eth/downloader: circumvent a fake blockchain attack

eth/downloader/downloader.go

@@ -90,7 +90,6 @@ func New(mux *event.TypeMux, hasBlock hashCheckFn, getBlock getBlockFn) *Downloa
 		mux:       mux,
 		queue:     newQueue(),
 		peers:     newPeerSet(),
-		checks:    make(map[common.Hash]time.Time),
 		hasBlock:  hasBlock,
 		getBlock:  getBlock,
 		newPeerCh: make(chan *peer, 1),
@@ -160,6 +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)
@@ -184,7 +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{})
@@ -259,8 +259,6 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			// 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", active.id)
-				d.queue.Reset()
-
 				return errEmptyHashSet
 			}
 			// Determine if we're done fetching hashes (queue up all pending), and continue if not done
@@ -277,8 +275,6 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			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)
-				d.queue.Reset()
-
 				return ErrBadPeer
 			}
 			if !done {
@@ -306,8 +302,13 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			if blockPack.peerId != active.id || len(blockPack.blocks) != 1 {
 				continue
 			}
-			hash := blockPack.blocks[0].Hash()
+			block := blockPack.blocks[0]
-			delete(d.checks, hash)
+			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
@@ -334,7 +335,6 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error {
 			// if all peers have been tried, abort the process entirely or if the hash is
 			// the zero hash.
 			if p == nil || (head == common.Hash{}) {
-				d.queue.Reset()
 				return ErrTimeout
 			}
 			// set p to the active peer. this will invalidate any hashes that may be returned
@@ -380,7 +380,6 @@ out:
 				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
-						d.queue.Reset()
 						return err
 					}
 					// Peer did deliver, but some blocks were off, penalize
@@ -414,7 +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
@@ -448,7 +446,6 @@ 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 errPeersUnavailable
 				}
 

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 := 0; i < len(hashes); i++ {
-	for i, hash := range hashes {
+		parent := knownHash
-		blocks[hash] = createBlock(len(hashes)-i, knownHash, hash)
+		if i < len(hashes)-1 {
+			parent = hashes[i+1]
+		}
+		blocks[hashes[i]] = createBlock(len(hashes)-i, parent, hashes[i])
 	}
-
 	return blocks
 }
 
@@ -136,6 +137,7 @@ func (dl *downloadTester) getHashes(head common.Hash) error {
 	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++
@@ -144,9 +146,11 @@ func (dl *downloadTester) getHashes(head common.Hash) error {
 		}
 	}
 	// Delay delivery a bit to allow attacks to unfold
+	id := dl.activePeerId
+	go func() {
 		time.Sleep(time.Millisecond)
-
+		dl.downloader.DeliverHashes(id, hashes)
-	dl.downloader.DeliverHashes(dl.activePeerId, hashes)
+	}()
 	return nil
 }
 
@@ -424,12 +428,15 @@ func TestInvalidHashOrderAttack(t *testing.T) {
 	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)
-	for i := len(hashes) / 4; i < 2*len(hashes)/4; i++ {
+	copy(reverse[blockCacheLimit:], chunk2)
-		reverse[i], reverse[len(hashes)-i-1] = reverse[len(hashes)-i-1], reverse[i]
-	}
 
 	// Try and sync with the malicious node and check that it fails
 	tester := newTester(t, reverse, blocks)
@@ -453,7 +460,6 @@ func TestMadeupHashChainAttack(t *testing.T) {
 
 	// Create a long chain of hashes without backing blocks
 	hashes := createHashes(0, 1024*blockCacheLimit)
-	hashes = hashes[:len(hashes)-1]
 
 	// Try and sync with the malicious node and check that it fails
 	tester := newTester(t, hashes, nil)
@@ -462,3 +468,31 @@ func TestMadeupHashChainAttack(t *testing.T) {
 		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)
+	}
+}