From 52db6d8be577669bd5ba659ac223acf61956b05a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?P=C3=A9ter=20Szil=C3=A1gyi?= Date: Thu, 21 May 2015 08:37:27 +0300 Subject: [PATCH] eth/downloader: circumvent a forged block chain with known parent attack --- eth/downloader/downloader.go | 33 +++++++++++++++++----------- eth/downloader/downloader_test.go | 36 ++++++++++++++++++++++++++++++- 2 files changed, 56 insertions(+), 13 deletions(-) diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go index f3a8664414..f0629a551e 100644 --- a/eth/downloader/downloader.go +++ b/eth/downloader/downloader.go @@ -61,13 +61,18 @@ type hashPack struct { hashes []common.Hash } +type crossCheck struct { + expire time.Time + parent common.Hash +} + type Downloader struct { mux *event.TypeMux 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 + queue *queue // Scheduler for selecting the hashes to download + peers *peerSet // Set of active peers from which download can proceed + checks map[common.Hash]*crossCheck // Pending cross checks to verify a hash chain // Callbacks hasBlock hashCheckFn @@ -158,7 +163,7 @@ 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) + d.checks = make(map[common.Hash]*crossCheck) // Retrieve the origin peer and initiate the downloading process p := d.peers.Peer(id) @@ -290,11 +295,15 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error { } // Try and fetch a random block to verify the hash batch // Skip the last hash as the cross check races with the next hash fetch - cross := inserts[rand.Intn(len(inserts)-1)] - glog.V(logger.Detail).Infof("Cross checking (%s) with %x", active.id, cross) + cross := rand.Intn(len(inserts) - 1) + origin, parent := inserts[cross], inserts[cross+1] + glog.V(logger.Detail).Infof("Cross checking (%s) with %x/%x", active.id, origin, parent) - d.checks[cross] = time.Now().Add(blockTTL) - active.getBlocks([]common.Hash{cross}) + d.checks[origin] = &crossCheck{ + expire: time.Now().Add(blockTTL), + parent: parent, + } + active.getBlocks([]common.Hash{origin}) // Also fetch a fresh active.getHashes(head) @@ -314,8 +323,8 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error { continue } block := blockPack.blocks[0] - if _, ok := d.checks[block.Hash()]; ok { - if !d.queue.Has(block.ParentHash()) { + if check, ok := d.checks[block.Hash()]; ok { + if block.ParentHash() != check.parent { return ErrCrossCheckFailed } delete(d.checks, block.Hash()) @@ -323,8 +332,8 @@ func (d *Downloader) fetchHashes(p *peer, h common.Hash) error { 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) { + for hash, check := range d.checks { + if time.Now().After(check.expire) { glog.V(logger.Debug).Infof("Cross check timeout for %x", hash) return ErrCrossCheckFailed } diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go index 8ed3289c69..d623a7c767 100644 --- a/eth/downloader/downloader_test.go +++ b/eth/downloader/downloader_test.go @@ -502,7 +502,7 @@ func TestMadeupBlockChainAttack(t *testing.T) { crossCheckCycle = 25 * time.Millisecond // Create a long chain of blocks and simulate an invalid chain by dropping every second - hashes := createHashes(0, 32*blockCacheLimit) + hashes := createHashes(0, 16*blockCacheLimit) blocks := createBlocksFromHashes(hashes) gapped := make([]common.Hash, len(hashes)/2) @@ -525,3 +525,37 @@ func TestMadeupBlockChainAttack(t *testing.T) { t.Fatalf("failed to synchronise blocks: %v", err) } } + +// Advanced form of the above forged blockchain attack, where not only does the +// attacker make up a valid hashes for random blocks, but also forges the block +// parents to point to existing hashes. +func TestMadeupParentBlockChainAttack(t *testing.T) { + defaultBlockTTL := blockTTL + defaultCrossCheckCycle := crossCheckCycle + + 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, 16*blockCacheLimit) + blocks := createBlocksFromHashes(hashes) + forges := createBlocksFromHashes(hashes) + for hash, block := range forges { + block.ParentHeaderHash = hash // Simulate pointing to already known hash + } + // Try and sync with the malicious node and check that it fails + tester := newTester(t, hashes, forges) + 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) + } + // Ensure that a valid chain can still pass sync + blockTTL = defaultBlockTTL + crossCheckCycle = defaultCrossCheckCycle + + tester.blocks = blocks + 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) + } +}