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.

2 files changed, 7 insertions, 3 deletions

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 beb4c6b73..84e3dfd78 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