eth, eth/downloader: detect and handle madeup hash attacks

3 files changed, 109 insertions, 42 deletions

diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 426da9beb..95dd37fd7 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -2,6 +2,7 @@ package downloader
 
 import (
 	"errors"
+	"math/rand"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -14,15 +15,19 @@ 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
+	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")
@@ -34,6 +39,7 @@ var (
 	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")
@@ -220,46 +226,47 @@ 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
+		crossChecks = make(map[common.Hash]time.Time) // running cross checks and their deadline
+		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)
+				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
 			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
@@ -267,25 +274,50 @@ out:
 			}
 			// Insert all the new hashes, but only continue if got something useful
 			inserts := d.queue.Insert(hashPack.hashes)
-			if inserts == 0 && !done {
-				glog.V(logger.Debug).Infof("Peer (%s) responded with stale hashes\n", activePeer.id)
+			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 {
-				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)
+
+				crossChecks[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 blockPack := <-d.blockCh:
+			// Cross check the block with the random verifications
+			if blockPack.peerId != active.id || len(blockPack.blocks) != 1 {
+				continue
+			}
+			hash := blockPack.blocks[0].Hash()
+			delete(crossChecks, hash)
+
+		case <-crossTicker.C:
+			// Iterate over all the cross checks and fail the hash chain if they're not verified
+			for hash, deadline := range crossChecks {
+				if time.Now().After(deadline) {
+					glog.V(logger.Debug).Infof("Cross check timeout for %x", hash)
+					return ErrCrossCheckFailed
+				}
+			}
 
-		case <-failureResponseTimer.C:
+		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
@@ -293,21 +325,21 @@ 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{}) {
+			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
 			// 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)
 		}
 	}
@@ -359,7 +391,7 @@ 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
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index 4b8ee93d2..60dcc06cd 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -130,8 +130,23 @@ 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 {
+			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
+	time.Sleep(time.Millisecond)
+
+	dl.downloader.DeliverHashes(dl.activePeerId, hashes)
 	return nil
 }
 
@@ -166,7 +181,7 @@ 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)
@@ -215,7 +230,7 @@ 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)
@@ -256,7 +271,7 @@ 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)
@@ -282,7 +297,7 @@ 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)
@@ -429,3 +444,21 @@ func TestInvalidHashOrderAttack(t *testing.T) {
 		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)
+	hashes = hashes[:len(hashes)-1]
+
+	// 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)
+	}
+}
diff --git a/eth/downloader/queue.go b/eth/downloader/queue.go
index aa48c521a..13ec9a520 100644
--- a/eth/downloader/queue.go
+++ b/eth/downloader/queue.go
@@ -123,13 +123,13 @@ func (q *queue) Has(hash common.Hash) bool {
 }
 
 // Insert adds a set of hashes for the download queue for scheduling, returning
-// the number of new hashes encountered.
-func (q *queue) Insert(hashes []common.Hash) int {
+// 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
-	inserts := 0
+	inserts := make([]common.Hash, 0, len(hashes))
 	for _, hash := range hashes {
 		// Skip anything we already have
 		if old, ok := q.hashPool[hash]; ok {
@@ -137,7 +137,9 @@ func (q *queue) Insert(hashes []common.Hash) int {
 			continue
 		}
 		// Update the counters and insert the hash
-		q.hashCounter, inserts = q.hashCounter+1, inserts+1
+		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
 	}