Fee on transfer tokens aren't supported by StakerVault's stakeFor

[code423n4]

Lines of code

https://github.com/code-423n4/2022-04-backd/blob/main/backd/contracts/StakerVault.sol#L340

Vulnerability details

Impact

If token is fee on transfer the stakeFor be unavailable: strict equality as deposit funds transfer success condition will not be met whenever the fee is positive.

Setting the severity to medium as the core functions become unavailable in this case.

Proof of Concept

StakerVault's stakeFor requires balance difference to be strictly equal to the amount sent:

https://github.com/code-423n4/2022-04-backd/blob/main/backd/contracts/StakerVault.sol#L340

Recommended Mitigation Steps

Cannot simply remove the condition or replace equality with >=, as == how protects from reentrancy:

https://github.com/code-423n4/2022-04-backd/blob/main/backd/contracts/StakerVault.sol#L338

So the recommendation is to add nonReentrant kind of modifier and remove the condition if fee on transfer tokens are in the scope.

[chase-manning]

The tokens that are staked are Backd LP Tokens. Backd LP Tokens to not have any fee on transfer type properties, so this vulnerability is not possible.