eth, eth/downloader: handle a potential unknown parent attack

eth/downloader/downloader.go

@@ -37,6 +37,7 @@ var (
 	errCancelHashFetch     = errors.New("hash fetching cancelled (requested)")
 	errCancelBlockFetch    = errors.New("block downloading cancelled (requested)")
 	errNoSyncActive        = errors.New("no sync active")
+	ErrUnknownParent       = errors.New("block has unknown parent")
 )
 
 type hashCheckFn func(common.Hash) bool
@@ -142,16 +143,19 @@ func (d *Downloader) Synchronise(id string, hash common.Hash) error {
 	return d.syncWithPeer(p, hash)
 }
 
-// TakeBlocks takes blocks from the queue and yields them to the blockTaker handler
-// it's possible it yields no blocks
-func (d *Downloader) TakeBlocks() types.Blocks {
-	// Check that there are blocks available and its parents are known
+// TakeBlocks takes blocks from the queue and yields them to the caller.
+func (d *Downloader) TakeBlocks() (types.Blocks, error) {
+	// If the head block is missing, no blocks are ready
 	head := d.queue.GetHeadBlock()
-	if head == nil || !d.hasBlock(head.ParentHash()) {
-		return nil
+	if head == nil {
+		return nil, nil
 	}
-	// Retrieve a full batch of blocks
-	return d.queue.TakeBlocks(head)
+	// If the parent hash of the head is unknown, notify the caller
+	if !d.hasBlock(head.ParentHash()) {
+		return nil, ErrUnknownParent
+	}
+	// Otherwise retrieve a full batch of blocks
+	return d.queue.TakeBlocks(head), nil
 }
 
 func (d *Downloader) Has(hash common.Hash) bool {

eth/downloader/downloader_test.go

@@ -10,7 +10,10 @@ import (
 	"github.com/ethereum/go-ethereum/core/types"
 )
 
-var knownHash = common.Hash{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
+var (
+	knownHash   = common.Hash{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
+	unknownHash = common.Hash{9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9}
+)
 
 func createHashes(start, amount int) (hashes []common.Hash) {
 	hashes = make([]common.Hash, amount+1)
@@ -27,7 +30,7 @@ func createBlock(i int, prevHash, hash common.Hash) *types.Block {
 	header := &types.Header{Number: big.NewInt(int64(i))}
 	block := types.NewBlockWithHeader(header)
 	block.HeaderHash = hash
-	block.ParentHeaderHash = knownHash
+	block.ParentHeaderHash = prevHash
 	return block
 }
 
@@ -42,9 +45,12 @@ func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
 }
 
 type downloadTester struct {
-	downloader   *Downloader
-	hashes       []common.Hash
-	blocks       map[common.Hash]*types.Block
+	downloader *Downloader
+
+	hashes []common.Hash                // Chain of hashes simulating
+	blocks map[common.Hash]*types.Block // Blocks associated with the hashes
+	chain  []common.Hash                // Block-chain being constructed
+
 	t            *testing.T
 	pcount       int
 	done         chan bool
@@ -52,7 +58,15 @@ type downloadTester struct {
 }
 
 func newTester(t *testing.T, hashes []common.Hash, blocks map[common.Hash]*types.Block) *downloadTester {
-	tester := &downloadTester{t: t, hashes: hashes, blocks: blocks, done: make(chan bool)}
+	tester := &downloadTester{
+		t: t,
+
+		hashes: hashes,
+		blocks: blocks,
+		chain:  []common.Hash{knownHash},
+
+		done: make(chan bool),
+	}
 	downloader := New(tester.hasBlock, tester.getBlock)
 	tester.downloader = downloader
 
@@ -64,9 +78,17 @@ func (dl *downloadTester) sync(peerId string, hash common.Hash) error {
 	return dl.downloader.Synchronise(peerId, hash)
 }
 
+func (dl *downloadTester) insertBlocks(blocks types.Blocks) {
+	for _, block := range blocks {
+		dl.chain = append(dl.chain, block.Hash())
+	}
+}
+
 func (dl *downloadTester) hasBlock(hash common.Hash) bool {
-	if knownHash == hash {
-		return true
+	for _, h := range dl.chain {
+		if h == hash {
+			return true
+		}
 	}
 	return false
 }
@@ -175,10 +197,12 @@ func TestTaking(t *testing.T) {
 	if err != nil {
 		t.Error("download error", err)
 	}
-
-	bs1 := tester.downloader.TakeBlocks()
-	if len(bs1) != 1000 {
-		t.Error("expected to take 1000, got", len(bs1))
+	bs, err := tester.downloader.TakeBlocks()
+	if err != nil {
+		t.Fatalf("failed to take blocks: %v", err)
+	}
+	if len(bs) != targetBlocks {
+		t.Error("retrieved block mismatch: have %v, want %v", len(bs), targetBlocks)
 	}
 }
 
