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DeFi YAM,一行代码如何蒸发数亿美元?

2020-08-13 慢雾区 来源:区块链网络

By :yudan @慢雾安全团队

前言


据链闻消息,2020 年 8 月 13 日,知名以太坊DeFi 项目 YAM 官方通过 Twitter 发文表明发现合约中存在漏洞,24 小时内价格暴跌 99% 。慢雾安全团队在收到情报后快速进行了相关的跟进及分析,以下是详细的技术细节。


发生了什么?


以上是YAM 官方对本次事件的简短说明(来源:

https://medium.com/@yamfinance/save-yam-245598d81cec)。


简单来说就是官方在合约中发现负责调整供应量的函数发生了问题,这个问题导致多余的 YAM 代币放进了 YAM 的 reserves 合约中,并且如果不修正这个问题,将会导致 YAM 的后续治理变为不可能。同时,官方给出了此次漏洞的具体问题代码,如下:


从上图可知,由于编码不规范,YAM 合约在调整 totalSupply 的时候,本应将最后的结果除以 BASE 变量,但是在实际开发过程中却忽略了,导致 totoalSupply 计算不正确,比原来的值要大 10^18 倍。但是代币供应量问题和治理是怎么扯上关系呢?这需要我们针对代码做进一步的分析。


YAM 会变成怎样?


为了深入了解此次漏洞造成的影响,需要对 YAM 项目代码进行深入的了解。根据官方给出的问题代码及项目 Github 地址(https://github.com/yam-finance/yam-protocol),可以定位出调整供应量的rebase函数位于 YAMDelegator.sol 合约中,具体代码如下:

function rebase( uint256 epoch, uint256 indexDelta, bool positive ) external returns (uint256) { epoch; indexDelta; positive; delegateAndReturn(); }

通过跟踪rebase函数,发现rebase函数最终调用了 delegateAndReturn 函数,代码如下:

function delegateAndReturn() private returns (bytes memory) { (bool success, ) = implementation.delegatecall(msg.data);
assembly { let free_mem_ptr := mload(0x40) returndatacopy(free_mem_ptr, 0, returndatasize)
switch success case 0 { revert(free_mem_ptr, returndatasize) } default { return(free_mem_ptr, returndatasize) } } }

通过分析代码,可以发现 delegateAndReturn 函数最终使用 delegatecall 的方式调用了 implementation 地址中的逻辑,也就是说,这是一个可升级的合约模型。而真正的rebase逻辑位于 YAM.sol 中, 继续跟进 rebase 函数的具体逻辑,如下:

function rebase( uint256 epoch, uint256 indexDelta, bool positive ) external onlyRebaser returns (uint256) { if (indexDelta == 0) { emit Rebase(epoch, yamsScalingFactor, yamsScalingFactor); return totalSupply; }
uint256 prevYamsScalingFactor = yamsScalingFactor;
if (!positive) { yamsScalingFactor = yamsScalingFactor.mul(BASE.sub(indexDelta)).div(BASE); } else { uint256 newScalingFactor = yamsScalingFactor.mul(BASE.add(indexDelta)).div(BASE); if (newScalingFactor < _maxScalingFactor()) { yamsScalingFactor = newScalingFactor; } else { yamsScalingFactor = _maxScalingFactor(); } } //SlowMist// 问题代码 totalSupply = initSupply.mul(yamsScalingFactor); emit Rebase(epoch, prevYamsScalingFactor, yamsScalingFactor); return totalSupply; }}

通过分析最终的rebase函数的逻辑,不难发现代码中根据 yamsScalingFactor 来对 totalSupply 进行调整,由于 yamsScalingFactor 是一个高精度的值,在调整完成后应当除以 BASE 来去除计算过程中的精度,获得正确的值。但是项目方在对 totalSupply 进行调整时,竟忘记了对计算结果进行调整,导致了 totalSupply 意外变大,计算出错误的结果。


分析到这里还没结束,要将漏洞和社区治理关联起来,需要对代码进行进一步的分析。通过观察rebase函数的修饰器,不难发现此处限定了只能是 rebaser 进行调用。而 rebaser 是 YAM 中用与实现供应量相关逻辑的合约,也就是说,是 rebaser 合约最终调用了 YAM.sol 合约中的rebase函数。通过跟踪相关代码,发现 rebaser 合约中对应供应量调整的逻辑为rebase函数,代码如下:

function rebase() public { // EOA only require(msg.sender == tx.origin); // ensure rebasing at correct time _inRebaseWindow();
// This comparison also ensures there is no reentrancy. require(lastRebaseTimestampSec.add(minRebaseTimeIntervalSec) < now);
// Snap the rebase time to the start of this window. lastRebaseTimestampSec = now.sub( now.mod(minRebaseTimeIntervalSec)).add(rebaseWindowOffsetSec);
epoch = epoch.add(1);
// get twap from uniswap v2; uint256 exchangeRate = getTWAP();
// calculates % change to supply (uint256 offPegPerc, bool positive) = computeOffPegPerc(exchangeRate);
uint256 indexDelta = offPegPerc;
// Apply the Dampening factor. indexDelta = indexDelta.div(rebaseLag);
YAMTokenInterface yam = YAMTokenInterface(yamAddress);
if (positive) { require(yam.yamsScalingFactor().mul(uint256(10**18).add(indexDelta)).div(10**18) < yam.maxScalingFactor(), "new scaling factor will be too big"); }
//SlowMist// 取当前 YAM 代币的供应量 uint256 currSupply = yam.totalSupply();
uint256 mintAmount; // reduce indexDelta to account for minting //SlowMist// 计算要调整的供应量 if (positive) { uint256 mintPerc = indexDelta.mul(rebaseMintPerc).div(10**18); indexDelta = indexDelta.sub(mintPerc); mintAmount = currSupply.mul(mintPerc).div(10**18); }
// rebase //SlowMist// 调用 YAM 的rebase 逻辑 uint256 supplyAfterRebase = yam.rebase(epoch, indexDelta, positive); assert(yam.yamsScalingFactor() <= yam.maxScalingFactor());
// perform actions after rebase //SlowMist// 进入调整逻辑 afterRebase(mintAmount, offPegPerc); }

通过分析代码,可以发现函数在进行了一系列的检查后,首先获取了当前 YAM 的供应量,计算此次的铸币数量,然后再调用 YAM.sol 中的rebase函数对 totalSupply 进行调整,也就是说 rebase 过后的对 totalSupply 的影响要在下一次调用 rebaser 合约的rebase函数才会生效。最后rebase函数调用了 afterRebase 函数。我们继续跟进 afterRebase 函数中的代码:

function afterRebase( uint256 mintAmount, uint256 offPegPerc ) internal { // update uniswap UniswapPair(uniswap_pair).sync(); //SlowMist// 通过 uniswap 购买 yCRV 代币 if (mintAmount > 0) { buyReserveAndTransfer( mintAmount, offPegPerc ); }
// call any extra functions //SlowMist// 社区管理调用 for (uint i = 0; i < transactions.length; i++) { Transaction storage t = transactions[i]; if (t.enabled) { bool result = externalCall(t.destination, t.data); if (!result) { emit TransactionFailed(t.destination, i, t.data); revert("Transaction Failed"); } } } }

通过分析发现, afterRebase 函数主要的逻辑在 buyReserveAndTransfer 函数中,此函数用于将增发出来的代币的一部分用于到 Uniswap 中购买 yCRV 代币。跟踪 buyReserveAndTransfer 函数,代码如下:

