User should be able to select slippage in GeneralVault

[code423n4]

Lines of code

https://github.com/code-423n4/2022-05-sturdy/blob/78f51a7a74ebe8adfd055bdbaedfddc05632566f/smart-contracts/GeneralVault.sol#L100

Vulnerability details

Impact

When withdrawing collateral, user may occur a loss because of the swapping of the staked asset back to the asset. There is a build-in slippage protection here but the user has no control over it. This could easily lead to loss of user funds if they are not aware of this functionality or would have like to use a lower slippage.

Proof of Concept

We've recently witness some panic movements of stETH and its depeg on Curve, which would have certainly lead to losses for Sturdy users if the vault was live without slippage protection.

Recommended Mitigation Steps

Pass the slippage as a parameter or add an other function to do so.

[sforman2000]

Duplicate of #133 (high risk)