core, eth: roll back uncertain headers in failed fast syncs

core/blockchain.go

@@ -245,7 +245,21 @@ func (bc *BlockChain) SetHead(head uint64) {
 	if bc.currentBlock == nil {
 		bc.currentBlock = bc.genesisBlock
 	}
-	bc.insert(bc.currentBlock)
+	if bc.currentHeader == nil {
+		bc.currentHeader = bc.genesisBlock.Header()
+	}
+	if bc.currentFastBlock == nil {
+		bc.currentFastBlock = bc.genesisBlock
+	}
+	if err := WriteHeadBlockHash(bc.chainDb, bc.currentBlock.Hash()); err != nil {
+		glog.Fatalf("failed to reset head block hash: %v", err)
+	}
+	if err := WriteHeadHeaderHash(bc.chainDb, bc.currentHeader.Hash()); err != nil {
+		glog.Fatalf("failed to reset head header hash: %v", err)
+	}
+	if err := WriteHeadFastBlockHash(bc.chainDb, bc.currentFastBlock.Hash()); err != nil {
+		glog.Fatalf("failed to reset head fast block hash: %v", err)
+	}
 	bc.loadLastState()
 }
 
@@ -790,6 +804,27 @@ func (self *BlockChain) InsertHeaderChain(chain []*types.Header, checkFreq int)
 	return 0, nil
 }
 
+// Rollback is designed to remove a chain of links from the database that aren't
+// certain enough to be valid.
+func (self *BlockChain) Rollback(chain []common.Hash) {
+	for i := len(chain) - 1; i >= 0; i-- {
+		hash := chain[i]
+
+		if self.currentHeader.Hash() == hash {
+			self.currentHeader = self.GetHeader(self.currentHeader.ParentHash)
+			WriteHeadHeaderHash(self.chainDb, self.currentHeader.Hash())
+		}
+		if self.currentFastBlock.Hash() == hash {
+			self.currentFastBlock = self.GetBlock(self.currentFastBlock.ParentHash())
+			WriteHeadFastBlockHash(self.chainDb, self.currentFastBlock.Hash())
+		}
+		if self.currentBlock.Hash() == hash {
+			self.currentBlock = self.GetBlock(self.currentBlock.ParentHash())
+			WriteHeadBlockHash(self.chainDb, self.currentBlock.Hash())
+		}
+	}
+}
+
 // InsertReceiptChain attempts to complete an already existing header chain with
 // transaction and receipt data.
 func (self *BlockChain) InsertReceiptChain(blockChain types.Blocks, receiptChain []types.Receipts) (int, error) {

core/blockchain_test.go

@@ -599,8 +599,8 @@ func testReorgBadHashes(t *testing.T, full bool) {
 			t.Errorf("last  block gasLimit mismatch: have: %x, want %x", ncm.GasLimit(), blocks[2].Header().GasLimit)
 		}
 	} else {
-		if ncm.CurrentHeader().Hash() != genesis.Hash() {
-			t.Errorf("last header hash mismatch: have: %x, want %x", ncm.CurrentHeader().Hash(), genesis.Hash())
+		if ncm.CurrentHeader().Hash() != headers[2].Hash() {
+			t.Errorf("last header hash mismatch: have: %x, want %x", ncm.CurrentHeader().Hash(), headers[2].Hash())
 		}
 	}
 }
@@ -775,6 +775,11 @@ func TestLightVsFastVsFullChainHeads(t *testing.T) {
 	height := uint64(1024)
 	blocks, receipts := GenerateChain(genesis, gendb, int(height), nil)
 
