IndexPool: Poor conversion from Balancer V1's corresponding functions

[code423n4]

Handle

GreyArt

Vulnerability details

Impact

A number of functions suffer from the erroneous conversion of Balancer V1's implementation.

• _compute() (equivalent to Balancer's [bpow()](https://github.com/balancer-labs/balancer-core/blob/master/contracts/BNum.sol#L108-L126))
  • if (remain == 0) output = wholePow; when a return statement should be used instead.
• _computeSingleOutGivenPoolIn() (equivalent to Balancer's [_calcSingleOutGivenPoolIn()](https://github.com/balancer-labs/balancer-core/blob/master/contracts/BMath.sol#L195-L224))
  • tokenOutRatio should be calculated with _compute() instead of _pow()
  • zaz should be calculated with _mul() instead of the native *
• _pow() (equivalent to Balancer's [bpowi()](https://github.com/balancer-labs/balancer-core/blob/master/contracts/BNum.sol#L89-L103))
  • Missing brackets {} for the for loop causes a different interpretation
  • _mul should be used instead of the native *

Recommended Mitigation Steps

The fixed implementation is provided below.

function _computeSingleOutGivenPoolIn(
  uint256 tokenOutBalance,
  uint256 tokenOutWeight,
  uint256 _totalSupply,
  uint256 _totalWeight,
  uint256 toBurn,
  uint256 _swapFee
) internal pure returns (uint256 amountOut) {
    uint256 normalizedWeight = _div(tokenOutWeight, _totalWeight);
    uint256 newPoolSupply = _totalSupply - toBurn;
    uint256 poolRatio = _div(newPoolSupply, _totalSupply);
    uint256 tokenOutRatio = _compute(poolRatio, _div(BASE, normalizedWeight));
    uint256 newBalanceOut = _mul(tokenOutRatio, tokenOutBalance);
    uint256 tokenAmountOutBeforeSwapFee = tokenOutBalance - newBalanceOut;
    uint256 zaz = _mul(BASE - normalizedWeight, _swapFee);
    amountOut = _mul(tokenAmountOutBeforeSwapFee, (BASE - zaz));
}

function _compute(uint256 base, uint256 exp) internal pure returns (uint256 output) {
  require(MIN_POW_BASE <= base && base <= MAX_POW_BASE, "INVALID_BASE");
  
  uint256 whole = (exp / BASE) * BASE;   
  uint256 remain = exp - whole;
  uint256 wholePow = _pow(base, whole / BASE);
  
  if (remain == 0) return wholePow;
  
  uint256 partialResult = _powApprox(base, remain, POW_PRECISION);
  output = _mul(wholePow, partialResult);
}

function _pow(uint256 a, uint256 n) internal pure returns (uint256 output) {
  output = n % 2 != 0 ? a : BASE;
  for (n /= 2; n != 0; n /= 2) {
		a = _mul(a, a);
    if (n % 2 != 0) output = _mul(output, a);
	}
}