以太坊合约审计 CheckList 之变量覆盖问题

作者：LoRexxar'@知道创宇404区块链安全研究团队

时间：2018年11月16日

系列文章：

《以太坊智能合约审计 CheckList》 《以太坊合约审计 CheckList 之“以太坊智能合约规范问题”影响分析报告》 《以太坊合约审计 CheckList 之“以太坊智能合约设计缺陷问题”影响分析报告》 《以太坊合约审计 CheckList 之“以太坊智能合约编码安全问题”影响分析报告》 《以太坊合约审计 CheckList 之“以太坊智能合约编码设计问题”影响分析报告》 《以太坊合约审计 CheckList 之“以太坊智能合约编码隐患”影响分析报告》

2018年11月6日，DVP上线了一场“地球OL真实盗币游戏”，其中第二题是一道智能合约题目，题目中涉及到的了一个很有趣的问题，这里拿出来详细说说看。

https://etherscan.io/address/0x5170a14aa36245a8a9698f23444045bdc4522e0a#code

Writeup pragma solidity ^0.4.21; library SafeMath { function mul(uint256 a, uint256 b) internal pure returns (uint256) { if (a == 0) { return 0; } uint256 c = a * b; assert(c / a == b); return c; } function div(uint256 a, uint256 b) internal pure returns (uint256) { uint256 c = a / b; return c; } function sub(uint256 a, uint256 b) internal pure returns (uint256) { assert(b <= a); return a - b; } function add(uint256 a, uint256 b) internal pure returns (uint256) { uint256 c = a + b; assert(c >= a); return c; } } contract ERC20Basic { function totalSupply() public view returns (uint256); function balanceOf(address who) public view returns (uint256); function transfer(address to, uint256 value) public returns (bool); event Transfer(address indexed from, address indexed to, uint256 value); } contract ERC20 is ERC20Basic { function allowance(address owner, address spender) public view returns (uint256); function transferFrom(address from, address to, uint256 value) public returns (bool); function approve(address spender, uint256 value) public returns (bool); event Approval( address indexed owner, address indexed spender, uint256 value ); } library SafeERC20 { function safeTransfer(ERC20Basic token, address to, uint256 value) internal { require(token.transfer(to, value)); } function safeTransferFrom( ERC20 token, address from, address to, uint256 value ) internal { require(token.transferFrom(from, to, value)); } function safeApprove(ERC20 token, address spender, uint256 value) internal { require(token.approve(spender, value)); } } contract DVPgame { ERC20 public token; uint256[] map; using SafeERC20 for ERC20; using SafeMath for uint256; constructor(address addr) payable{ token = ERC20(addr); } function (){ if(map.length>=uint256(msg.sender)){ require(map[uint256(msg.sender)]!=1); } if(token.balanceOf(this)==0){ //airdrop is over selfdestruct(msg.sender); }else{ token.safeTransfer(msg.sender,100); if (map.length <= uint256(msg.sender)) { map.length = uint256(msg.sender) + 1; } map[uint256(msg.sender)] = 1; } } //Guess the value(param:x) of the keccak256 value modulo 10000 of the future block (param:blockNum) function guess(uint256 x,uint256 blockNum) public payable { require(msg.value == 0.001 ether || token.allowance(msg.sender,address(this))>=1*(10**18)); require(blockNum>block.number); if(token.allowance(msg.sender,address(this))>0){ token.safeTransferFrom(msg.sender,address(this),1*(10**18)); } if (map.length <= uint256(msg.sender)+x) { map.length = uint256(msg.sender)+x + 1; } map[uint256(msg.sender)+x] = blockNum; } //Run a lottery function lottery(uint256 x) public { require(map[uint256(msg.sender)+x]!=0); require(block.number > map[uint256(msg.sender)+x]); require(block.blockhash(map[uint256(msg.sender)+x])!=0); uint256 answer = uint256(keccak256(block.blockhash(map[uint256(msg.sender)+x])))%10000; if (x == answer) { token.safeTransfer(msg.sender,token.balanceOf(address(this))); selfdestruct(msg.sender); } } }

看着代码那么长，但其实核心代码就后面这点。

fallback函数

<span class="kd">function</span> <span class="p">(){</span> <span class="k">if</span><span class="p">(</span><span class="nx">map</span><span class="p">.</span><span class="nx">length</span><span class="o">>=</span><span class="nx">uint256</span><span class="p">(</span><span class="nx">msg</span><span class="p">.</span><span class="nx">sender</span><span class="p">)){</span> <span class="nx">require</span><span class="p">(</span><span class="nx">map</span><span class="p">[</span><span class="nx">uint256</span><span class="p">(</span><span class="nx">msg</span><span class="p">.</span><span class="nx">sender</span><span class="p">)]</span><span class="o">!=</span><span class="mi">1</span><span class="p">);</span> <span class="p">}</span> <span class="k">if</span><span class="p">(</span><span class="nx">token</span><span class="p">.</span><span class="nx">balanceOf</span><span class="p">(</span><span class="k">this</span><span class="p">)</span><span class="o">==</span><span class="mi">0</span><span class="p">){</span> <span class="c1">//airdrop is over</span> <span class="nx">selfdestruct</span><span class="p">(</span><span class="nx">msg</span><span class="p">.</span><span class="nx">sender</span><span class="p">);</span> <span class="c1">//如果token花完了，就会自动销毁自己发送余额</span> <span class="p">}</span><span class="k">else</span><span class="p">{</span> <span class="nx">token</span><span class="p">.</span><span class="nx">safeTransfer</span><span class="p">(</span><span class="nx">msg</span><span class="p">.</span><span class="nx">sender</span><span class="p">,</span><span class="mi">100</span><span class="p">);</span> <span class="c1">// 否则就给你转100token</span> <span class="k">if</span> <span class="p">(</span><span class="nx">map</span><span class="p">.</span><span class="nx">length</span> <span class="o"><=</span> <span class="nx">uint256</span><span class="p">(</span><span class="nx">msg</span><span class="p">.</span><span class="nx">sender</span><span class="p">))</span> <span class="p">{</span> <span class="nx">map</span><span class="p">.</span><span class="nx">length</span> <span class="o">=</span> <span class="nx">uint256</span><span class="p">(</span><span class="nx">msg</span