@@ -248,17 +272,18 @@ func TestThrottling(t *testing.T) {
 	done := make(chan struct{})
 	took := []*types.Block{}
 	go func() {
-		for {
+		for running := true; running; {
 			select {
 			case <-done:
-				took = append(took, tester.downloader.TakeBlocks()...)
-				done <- struct{}{}
-				return
+				running = false
 			default:
-				took = append(took, tester.downloader.TakeBlocks()...)
 				time.Sleep(time.Millisecond)
 			}
+			// Take a batch of blocks and accumulate
+			blocks, _ := tester.downloader.TakeBlocks()
+			took = append(took, blocks...)
 		}
+		done <- struct{}{}
 	}()
 
 	// Synchronise the two threads and verify
@@ -273,3 +298,44 @@ func TestThrottling(t *testing.T) {
 		t.Fatalf("downloaded block mismatch: have %v, want %v", len(took), targetBlocks)
 	}
 }
+
+// Tests that if a peer returns an invalid chain with a block pointing to a non-
+// existing parent, it is correctly detected and handled.
+func TestNonExistingParentAttack(t *testing.T) {
+	// Forge a single-link chain with a forged header
+	hashes := createHashes(0, 1)
+	blocks := createBlocksFromHashes(hashes)
+
+	forged := blocks[hashes[0]]
+	forged.ParentHeaderHash = 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 != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	bs, err := tester.downloader.TakeBlocks()
+	if err != ErrUnknownParent {
+		t.Fatalf("take error mismatch: have %v, want %v", err, ErrUnknownParent)
+	}
+	if len(bs) != 0 {
+		t.Error("retrieved block mismatch: have %v, want %v", len(bs), 0)
+	}
+	// Cancel the download due to the parent attack
+	tester.downloader.Cancel()
+
+	// Reconstruct a valid chain, and try to synchronize with it
+	forged.ParentHeaderHash = knownHash
+	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)
+	}
+	bs, err = tester.downloader.TakeBlocks()
+	if err != nil {
+		t.Fatalf("failed to retrieve blocks: %v", err)
+	}
+	if len(bs) != 1 {
+		t.Error("retrieved block mismatch: have %v, want %v", len(bs), 1)
+	}
+}

eth/sync.go

@@ -2,6 +2,7 @@ package eth
 
 import (
 	"math"
+	"sync/atomic"
 	"time"
 
 	"github.com/ethereum/go-ethereum/eth/downloader"
@@ -14,6 +15,7 @@ import (
 func (pm *ProtocolManager) update() {
 	forceSync := time.Tick(forceSyncCycle)
 	blockProc := time.Tick(blockProcCycle)
+	blockProcPend := int32(0)
 
 	for {
 		select {
@@ -36,7 +38,14 @@ func (pm *ProtocolManager) update() {
 			}
 		case <-blockProc:
 			// Try to pull some blocks from the downloaded
-			go pm.processBlocks()
+			if atomic.CompareAndSwapInt32(&blockProcPend, 0, 1) {
+				go func() {
+					if err := pm.processBlocks(); err != nil {
+						pm.downloader.Cancel()
+					}
+					atomic.StoreInt32(&blockProcPend, 0)
+				}()
+			}
 
 		case <-pm.quitSync:
 			return
@@ -52,8 +61,12 @@ func (pm *ProtocolManager) processBlocks() error {
 	pm.wg.Add(1)
 	defer pm.wg.Done()
 
-	// Take a batch of blocks (will return nil if a previous batch has not reached the chain yet)
-	blocks := pm.downloader.TakeBlocks()
+	// Take a batch of blocks, but abort if there's an invalid head or if the chain's empty
+	blocks, err := pm.downloader.TakeBlocks()
+	if err != nil {
+		glog.V(logger.Warn).Infof("Block processing failed: %v", err)
+		return err
+	}
 	if len(blocks) == 0 {
 		return nil
 	}
@@ -63,9 +76,7 @@ func (pm *ProtocolManager) processBlocks() error {
 		max := int(math.Min(float64(len(blocks)), float64(blockProcAmount)))
 		_, err := pm.chainman.InsertChain(blocks[:max])
 		if err != nil {
-			// cancel download process
-			pm.downloader.Cancel()
-
+			glog.V(logger.Warn).Infof("Block insertion failed: %v", err)
 			return err
 		}
 		blocks = blocks[max:]