+	// Configure a subchain to roll back
+	remove := []common.Hash{}
+	for _, block := range blocks[height/2:] {
+		remove = append(remove, block.Hash())
+	}
 	// Create a small assertion method to check the three heads
 	assert := func(t *testing.T, kind string, chain *BlockChain, header uint64, fast uint64, block uint64) {
 		if num := chain.CurrentBlock().NumberU64(); num != block {
@@ -798,6 +803,8 @@ func TestLightVsFastVsFullChainHeads(t *testing.T) {
 		t.Fatalf("failed to process block %d: %v", n, err)
 	}
 	assert(t, "archive", archive, height, height, height)
+	archive.Rollback(remove)
+	assert(t, "archive", archive, height/2, height/2, height/2)
 
 	// Import the chain as a non-archive node and ensure all pointers are updated
 	fastDb, _ := ethdb.NewMemDatabase()
@@ -816,6 +823,8 @@ func TestLightVsFastVsFullChainHeads(t *testing.T) {
 		t.Fatalf("failed to insert receipt %d: %v", n, err)
 	}
 	assert(t, "fast", fast, height, height, 0)
+	fast.Rollback(remove)
+	assert(t, "fast", fast, height/2, height/2, 0)
 
 	// Import the chain as a light node and ensure all pointers are updated
 	lightDb, _ := ethdb.NewMemDatabase()
@@ -827,6 +836,8 @@ func TestLightVsFastVsFullChainHeads(t *testing.T) {
 		t.Fatalf("failed to insert header %d: %v", n, err)
 	}
 	assert(t, "light", light, height, 0, 0)
+	light.Rollback(remove)
+	assert(t, "light", light, height/2, 0, 0)
 }
 
 // Tests that chain reorganizations handle transaction removals and reinsertions.

eth/downloader/downloader.go

@@ -59,8 +59,8 @@ var (
 	maxQueuedStates   = 256 * 1024 // [eth/63] Maximum number of state requests to queue (DOS protection)
 	maxResultsProcess = 256        // Number of download results to import at once into the chain
 
-	headerCheckFrequency = 64   // Verification frequency of the downloaded headers during fast sync
-	minCheckedHeaders    = 1024 // Number of headers to verify fully when approaching the chain head
+	headerCheckFrequency = 100  // Verification frequency of the downloaded headers during fast sync
+	minCheckedHeaders    = 2048 // Number of headers to verify fully when approaching the chain head
 	minFullBlocks        = 1024 // Number of blocks to retrieve fully even in fast sync
 )
 
@@ -117,6 +117,7 @@ type Downloader struct {
 	insertHeaders   headerChainInsertFn      // Injects a batch of headers into the chain
 	insertBlocks    blockChainInsertFn       // Injects a batch of blocks into the chain
 	insertReceipts  receiptChainInsertFn     // Injects a batch of blocks and their receipts into the chain
+	rollback        chainRollbackFn          // Removes a batch of recently added chain links
 	dropPeer        peerDropFn               // Drops a peer for misbehaving
 
 	// Status
@@ -152,7 +153,7 @@ type Downloader struct {
 func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, hasHeader headerCheckFn, hasBlock blockCheckFn, getHeader headerRetrievalFn,
 	getBlock blockRetrievalFn, headHeader headHeaderRetrievalFn, headBlock headBlockRetrievalFn, headFastBlock headFastBlockRetrievalFn,
 	commitHeadBlock headBlockCommitterFn, getTd tdRetrievalFn, insertHeaders headerChainInsertFn, insertBlocks blockChainInsertFn,
-	insertReceipts receiptChainInsertFn, dropPeer peerDropFn) *Downloader {
+	insertReceipts receiptChainInsertFn, rollback chainRollbackFn, dropPeer peerDropFn) *Downloader {
 