function buyReserveAndTransfer( uint256 mintAmount, uint256 offPegPerc ) internal { UniswapPair pair = UniswapPair(uniswap_pair);
YAMTokenInterface yam = YAMTokenInterface(yamAddress);
// get reserves (uint256 token0Reserves, uint256 token1Reserves, ) = pair.getReserves();
// check if protocol has excess yam in the reserve uint256 excess = yam.balanceOf(reservesContract);
//SlowMist// 计算用于 Uniswap 中兑换的 YAM 数量 uint256 tokens_to_max_slippage = uniswapMaxSlippage(token0Reserves, token1Reserves, offPegPerc);
UniVars memory uniVars = UniVars({ yamsToUni: tokens_to_max_slippage, // how many yams uniswap needs amountFromReserves: excess, // how much of yamsToUni comes from reserves mintToReserves: 0 // how much yams protocol mints to reserves });
// tries to sell all mint + excess // falls back to selling some of mint and all of excess // if all else fails, sells portion of excess // upon pair.swap, `uniswapV2Call` is called by the uniswap pair contract if (isToken0) { if (tokens_to_max_slippage > mintAmount.add(excess)) { // we already have performed a safemath check on mintAmount+excess // so we dont need to continue using it in this code path
// can handle selling all of reserves and mint uint256 buyTokens = getAmountOut(mintAmount + excess, token0Reserves, token1Reserves); uniVars.yamsToUni = mintAmount + excess; uniVars.amountFromReserves = excess; // call swap using entire mint amount and excess; mint 0 to reserves pair.swap(0, buyTokens, address(this), abi.encode(uniVars)); } else { if (tokens_to_max_slippage > excess) { // uniswap can handle entire reserves uint256 buyTokens = getAmountOut(tokens_to_max_slippage, token0Reserves, token1Reserves);
// swap up to slippage limit, taking entire yam reserves, and minting part of total //SlowMist// 将多余代币铸给 reserves 合约 uniVars.mintToReserves = mintAmount.sub((tokens_to_max_slippage - excess)); //SlowMist// Uniswap代币交换 pair.swap(0, buyTokens, address(this), abi.encode(uniVars)); } else { // uniswap cant handle all of excess uint256 buyTokens = getAmountOut(tokens_to_max_slippage, token0Reserves, token1Reserves); uniVars.amountFromReserves = tokens_to_max_slippage; uniVars.mintToReserves = mintAmount; // swap up to slippage limit, taking excess - remainingExcess from reserves, and minting full amount // to reserves pair.swap(0, buyTokens, address(this), abi.encode(uniVars)); } } } else { if (tokens_to_max_slippage > mintAmount.add(excess)) { // can handle all of reserves and mint uint256 buyTokens = getAmountOut(mintAmount + excess, token1Reserves, token0Reserves); uniVars.yamsToUni = mintAmount + excess; uniVars.amountFromReserves = excess; // call swap using entire mint amount and excess; mint 0 to reserves pair.swap(buyTokens, 0, address(this), abi.encode(uniVars)); } else { if (tokens_to_max_slippage > excess) { // uniswap can handle entire reserves uint256 buyTokens = getAmountOut(tokens_to_max_slippage, token1Reserves, token0Reserves);
// swap up to slippage limit, taking entire yam reserves, and minting part of total //SlowMist// 增发的多余的代币给 reserves 合约 uniVars.mintToReserves = mintAmount.sub( (tokens_to_max_slippage - excess)); // swap up to slippage limit, taking entire yam reserves, and minting part of total //Slowist// 在 uniswap 中进行兑换,并最终调用 rebase 合约的 uniswapV2Call 函数 pair.swap(buyTokens, 0, address(this), abi.encode(uniVars)); } else { // uniswap cant handle all of excess uint256 buyTokens = getAmountOut(tokens_to_max_slippage, token1Reserves, token0Reserves); uniVars.amountFromReserves = tokens_to_max_slippage; uniVars.mintToReserves = mintAmount; // swap up to slippage limit, taking excess - remainingExcess from reserves, and minting full amount // to reserves pair.swap(buyTokens, 0, address(this), abi.encode(uniVars)); } } } }

通过对代码分析,buyReserveAndTransfer 首先会计算在 Uniswap 中用于兑换 yCRV 的 YAM 的数量,如果该数量少于 YAM 的铸币数量,则会将多余的增发的 YAM 币给 reserves 合约,这一步是通过 Uniswap 合约调用rebase合约的 uniswapV2Call 函数实现的,具体的代码如下:

function uniswapV2Call( address sender, uint256 amount0, uint256 amount1, bytes memory data ) public { // enforce that it is coming from uniswap require(msg.sender == uniswap_pair, "bad msg.sender"); // enforce that this contract called uniswap require(sender == address(this), "bad origin"); (UniVars memory uniVars) = abi.decode(data, (UniVars));
YAMTokenInterface yam = YAMTokenInterface(yamAddress);
if (uniVars.amountFromReserves > 0) { // transfer from reserves and mint to uniswap yam.transferFrom(reservesContract, uniswap_pair, uniVars.amountFromReserves); if (uniVars.amountFromReserves < uniVars.yamsToUni) { // if the amount from reserves > yamsToUni, we have fully paid for the yCRV tokens // thus this number would be 0 so no need to mint yam.mint(uniswap_pair, uniVars.yamsToUni.sub(uniVars.amountFromReserves)); } } else { // mint to uniswap yam.mint(uniswap_pair, uniVars.yamsToUni); }
// mint unsold to mintAmount //SlowMist// 将多余的 YAM 代币分发给 reserves 合约 if (uniVars.mintToReserves > 0) { yam.mint(reservesContract, uniVars.mintToReserves); }
// transfer reserve token to reserves if (isToken0) { SafeERC20.safeTransfer(IERC20(reserveToken), reservesContract, amount1); emit TreasuryIncreased(amount1, uniVars.yamsToUni, uniVars.amountFromReserves, uniVars.mintToReserves); } else { SafeERC20.safeTransfer(IERC20(reserveToken), reservesContract, amount0); emit TreasuryIncreased(amount0, uniVars.yamsToUni, uniVars.amountFromReserves, uniVars.mintToReserves); } }

分析到这里,一个完整的rebase流程就完成了,你可能看得很懵,我们用简单的流程图简化下:



也就是说,每次的rebase,如果有多余的 YAM 代币,这些代币将会流到 reserves 合约中,那这和社区治理的关系是什么呢?


