J4de - USSDRebalancer.sol#BuyUSSDSellCollateral has a loss of precision

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J4de

medium

USSDRebalancer.sol#BuyUSSDSellCollateral has a loss of precision

Summary

The BuyUSSDSellCollateral has a loss of precision causing collateral sells less than expected

Vulnerability Detail

File: USSDRebalancer.sol
117         if (collateralval > amountToBuyLeftUSD) {
118           // sell a portion of collateral and exit
119           if (collateral[i].pathsell.length > 0) {
120             uint256 amountBefore = IERC20Upgradeable(baseAsset).balanceOf(USSD);
121 >>          uint256 amountToSellUnits = IERC20Upgradeable(collateral[i].token).balanceOf(USSD) * ((amountToBuyLeftUSD * 1e18 / collateralval) / 1e18) / 1e18;
122             IUSSD(USSD).UniV3SwapInput(collateral[i].pathsell, amountToSellUnits);
123             amountToBuyLeftUSD -= (IERC20Upgradeable(baseAsset).balanceOf(USSD) - amountBefore);
124             DAItosell += (IERC20Upgradeable(baseAsset).balanceOf(USSD) - amountBefore);
125           } else {

If the collateralval is large and the amountToBuyLeftUSD is small, the result of ((amountToBuyLeftUSD * 1e18 / collateralval) / 1e18) is 0.

Impact

Collateral sells less than expected causing may depeg of USSD prices.

Code Snippet

https://github.com/USSDofficial/ussd-contracts/blob/f44c726371f3152634bcf0a3e630802e39dec49c/contracts/USSDRebalancer.sol#L121

Tool used

Manual Review

Recommendation

It is recommended to change the order of calculation

            uint256 amountBefore = IERC20Upgradeable(baseAsset).balanceOf(USSD);
-           uint256 amountToSellUnits = IERC20Upgradeable(collateral[i].token).balanceOf(USSD) * ((amountToBuyLeftUSD * 1e18 / collateralval) / 1e18) / 1e18;
+           uint256 amountToSellUnits = IERC20Upgradeable(collateral[i].token).balanceOf(USSD) *
+               amountToBuyLeftUSD * 1e18 / collateralval / 1e18 / 1e18;
            IUSSD(USSD).UniV3SwapInput(collateral[i].pathsell, amountToSellUnits);
            amountToBuyLeftUSD -= (IERC20Upgradeable(baseAsset).balanceOf(USSD) - amountBefore);
            DAItosell += (IERC20Upgradeable(baseAsset).balanceOf(USSD) - amountBefore);

Duplicate of #656