, hashes, blocks)
+	tester.newPeer("attack", big.NewInt(10000), hashes[0])
+	if err := tester.sync("attack", hashes[0]); err != errPeersUnavailable {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errPeersUnavailable)
+	}
+	// Ensure that a valid chain can still pass sync
+	hashes[len(hashes)/2] = origin
+	tester.newPeer("valid", big.NewInt(20000), hashes[0])
+	if err := tester.sync("valid", hashes[0]); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+}
+
+// Tests that if a malicious peer is returning hashes in a weird order, that the
+// sync throttler doesn't choke on them waiting for the valid blocks.
+func TestInvalidHashOrderAttack(t *testing.T) {
+	// Create a valid long chain, but reverse some hashes within
+	hashes := createHashes(0, 4*blockCacheLimit)
+	blocks := createBlocksFromHashes(hashes)
+
+	chunk1 := make([]common.Hash, blockCacheLimit)
+	chunk2 := make([]common.Hash, blockCacheLimit)
+	copy(chunk1, hashes[blockCacheLimit:2*blockCacheLimit])
+	copy(chunk2, hashes[2*blockCacheLimit:3*blockCacheLimit])
+
+	reverse := make([]common.Hash, len(hashes))
+	copy(reverse, hashes)
+	copy(reverse[2*blockCacheLimit:], chunk1)
+	copy(reverse[blockCacheLimit:], chunk2)
+
+	// Try and sync with the malicious node and check that it fails
+	tester := newTester(t, reverse, blocks)
+	tester.newPeer("attack", big.NewInt(10000), reverse[0])
+	if _, err := tester.syncTake("attack", reverse[0]); err != ErrInvalidChain {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrInvalidChain)
+	}
+	// Ensure that a valid chain can still pass sync
+	tester.hashes = hashes
+	tester.newPeer("valid", big.NewInt(20000), hashes[0])
+	if _, err := tester.syncTake("valid", hashes[0]); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+}
+
+// Tests that if a malicious peer makes up a random hash chain and tries to push
+// indefinitely, it actually gets caught with it.
+func TestMadeupHashChainAttack(t *testing.T) {
+	blockTTL = 100 * time.Millisecond
+	crossCheckCycle = 25 * time.Millisecond
+
+	// Create a long chain of hashes without backing blocks
+	hashes := createHashes(0, 1024*blockCacheLimit)
+
+	// Try and sync with the malicious node and check that it fails
+	tester := newTester(t, hashes, nil)
+	tester.newPeer("attack", big.NewInt(10000), hashes[0])
+	if _, err := tester.syncTake("attack", hashes[0]); err != ErrCrossCheckFailed {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrCrossCheckFailed)
+	}
+}
+
+// Tests that if a malicious peer makes up a random block chain, and tried to
+// push indefinitely, it actually gets caught with it.
+func TestMadeupBlockChainAttack(t *testing.T) {
+	blockTTL = 100 * time.Millisecond
+	crossCheckCycle = 25 * time.Millisecond
+
+	// Create a long chain of blocks and simulate an invalid chain by dropping every second
+	hashes := createHashes(0, 32*blockCacheLimit)
+	blocks := createBlocksFromHashes(hashes)
+
+	gapped := make([]common.Hash, len(hashes)/2)
+	for i := 0; i < len(gapped); i++ {
+		gapped[i] = hashes[2*i]
+	}
+	// Try and sync with the malicious node and check that it fails
+	tester := newTester(t, gapped, blocks)
+	tester.newPeer("attack", big.NewInt(10000), gapped[0])
+	if _, err := tester.syncTake("attack", gapped[0]); err != ErrCrossCheckFailed {
+		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, ErrCrossCheckFailed)
+	}
+	// Ensure that a valid chain can still pass sync
+	tester.hashes = hashes
+	tester.newPeer("valid", big.NewInt(20000), hashes[0])
+	if _, err := tester.syncTake("valid", hashes[0]); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+}
diff --git a/eth/downloader/queue.go b/eth/downloader/queue.go
index 6ad915757..13ec9a520 100644
--- a/eth/downloader/queue.go
+++ b/eth/downloader/queue.go
@@ -122,24 +122,28 @@ func (q *queue) Has(hash common.Hash) bool {
 	return false
 }
 
