eth: don't import propagated blocks during fastsync

eth/handler.go

@@ -171,6 +171,11 @@ func NewProtocolManager(config *params.ChainConfig, mode downloader.SyncMode, ne
 		return blockchain.CurrentBlock().NumberU64()
 	}
 	inserter := func(blocks types.Blocks) (int, error) {
+		// If fast sync is running, deny importing weird blocks
+		if atomic.LoadUint32(&manager.fastSync) == 1 {
+			log.Warn("Discarded bad propagated block", "number", blocks[0].Number(), "hash", blocks[0].Hash())
+			return 0, nil
+		}
 		atomic.StoreUint32(&manager.acceptTxs, 1) // Mark initial sync done on any fetcher import
 		return manager.blockchain.InsertChain(blocks)
 	}