 	return &Downloader{
 		mode:            mode,
@@ -171,6 +172,7 @@ func New(mode SyncMode, stateDb ethdb.Database, mux *event.TypeMux, hasHeader he
 		insertHeaders:   insertHeaders,
 		insertBlocks:    insertBlocks,
 		insertReceipts:  insertReceipts,
+		rollback:        rollback,
 		dropPeer:        dropPeer,
 		newPeerCh:       make(chan *peer, 1),
 		hashCh:          make(chan dataPack, 1),
@@ -394,7 +396,7 @@ func (d *Downloader) syncWithPeer(p *peer, hash common.Hash, td *big.Int) (err e
 			d.syncInitHook(origin, latest)
 		}
 		errc := make(chan error, 4)
-		go func() { errc <- d.fetchHeaders(p, td, origin+1) }() // Headers are always retrieved
+		go func() { errc <- d.fetchHeaders(p, td, origin+1, latest) }() // Headers are always retrieved
 		go func() { errc <- d.fetchBodies(origin + 1) }()               // Bodies are retrieved during normal and fast sync
 		go func() { errc <- d.fetchReceipts(origin + 1) }()             // Receipts are retrieved during fast sync
 		go func() { errc <- d.fetchNodeData() }()                       // Node state data is retrieved during fast sync
@@ -1049,10 +1051,28 @@ func (d *Downloader) findAncestor(p *peer) (uint64, error) {
 //
 // The queue parameter can be used to switch between queuing headers for block
 // body download too, or directly import as pure header chains.
-func (d *Downloader) fetchHeaders(p *peer, td *big.Int, from uint64) error {
+func (d *Downloader) fetchHeaders(p *peer, td *big.Int, from, latest uint64) error {
 	glog.V(logger.Debug).Infof("%v: downloading headers from #%d", p, from)
 	defer glog.V(logger.Debug).Infof("%v: header download terminated", p)
 
+	// Keep a count of uncertain headers to roll back
+	rollback := []*types.Header{}
+	defer func() {
+		if len(rollback) > 0 {
+			hashes := make([]common.Hash, len(rollback))
+			for i, header := range rollback {
+				hashes[i] = header.Hash()
+			}
+			d.rollback(hashes)
+		}
+	}()
+	// Calculate the pivoting point for switching from fast to slow sync
+	pivot := uint64(0)
+	if d.mode == FastSync && latest > uint64(minFullBlocks) {
+		pivot = latest - uint64(minFullBlocks)
+	} else if d.mode == LightSync {
+		pivot = latest
+	}
 	// Create a timeout timer, and the associated hash fetcher
 	request := time.Now()       // time of the last fetch request
 	timeout := time.NewTimer(0) // timer to dump a non-responsive active peer
@@ -1124,10 +1144,30 @@ func (d *Downloader) fetchHeaders(p *peer, td *big.Int, from uint64) error {
 			glog.V(logger.Detail).Infof("%v: schedule %d headers from #%d", p, len(headers), from)
 
 			if d.mode == FastSync || d.mode == LightSync {
-				if n, err := d.insertHeaders(headers, headerCheckFrequency); err != nil {
+				// Collect the yet unknown headers to mark them as uncertain
+				unknown := make([]*types.Header, 0, len(headers))
+				for _, header := range headers {
+					if !d.hasHeader(header.Hash()) {
+						unknown = append(unknown, header)
+					}
+				}
+				// If we're importing pure headers, verify based on their recentness
+				frequency := headerCheckFrequency
+				if headers[len(headers)-1].Number.Uint64()+uint64(minCheckedHeaders) > pivot {
+					frequency = 1
+				}
+				if n, err := d.insertHeaders(headers, frequency); err != nil {
 					glog.V(logger.Debug).Infof("%v: invalid header #%d [%x…]: %v", p, headers[n].Number, headers[n].Hash().Bytes()[:4], err)
 					return errInvalidChain
 				}
+				// All verifications passed, store newly found uncertain headers
+				rollback = append(rollback, unknown...)
+				if len(rollback) > minCheckedHeaders {
+					rollback = append(rollback[:0], rollback[len(rollback)-minCheckedHeaders:]...)
+				}
+				if headers[len(headers)-1].Number.Uint64() >= pivot {
+					rollback = rollback[:0]
+				}
 			}
 			if d.mode == FullSync || d.mode == FastSync {
 				inserts := d.queue.Schedule(headers, from)

