Having no deposits in StakedEXA will lead to stuck rewards when harvesting

Summary

The provider in StakedEXA is free to distributed rewards as it pleases and there is an automated way to distribute via StakedEXA::harvest(). The harvest setup may be started but no actual deposits have been made to StakedEXA or users may withdraw from StakedEXA while automated ongoing rewards are happening, which will lead to stuck rewards inside StakedEXA.

Root Cause

In StakedEXA::globalIndex() it returns a stale index if the totalSupply() is null (no deposits), but it should instead calculate the amount (rate x deltaTime) and send it to savings. This way rewards will never be stuck.

Internal pre-conditions

1. No deposits are present in StakedEXA.

External pre-conditions

None.

Attack Path

1. Deposits were not yet made and rewards are harvested or users withdraw all deposits while rewards are rolling out.

Impact

Stuck rewards as the index is not increased but rewardData.updatedAt increases.

PoC

In StakedEXA::updateIndex(), the index is updated to globalIndex(reward); and rewardData.updatedAt to the current block.timestamp or rewardData.finishAt.

function updateIndex(IERC20 reward) internal {
  RewardData storage rewardData = rewards[reward];
  rewardData.index = globalIndex(reward);
  rewardData.updatedAt = uint40(lastTimeRewardApplicable(rewardData.finishAt));
}

In StakedEXA::globalIndex(), the index is not increased if the total supply is null:

function globalIndex(IERC20 reward) public view returns (uint256) {
  RewardData storage rewardData = rewards[reward];
  if (totalSupply() == 0) return rewardData.index;

  return
    rewardData.index +
    (rewardData.rate * (lastTimeRewardApplicable(rewardData.finishAt) - rewardData.updatedAt)).divWadDown(
      totalSupply()
    );
}

Thus, as can be seen, the index will not be updated by rewardData.updatedAt is increased, losing these rewards forever.

Mitigation

If the total supply is null the amount not distributed can be calculated by doing rate x deltaTime and send to savings.