From 0bb42a73d6e38f590cc61bf13a5cb9882c4ef6d8 Mon Sep 17 00:00:00 2001
From: Wei-Ning Huang <w@dexon.org>
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(-)
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
--
cgit