eth/downloader/downloader_test.go

@@ -152,7 +152,7 @@ func newTester(mode SyncMode) *downloadTester {
 	tester.stateDb, _ = ethdb.NewMemDatabase()
 	tester.downloader = New(mode, tester.stateDb, new(event.TypeMux), tester.hasHeader, tester.hasBlock, tester.getHeader,
 		tester.getBlock, tester.headHeader, tester.headBlock, tester.headFastBlock, tester.commitHeadBlock, tester.getTd,
-		tester.insertHeaders, tester.insertBlocks, tester.insertReceipts, tester.dropPeer)
+		tester.insertHeaders, tester.insertBlocks, tester.insertReceipts, tester.rollback, tester.dropPeer)
 
 	return tester
 }
@@ -272,6 +272,16 @@ func (dl *downloadTester) insertHeaders(headers []*types.Header, checkFreq int)
 	dl.lock.Lock()
 	defer dl.lock.Unlock()
 
+	// Do a quick check, as the blockchain.InsertHeaderChain doesn't insert anthing in case of errors
+	if _, ok := dl.ownHeaders[headers[0].ParentHash]; !ok {
+		return 0, errors.New("unknown parent")
+	}
+	for i := 1; i < len(headers); i++ {
+		if headers[i].ParentHash != headers[i-1].Hash() {
+			return i, errors.New("unknown parent")
+		}
+	}
+	// Do a full insert if pre-checks passed
 	for i, header := range headers {
 		if _, ok := dl.ownHeaders[header.Hash()]; ok {
 			continue
@@ -322,6 +332,22 @@ func (dl *downloadTester) insertReceipts(blocks types.Blocks, receipts []types.R
 	return len(blocks), nil
 }
 
+// rollback removes some recently added elements from the chain.
+func (dl *downloadTester) rollback(hashes []common.Hash) {
+	dl.lock.Lock()
+	defer dl.lock.Unlock()
+
+	for i := len(hashes) - 1; i >= 0; i-- {
+		if dl.ownHashes[len(dl.ownHashes)-1] == hashes[i] {
+			dl.ownHashes = dl.ownHashes[:len(dl.ownHashes)-1]
+		}
+		delete(dl.ownChainTd, hashes[i])
+		delete(dl.ownHeaders, hashes[i])
+		delete(dl.ownReceipts, hashes[i])
+		delete(dl.ownBlocks, hashes[i])
+	}
+}
+
 // newPeer registers a new block download source into the downloader.
 func (dl *downloadTester) newPeer(id string, version int, hashes []common.Hash, headers map[common.Hash]*types.Header, blocks map[common.Hash]*types.Block, receipts map[common.Hash]types.Receipts) error {
 	return dl.newSlowPeer(id, version, hashes, headers, blocks, receipts, 0)
@@ -1031,6 +1057,56 @@ func testShiftedHeaderAttack(t *testing.T, protocol int, mode SyncMode) {
 	assertOwnChain(t, tester, targetBlocks+1)
 }
 
