eth/downloader: bypass peer validation if remote peer is far away (#1219)

* eth/downloader: bypass peer validation if remote peer is far away

* eth/downloader: parameterise threshold, fix condition

* eth: add tests for bypassing validation

* eth/downloader: declare max validation threshold instead of using direct value

* address comments

* simplify

eth/downloader/downloader.go

@@ -57,6 +57,8 @@ var (
 	fsHeaderSafetyNet = 2048            // Number of headers to discard in case a chain violation is detected
 	fsHeaderContCheck = 3 * time.Second // Time interval to check for header continuations during state download
 	fsMinFullBlocks   = 64              // Number of blocks to retrieve fully even in snap sync
+
+	maxValidationThreshold = uint64(1024) // Number of block difference from remote peer to start validation
 )
 
 var (
@@ -147,7 +149,9 @@ type Downloader struct {
 	quitCh   chan struct{} // Quit channel to signal termination
 	quitLock sync.Mutex    // Lock to prevent double closes
 
+	// Validation
 	ethereum.ChainValidator
+	maxValidationThreshold uint64 // Number of block difference from remote peer to start validation
 
 	// Testing hooks
 	syncInitHook     func(uint64, uint64)  // Method to call upon initiating a new sync run
@@ -241,6 +245,7 @@ func New(stateDb ethdb.Database, mux *event.TypeMux, chain BlockChain, lightchai
 		stateSyncStart:         make(chan *stateSync),
 		syncStartBlock:         chain.CurrentSnapBlock().Number.Uint64(),
 		ChainValidator:         whitelistService,
+		maxValidationThreshold: maxValidationThreshold,
 	}
 	// Create the post-merge skeleton syncer and start the process
 	dl.skeleton = newSkeleton(stateDb, dl.peers, dropPeer, newBeaconBackfiller(dl, success))
@@ -893,13 +898,6 @@ func (d *Downloader) getFetchHeadersByNumber(p *peerConnection) func(number uint
 // In the rare scenario when we ended up on a long reorganisation (i.e. none of
 // the head links match), we do a binary search to find the common ancestor.
 func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header) (uint64, error) {
-	// Check the validity of peer from which the chain is to be downloaded
-	if d.ChainValidator != nil {
-		if _, err := d.IsValidPeer(d.getFetchHeadersByNumber(p)); err != nil {
-			return 0, err
-		}
-	}
-
 	// Figure out the valid ancestor range to prevent rewrite attacks
 	var (
 		floor        = int64(-1)
@@ -917,6 +915,25 @@ func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header)
 		localHeight = d.lightchain.CurrentHeader().Number.Uint64()
 	}
 
+	// Check the validity of peer from which the chain is to be downloaded
+	if d.ChainValidator != nil {
+		_, err := d.IsValidPeer(d.getFetchHeadersByNumber(p))
+		if errors.Is(err, whitelist.ErrMismatch) {
+			return 0, err
+		}
+
+		if errors.Is(err, whitelist.ErrNoRemote) {
+			// Don't validate the peer against whitelisted milestones until the different of
+			// our local height and remote peer's height is less than `maxValidationThreshold`
+			if localHeight >= remoteHeight-d.maxValidationThreshold {
+				log.Info("Remote peer didn't respond", "id", p.id, "local", localHeight, "remote", remoteHeight, "err", err)
+				return 0, err
+			}
+
+			log.Info("Remote peer didn't respond but is far ahead, skipping validation", "id", p.id, "local", localHeight, "remote", remoteHeight, "err", err)
+		}
+	}
+
 	p.log.Debug("Looking for common ancestor", "local", localHeight, "remote", remoteHeight)
 
 	// Recap floor value for binary search

eth/downloader/downloader_test.go

@@ -1649,14 +1649,42 @@ func TestFakedSyncProgress67NoRemoteCheckpoint(t *testing.T) {
 	chainA := testChainForkLightA.blocks
 	tester.newPeer("light", protocol, chainA[1:])
 
+	// Set the max validation threshold equal to chain length to enforce validation
+	tester.downloader.maxValidationThreshold = uint64(len(chainA) - 1)
+
 	// Synchronise with the peer and make sure all blocks were retrieved
 	// Should fail in first attempt
-	if err := tester.sync("light", nil, mode); err != nil {
+	err := tester.sync("light", nil, mode)
 	assert.Equal(t, whitelist.ErrNoRemote, err, "failed synchronisation")
-	}
 
 	// Try syncing again, should succeed
 	if err := tester.sync("light", nil, mode); err != nil {
 		t.Fatal("succeeded attacker synchronisation")
 	}
 }
+
+// TestFakedSyncProgress67BypassWhitelistValidation tests if peer validation
+// via whitelist is bypassed when remote peer is far away or not
+func TestFakedSyncProgress67BypassWhitelistValidation(t *testing.T) {
+	protocol := uint(eth.ETH67)
+	mode := FullSync
+
+	tester := newTester(t)
+	validate := func(count int) (bool, error) {
+		return false, whitelist.ErrNoRemote
+	}
+
+	tester.downloader.ChainValidator = newWhitelistFake(validate)
+
+	defer tester.terminate()
+
+	// 1223 length chain
+	chainA := testChainBase.blocks
+	tester.newPeer("light", protocol, chainA[1:])
+
+	// Although the validate function above returns an error (which says that
+	// remote peer doesn't have that block), sync will go through as the chain
+	// import length is 1223 which is more than the default threshold of 1024
+	err := tester.sync("light", nil, mode)
+	assert.NoError(t, err, "failed synchronisation")
+}