duc - `leverageAmount` is incorrect in `SGLLeverage.sellCollateral` function due to calculation based on the new states of YieldBox after withdrawal

[sherlock-admin4]

duc

medium

leverageAmount is incorrect in SGLLeverage.sellCollateral function due to calculation based on the new states of YieldBox after withdrawal

Summary

See vulnerability detail

Vulnerability Detail

SGLLeverage.sellCollateral function attempts to remove the user's collateral in shares of YieldBox, then withdraws those collateral shares to collect collateral tokens. Subsequently, the received collateral tokens can be used to swap for asset tokens.

However, the leverageAmount variable in this function does not represent the actual withdrawn tokens from the provided shares because it is calculated after the withdrawal.

yieldBox.withdraw(collateralId, address(this), address(leverageExecutor), 0, calldata_.share);
uint256 leverageAmount = yieldBox.toAmount(collateralId, calldata_.share, false);

amountOut = leverageExecutor.getAsset(
    assetId, address(collateral), address(asset), leverageAmount, calldata_.from, calldata_.data
);

yieldBox.toAmount after withdrawal may return different from the actual withdrawn token amount, because the states of YieldBox has changed. Because the token amount is calculated with rounding down in YieldBox, leverageAmount will be higher than the actual withdrawn amount.

For example, before the withdrawal, YieldBox had 100 total shares and 109 total tokens. Now this function attempt to withdraw 10 shares (calldata_.share = 10) -> the actual withdrawn amount = 10 109 / 100 = 10 tokens After that, leverageAmount will be calculated based on the new yieldBox's total shares and total tokens -> leverageAmount = 10 (109 - 10) / (100 - 10) = 11 tokens

The same vulnerability exists in BBLeverage.sellCollateral function.

Impact

Because leverageAmount can be higher than the actual withdrawn collateral tokens, leverageExecutor.getAsset() will revert due to not having enough tokens in the contract to pull. This results in a DOS of sellCollateral, break this functionality.

Code Snippet

https://github.com/sherlock-audit/2024-02-tapioca/blob/main/Tapioca-bar/contracts/markets/singularity/SGLLeverage.sol#L127-L128

Tool used

Manual Review

Recommendation

leverageAmount should be obtained from the return value of YieldBox.withdraw:

(uint256 leverageAmount, ) = yieldBox.withdraw(collateralId, address(this), address(leverageExecutor), 0, calldata_.share);

[cryptotechmaker]

Fixed by https://github.com/Tapioca-DAO/Tapioca-bar/pull/356

[sherlock-admin4]

The protocol team fixed this issue in PR/commit https://github.com/Tapioca-DAO/Tapioca-bar/pull/356.