-// Insert adds a set of hashes for the download queue for scheduling.
-func (q *queue) Insert(hashes []common.Hash) {
+// Insert adds a set of hashes for the download queue for scheduling, returning
+// the new hashes encountered.
+func (q *queue) Insert(hashes []common.Hash) []common.Hash {
 	q.lock.Lock()
 	defer q.lock.Unlock()
 
 	// Insert all the hashes prioritized in the arrival order
-	for i, hash := range hashes {
-		index := q.hashCounter + i
-
+	inserts := make([]common.Hash, 0, len(hashes))
+	for _, hash := range hashes {
+		// Skip anything we already have
 		if old, ok := q.hashPool[hash]; ok {
 			glog.V(logger.Warn).Infof("Hash %x already scheduled at index %v", hash, old)
 			continue
 		}
-		q.hashPool[hash] = index
-		q.hashQueue.Push(hash, float32(index)) // Highest gets schedules first
+		// Update the counters and insert the hash
+		q.hashCounter = q.hashCounter + 1
+		inserts = append(inserts, hash)
+
+		q.hashPool[hash] = q.hashCounter
+		q.hashQueue.Push(hash, float32(q.hashCounter)) // Highest gets schedules first
 	}
-	// Update the hash counter for the next batch of inserts
-	q.hashCounter += len(hashes)
+	return inserts
 }
 
 // GetHeadBlock retrieves the first block from the cache, or nil if it hasn't
@@ -296,18 +300,17 @@ func (q *queue) Deliver(id string, blocks []*types.Block) (err error) {
 	// Iterate over the downloaded blocks and add each of them
 	errs := make([]error, 0)
 	for _, block := range blocks {
-		// Skip any blocks that fall outside the cache range
-		index := int(block.NumberU64()) - q.blockOffset
-		if index >= len(q.blockCache) || index < 0 {
-			//fmt.Printf("block cache overflown (N=%v O=%v, C=%v)", block.Number(), q.blockOffset, len(q.blockCache))
-			continue
-		}
 		// Skip any blocks that were not requested
 		hash := block.Hash()
 		if _, ok := request.Hashes[hash]; !ok {
 			errs = append(errs, fmt.Errorf("non-requested block %v", hash))
 			continue
 		}
+		// If a requested block falls out of the range, the hash chain is invalid
+		index := int(block.NumberU64()) - q.blockOffset
+		if index >= len(q.blockCache) || index < 0 {
+			return ErrInvalidChain
+		}
 		// Otherwise merge the block and mark the hash block
 		q.blockCache[index] = block
 
diff --git a/eth/sync.go b/eth/sync.go
index c89f34596..aa7ebc77b 100644
--- a/eth/sync.go
+++ b/eth/sync.go
@@ -101,11 +101,13 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 	case downloader.ErrBusy:
 		glog.V(logger.Debug).Infof("Synchronisation already in progress")
 
-	case downloader.ErrTimeout:
-		glog.V(logger.Debug).Infof("Removing peer %v due to sync timeout", peer.id)
+	case downloader.ErrTimeout, downloader.ErrBadPeer, downloader.ErrInvalidChain, downloader.ErrCrossCheckFailed:
+		glog.V(logger.Debug).Infof("Removing peer %v: %v", peer.id, err)
 		pm.removePeer(peer)
+
 	case downloader.ErrPendingQueue:
 		glog.V(logger.Debug).Infoln("Synchronisation aborted:", err)
+
 	default:
 		glog.V(logger.Warn).Infof("Synchronisation failed: %v", err)
 	}