通过分析项目代码,发现治理相关的逻辑在 YAMGovernorAlpha.sol 中,其中发起提案的函数为 propose,具体代码如下:

function propose( address[] memory targets, uint[] memory values, string[] memory signatures, bytes[] memory calldatas, string memory description ) public returns (uint256) { //SlowMist// 校验提案发起者的票数占比 require(yam.getPriorVotes(msg.sender, sub256(block.number, 1)) > proposalThreshold(), "GovernorAlpha::propose: proposer votes below proposal threshold"); require(targets.length == values.length && targets.length == signatures.length && targets.length == calldatas.length, "GovernorAlpha::propose: proposal function information arity mismatch"); require(targets.length != 0, "GovernorAlpha::propose: must provide actions"); require(targets.length <= proposalMaxOperations(), "GovernorAlpha::propose: too many actions");
uint256 latestProposalId = latestProposalIds[msg.sender]; if (latestProposalId != 0) { ProposalState proposersLatestProposalState = state(latestProposalId); require(proposersLatestProposalState != ProposalState.Active, "GovernorAlpha::propose: one live proposal per proposer, found an already active proposal"); require(proposersLatestProposalState != ProposalState.Pending, "GovernorAlpha::propose: one live proposal per proposer, found an already pending proposal"); }
uint256 startBlock = add256(block.number, votingDelay()); uint256 endBlock = add256(startBlock, votingPeriod());
proposalCount++; Proposal memory newProposal = Proposal({ id: proposalCount, proposer: msg.sender, eta: 0, targets: targets, values: values, signatures: signatures, calldatas: calldatas, startBlock: startBlock, endBlock: endBlock, forVotes: 0, againstVotes: 0, canceled: false, executed: false });
proposals[newProposal.id] = newProposal; latestProposalIds[newProposal.proposer] = newProposal.id;
emit ProposalCreated( newProposal.id, msg.sender, targets, values, signatures, calldatas, startBlock, endBlock, description ); return newProposal.id; }

通过分析代码,可以发现在发起提案时,需要提案发起人拥有一定额度的票权利,这个值必须大于 proposalThreshold 计算得来的值,具体代码如下:

functionproposalThreshold()publicviewreturns(uint256){ returnSafeMath.div(yam.initSupply(),100);}//1%ofYAM

也就是说提案发起人的票权必须大于 initSupply 的 1% 才能发起提案。那 initSupply 受什么影响呢?答案是 YAM 代币的 mint 函数,代码如下:

function mint(address to, uint256 amount) external onlyMinter returns (bool) { _mint(to, amount); return true; }
function _mint(address to, uint256 amount) internal { // increase totalSupply totalSupply = totalSupply.add(amount);
// get underlying value uint256 yamValue = amount.mul(internalDecimals).div(yamsScalingFactor);
// increase initSupply initSupply = initSupply.add(yamValue);
// make sure the mint didnt push maxScalingFactor too low require(yamsScalingFactor <= _maxScalingFactor(), "max scaling factor too low");
// add balance _yamBalances[to] = _yamBalances[to].add(yamValue);
// add delegates to the minter _moveDelegates(address(0), _delegates[to], yamValue); emit Mint(to, amount); }

从代码可知,mint 函数在每次铸币时都会更新 initSupply 的值,而这个值是根据 amount 的值来计算的,也就是铸币的数量。


现在,我们已经分析完所有的流程了,剩下的就是把所有的分析串起来,看看这次的漏洞对 YAM 产生了什么影响,对上文的流程图做拓展,变成下面这样:

整个事件的分析如上图,由于rebase的时候取的是上一次的 totalSupply 的值,所以计算错误的 totalSupply 的值并不会立即通过 mint 作用到 initSupply 上,所以在下一次rebase前,社区仍有机会挽回这个错误,减少损失。但是一旦下一次rebase执行,整个失误将会变得无法挽回。


通过查询 Etherscan 上 YAM 代币合约的相关信息,可以看到 totalSupply 已经到了一个非常大的值,而 initSupply 还未受到影响。



前车之鉴


这次事件中官方已经给出了具体的修复方案,这里不再赘述。这次的事件充分暴露了未经审计 DeFi 合约中隐藏的巨大风险,虽然 YAM 开发者已经在 Github 中表明 YAM 合约的很多代码是参考了经过充分审计的 DeFi 项目如 Compound、Ampleforth、Synthetix 及 YEarn/YFI,但是仍无可避免地发生了意料之外的风险。


DeFi 项目 Yam Finance(YAM) 核心开发者 belmore 在推特上表示:“对不起,大家。我失败了。谢谢你们今天的大力支持。我太难过了。”,但是覆水已经难收,在此,慢雾安全团队给出如下建议


1、由于 DeFi 合约的高度复杂性,任何 DeFi 项目都需在经过专业的安全团队充分审计后再进行上线,降低合约发生意外的风险。审计可联系慢雾安全团队([email protected]


2、项目中去中心化治理应循序渐进,在项目开始阶段,需要设置适当的权限以防发生黑天鹅事件。

—-

编译者/作者:慢雾区

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