Add a size limit to block_cache_unverified.

[ShawnYun]

Summary or problem description

If we received a block, OnNewBlock will be called to add the block.

321         private VerifyResult OnNewBlock(Block block)
322         {
323             if (block.Index <= Height)
324                 return VerifyResult.AlreadyExists;
325             if (block.Index - 1 > Height)
326             {
327                 AddUnverifiedBlockToCache(block);
328                 return VerifyResult.UnableToVerify;
329             }
330             if (block.Index == Height + 1)
331             {
332                 if (!block.Verify(currentSnapshot))
333                     return VerifyResult.Invalid;
334                 block_cache.TryAdd(block.Hash, block);
335                 block_cache_unverified.Remove(block.Index);
336                 // We can store the new block in block_cache and tell the new height to other nodes before Persist().
337                 system.LocalNode.Tell(Message.Create(MessageCommand.Ping, PingPayload.Create(Singleton.Height + 1)));
338                 Persist(block);
339                 SaveHeaderHashList();
340                 if (block_cache_unverified.TryGetValue(Height + 1, out LinkedList<Block> unverifiedBlocks))
341                 {
342                     foreach (var unverifiedBlock in unverifiedBlocks)
343                         Self.Tell(unverifiedBlock, ActorRefs.NoSender);
344                     block_cache_unverified.Remove(Height + 1);
345                 }
346             }
347             return VerifyResult.Succeed;
348         }

If the block index is bigger than current block height plus 1, it will be added to unverified cache directly by calling AddUnverifiedBlockToCache.

Specifically, an attacker can keep sending blocks with large block index to flood the block_cache_unverified and cause the node Out-of-Memory. Further more, since the block_cache_unverified stores blocks with same index but different hashes into a LinkedList. Attacker can also flood the node with blocks that has same index but different hashes.

Do you have any solution you want to propose?

1. Add a size limit to block_cache_unverified.

Neo Version

• Neo 3

Where in the software does this update applies to?

• Ledger
• P2P (TCP)