Calculating new rewards is susceptible to precision loss due to division before multiplication

[code423n4]

Lines of code

https://github.com/code-423n4/2023-05-ajna/blob/main/ajna-core/src/RewardsManager.sol#L519-L541

Vulnerability details

This issue is similar to https://github.com/ajna-finance/audits/blob/main/sherlock/Contest1.md#issue-m-7-calculating-new-rewards-is-susceptible-to-precision-loss-due-to-division-before-multiplication which is not fixed properly. Still, the final multiplication is being performed after the division.

Impact

Rewards may be lost (0) due to division before multiplication precision issues.

Proof of Concept

The RewardsManager._calculateNewRewards function calculates the new rewards for a staker by first multiplying interestEarned_ by totalBurnedInPeriod and then dividing by totalInterestEarnedInPeriod and then again multiplying by REWARD_FACTOR Since the division is being performed before the final multiplication, this can lead to precision loss.

    function _calculateNewRewards(
        address ajnaPool_,
        uint256 interestEarned_,
        uint256 nextEpoch_,
        uint256 epoch_,
        uint256 rewardsClaimedInEpoch_
    ) internal view returns (uint256 newRewards_) {
        (
            ,
            // total interest accumulated by the pool over the claim period
            uint256 totalBurnedInPeriod,
            // total tokens burned over the claim period
            uint256 totalInterestEarnedInPeriod
        ) = _getPoolAccumulators(ajnaPool_, nextEpoch_, epoch_);

        // calculate rewards earned 
        newRewards_ = totalInterestEarnedInPeriod == 0 ? 0 : Maths.wmul(
            REWARD_FACTOR,
            Maths.wdiv(
                Maths.wmul(interestEarned_, totalBurnedInPeriod), 
                totalInterestEarnedInPeriod
            )
        );

Recommended Mitigation Steps

All the multiplication should be performed in step 1 and then division at the end.

Assessed type

Math

[c4-judge]

Picodes marked the issue as primary issue

[c4-sponsor]

MikeHathaway marked the issue as sponsor confirmed

[c4-judge]

Picodes marked the issue as satisfactory

[c4-judge]

Picodes marked the issue as selected for report