eth/downloader: retrieve pivot header from local chain if necessary

eth/downloader/beaconsync.go

@@ -265,7 +265,11 @@ func (d *Downloader) fetchBeaconHeaders(from uint64) error {
 			hashes  = make([]common.Hash, 0, maxHeadersProcess)
 		)
 		for i := 0; i < maxHeadersProcess && from <= head.Number.Uint64(); i++ {
-			headers = append(headers, d.skeleton.Header(from))
+			header := d.skeleton.Header(from)
+			if header == nil {
+				header = d.lightchain.GetHeaderByNumber(from)
+			}
+			headers = append(headers, header)
 			hashes = append(hashes, headers[i].Hash())
 			from++
 		}

eth/downloader/downloader.go

@@ -78,6 +78,7 @@ var (
 	errCanceled                = errors.New("syncing canceled (requested)")
 	errTooOld                  = errors.New("peer's protocol version too old")
 	errNoAncestorFound         = errors.New("no common ancestor found")
+	errNoPivotHeader           = errors.New("pivot header is not found")
 	ErrMergeTransition         = errors.New("legacy sync reached the merge")
 )
 
@@ -158,6 +159,9 @@ type LightChain interface {
 	// GetHeaderByHash retrieves a header from the local chain.
 	GetHeaderByHash(common.Hash) *types.Header
 
+	// GetHeaderByNumber retrieves a block header from the local chain by number.
+	GetHeaderByNumber(number uint64) *types.Header
+
 	// CurrentHeader retrieves the head header from the local chain.
 	CurrentHeader() *types.Header
 
@@ -477,7 +481,20 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td, ttd *
 			return err
 		}
 		if latest.Number.Uint64() > uint64(fsMinFullBlocks) {
-			pivot = d.skeleton.Header(latest.Number.Uint64() - uint64(fsMinFullBlocks))
+			number := latest.Number.Uint64() - uint64(fsMinFullBlocks)
+
+			// Retrieve the pivot header from the skeleton chain segment but
+			// fallback to local chain if it's not found in skeleton space.
+			if pivot = d.skeleton.Header(number); pivot == nil {
+				pivot = d.lightchain.GetHeaderByNumber(number)
+			}
+			// Print an error log and return directly in case the pivot header
+			// is still not found. It means the skeleton chain is not linked
+			// correctly with local chain.
+			if pivot == nil {
+				log.Error("Pivot header is not found", "number", number)
+				return errNoPivotHeader
+			}
 		}
 	}
 	// If no pivot block was returned, the head is below the min full block

eth/downloader/downloader_test.go

@@ -56,6 +56,11 @@ type downloadTester struct {
 
 // newTester creates a new downloader test mocker.
 func newTester() *downloadTester {
+	return newTesterWithNotification(nil)
+}
+
+// newTester creates a new downloader test mocker.
+func newTesterWithNotification(success func()) *downloadTester {
 	freezer, err := ioutil.TempDir("", "")
 	if err != nil {
 		panic(err)
@@ -75,7 +80,7 @@ func newTester() *downloadTester {
 		chain:   chain,
 		peers:   make(map[string]*downloadTesterPeer),
 	}
-	tester.downloader = New(0, db, new(event.TypeMux), tester.chain, nil, tester.dropPeer, nil)
+	tester.downloader = New(0, db, new(event.TypeMux), tester.chain, nil, tester.dropPeer, success)
 	return tester
 }
 
@@ -1368,3 +1373,51 @@ func testCheckpointEnforcement(t *testing.T, protocol uint, mode SyncMode) {
 		assertOwnChain(t, tester, len(chain.blocks))
 	}
 }
+
+// Tests that peers below a pre-configured checkpoint block are prevented from
+// being fast-synced from, avoiding potential cheap eclipse attacks.
+func TestBeaconSync66Full(t *testing.T) { testBeaconSync(t, eth.ETH66, FullSync) }
+func TestBeaconSync66Snap(t *testing.T) { testBeaconSync(t, eth.ETH66, SnapSync) }
+
+func testBeaconSync(t *testing.T, protocol uint, mode SyncMode) {
+	//log.Root().SetHandler(log.LvlFilterHandler(log.LvlInfo, log.StreamHandler(os.Stderr, log.TerminalFormat(true))))
+
+	var cases = []struct {
+		name  string // The name of testing scenario
+		local int    // The length of local chain(canonical chain assumed), 0 means genesis is the head
+	}{
+		{name: "Beacon sync since genesis", local: 0},
+		{name: "Beacon sync with short local chain", local: 1},
+		{name: "Beacon sync with long local chain", local: blockCacheMaxItems - 15 - fsMinFullBlocks/2},
+		{name: "Beacon sync with full local chain", local: blockCacheMaxItems - 15 - 1},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			success := make(chan struct{})
+			tester := newTesterWithNotification(func() {
+				close(success)
+			})
+			defer tester.terminate()
+
+			chain := testChainBase.shorten(blockCacheMaxItems - 15)
+			tester.newPeer("peer", protocol, chain.blocks[1:])
+
+			// Build the local chain segment if it's required
+			if c.local > 0 {
+				tester.chain.InsertChain(chain.blocks[1 : c.local+1])
+			}
+			if err := tester.downloader.BeaconSync(mode, chain.blocks[len(chain.blocks)-1].Header()); err != nil {
+				t.Fatalf("Failed to beacon sync chain %v %v", c.name, err)
+			}
+			select {
+			case <-success:
+				// Ok, downloader fully cancelled after sync cycle
+				if bs := int(tester.chain.CurrentBlock().NumberU64()) + 1; bs != len(chain.blocks) {
+					t.Fatalf("synchronised blocks mismatch: have %v, want %v", bs, len(chain.blocks))
+				}
+			case <-time.NewTimer(time.Second * 3).C:
+				t.Fatalf("Failed to sync chain in three seconds")
+			}
+		})
+	}
+}