Beedle - Oracle free perpetual lending

Summary

As we know division in solidity can result in rounding down, and to minimize precision loss we always need to do multiplication before division. Yet even when we do this some precision loss can occur especially when dealing with small numbers, and rounding down to zero can be a source of major error if not handled correctly.

Vulnerability Details

The fees calculation in the Lender.sol#borrow() function rounds down to zero if the debt or borrowerFee amount is small enough. So, if the fee is 0 amout, this allows iteratively borrowing a loan without paying any fee.
Also, once the fees are calculated, they are transferred to feeReceiver. However many ERC20 tokens revert on zero value transfer. more details

The same kind of problem exists with the calculation of govFee in the Lender.sol#seizeLoan().

Impact

The Lender.sol#borrow() function can revert due to underflowing the fees calculation if the borrow() function is
(iteratively) called with small amounts of debt tokens. (or borrowerFee is very small)

The Lender.sol#seizeLoan() function can revert due to underflowing the fees calculation if the seizeLoan() function is (iteratively) called with loan that have small amounts of loan.collateral tokens. (or borrowerFee is very small)

Tools Used

Manual Review

Recommendations

• Consider preventing the loan from being borrow if the amount of debt or borrowFee tokens is too small (i.e., fees == 0).

• Consider preventing the loan from being seize if the amount of collateral or borrowFee tokens is too small (i.e., govFee == 0).