Need to adjust price of opcode PACK/UNPACK, CONVERT to prevent DoS attack

[Qiao-Jin]

Summary or problem description
We have discussed about relationship between opcode functionalities & prices in #2004, #1875, etc. However, there still exist some opcodes whose prices are lower than their execution costs in worst cases, which might be utilized in DoS attack.

PACK/UNPACK

According to test result in #2004 the execution time cost of PACK/UNPACK in worst cases are 3874/1656 times of NOP. But the price of both the 2 opcodes are currently 512 times of NOP. Besides, test result shows that a malicious node can use this to delay about 7 seconds with 1 GAS's cost.

        public void TestDosPacking()
        {
            using var script = new ScriptBuilder();
            script.EmitPush((2 * 1024) - 1);
            script.Emit(OpCode.NEWARRAY_T, new byte[] { (byte)StackItemType.Integer });
            script.Emit(OpCode.UNPACK);
            script.Emit(OpCode.PACK);
            script.EmitJump(OpCode.JMP, -2);
            var sc = script.ToArray();
            var snapshot = TestBlockchain.TheNeoSystem.GetSnapshot();
            for (int x = 0; x < 10; x++)
            {
                using var engine = ApplicationEngine.Run(sc, snapshot: snapshot, gas:
                1_00000000);
                engine.LoadScript(script.ToArray(), -1, 0);
                engine.Execute();
            }
        }

CONVERT

Similarly, for opcode CONVERT test result shows that a malicious node can use this to delay about 9.2 seconds with 1 GAS's cost.

        public void TestDosBuffer()
        {
            using var script = new ScriptBuilder();
            script.EmitPush(1024 * 1024);
            script.Emit(OpCode.NEWBUFFER);
            script.Emit(OpCode.CONVERT, new byte[] { (byte)StackItemType.ByteString });
            script.Emit(OpCode.CONVERT, new byte[] { (byte)StackItemType.Buffer });
            script.EmitJump(OpCode.JMP, -4); // Infinite loop
            var sc = script.ToArray();
            var snapshot = TestBlockchain.TheNeoSystem.GetSnapshot();
            for (int x = 0; x < 10; x++)
            {
                using var engine = ApplicationEngine.Run(sc, snapshot: snapshot, gas:
                1_00000000);
                engine.LoadScript(script.ToArray(), -1, 0);
                engine.Execute();
            }
        }

Do you have any solution you want to propose?
Redefine the prices of the 3 opcodes.

Neo Version

• Neo 3