From 730751e28ee246c7ba082e2d10e782408fbadda8 Mon Sep 17 00:00:00 2001 From: Wei-Ning Huang Date: Thu, 24 Jan 2019 17:30:03 +0800 Subject: core: vm: modify randomness calculation algorithm (#173) The original algorithm used for calculating algorithm is vulnerable to cross context re-entry attack. Example as follows: contract B { event Value(uint256 value); uint256 public value; function call() public { value = rand; emit Value(value); } } contract A { function randTwice(address bAddr) public { B b = B(bAddr); b.call.gas(100000)(); b.call.gas(100000)(); } } The two `b.call` will result in the same randomness value. This commit fix the issue by recording a called index used to store how many times opRand is called, and use it as argument to the Keccak call. --- core/vm/evm.go | 2 ++ core/vm/instructions.go | 8 +++++--- 2 files changed, 7 insertions(+), 3 deletions(-) (limited to 'core') diff --git a/core/vm/evm.go b/core/vm/evm.go index 64f71e530..2eba9c2cb 100644 --- a/core/vm/evm.go +++ b/core/vm/evm.go @@ -101,6 +101,8 @@ type Context struct { Time *big.Int // Provides information for TIME Randomness []byte // Provides information for RAND Difficulty *big.Int // Provides information for DIFFICULTY + + RandCallIndex uint64 // Number of times opRand is called } // EVM is the Ethereum Virtual Machine base object and provides diff --git a/core/vm/instructions.go b/core/vm/instructions.go index 3a82190da..3d17287ed 100644 --- a/core/vm/instructions.go +++ b/core/vm/instructions.go @@ -417,14 +417,16 @@ func opRand(pc *uint64, interpreter *EVMInterpreter, contract *Contract, memory binaryNonce := make([]byte, binary.MaxVarintLen64) binary.PutUvarint(binaryNonce, nonce) - binaryGas := make([]byte, binary.MaxVarintLen64) - binary.PutUvarint(binaryGas, contract.Gas) + binaryUsedIndex := make([]byte, binary.MaxVarintLen64) + binary.PutUvarint(binaryUsedIndex, evm.RandCallIndex) + + evm.RandCallIndex += 1 hash := crypto.Keccak256( evm.Randomness, contract.Caller().Bytes(), binaryNonce, - binaryGas) + binaryUsedIndex) stack.push(interpreter.intPool.get().SetBytes(hash)) return nil, nil -- cgit