+// Tests that upon detecting an invalid header, the recent ones are rolled back
+func TestInvalidHeaderRollback63Fast(t *testing.T)  { testInvalidHeaderRollback(t, 63, FastSync) }
+func TestInvalidHeaderRollback64Fast(t *testing.T)  { testInvalidHeaderRollback(t, 64, FastSync) }
+func TestInvalidHeaderRollback64Light(t *testing.T) { testInvalidHeaderRollback(t, 64, LightSync) }
+
+func testInvalidHeaderRollback(t *testing.T, protocol int, mode SyncMode) {
+	// Create a small enough block chain to download
+	targetBlocks := 3*minCheckedHeaders + minFullBlocks
+	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil)
+
+	tester := newTester(mode)
+
+	// Attempt to sync with an attacker that feeds junk during the fast sync phase
+	tester.newPeer("fast-attack", protocol, hashes, headers, blocks, receipts)
+	missing := minCheckedHeaders + MaxHeaderFetch + 1
+	delete(tester.peerHeaders["fast-attack"], hashes[len(hashes)-missing])
+
+	if err := tester.sync("fast-attack", nil); err == nil {
+		t.Fatalf("succeeded fast attacker synchronisation")
+	}
+	if head := tester.headHeader().Number.Int64(); int(head) > MaxHeaderFetch {
+		t.Fatalf("rollback head mismatch: have %v, want at most %v", head, MaxHeaderFetch)
+	}
+	// Attempt to sync with an attacker that feeds junk during the block import phase
+	tester.newPeer("block-attack", protocol, hashes, headers, blocks, receipts)
+	missing = 3*minCheckedHeaders + MaxHeaderFetch + 1
+	delete(tester.peerHeaders["block-attack"], hashes[len(hashes)-missing])
+
+	if err := tester.sync("block-attack", nil); err == nil {
+		t.Fatalf("succeeded block attacker synchronisation")
+	}
+	if mode == FastSync {
+		// Fast sync should not discard anything below the verified pivot point
+		if head := tester.headHeader().Number.Int64(); int(head) < 3*minCheckedHeaders {
+			t.Fatalf("rollback head mismatch: have %v, want at least %v", head, 3*minCheckedHeaders)
+		}
+	} else if mode == LightSync {
+		// Light sync should still discard data as before
+		if head := tester.headHeader().Number.Int64(); int(head) > 3*minCheckedHeaders {
+			t.Fatalf("rollback head mismatch: have %v, want at most %v", head, 3*minCheckedHeaders)
+		}
+	}
+	// Synchronise with the valid peer and make sure sync succeeds
+	tester.newPeer("valid", protocol, hashes, headers, blocks, receipts)
+	if err := tester.sync("valid", nil); err != nil {
+		t.Fatalf("failed to synchronise blocks: %v", err)
+	}
+	assertOwnChain(t, tester, targetBlocks+1)
+}
+
 // Tests that if a peer sends an invalid block piece (body or receipt) for a
 // requested block, it gets dropped immediately by the downloader.
 func TestInvalidContentAttack62(t *testing.T)      { testInvalidContentAttack(t, 62, FullSync) }

eth/downloader/types.go

@@ -60,6 +60,9 @@ type blockChainInsertFn func(types.Blocks) (int, error)
 // receiptChainInsertFn is a callback type to insert a batch of receipts into the local chain.
 type receiptChainInsertFn func(types.Blocks, []types.Receipts) (int, error)
 
+// chainRollbackFn is a callback type to remove a few recently added elements from the local chain.
+type chainRollbackFn func([]common.Hash)
+
 // peerDropFn is a callback type for dropping a peer detected as malicious.
 type peerDropFn func(id string)
 

eth/handler.go

@@ -131,7 +131,7 @@ func NewProtocolManager(mode Mode, networkId int, mux *event.TypeMux, txpool txP
 	}
 	manager.downloader = downloader.New(syncMode, chaindb, manager.eventMux, blockchain.HasHeader, blockchain.HasBlock, blockchain.GetHeader,
 		blockchain.GetBlock, blockchain.CurrentHeader, blockchain.CurrentBlock, blockchain.CurrentFastBlock, blockchain.FastSyncCommitHead,
-		blockchain.GetTd, blockchain.InsertHeaderChain, blockchain.InsertChain, blockchain.InsertReceiptChain, manager.removePeer)
+		blockchain.GetTd, blockchain.InsertHeaderChain, blockchain.InsertChain, blockchain.InsertReceiptChain, blockchain.Rollback, manager.removePeer)
 
 	validator := func(block *types.Block, parent *types.Block) error {
 		return core.ValidateHeader(pow, block.Header(), parent.Header(), true, false)