Summary
The Protocol allowed the owner to remove the dust amount if not backed by rToken , However to Find the totalSupply of rToken the rayMul is applied twice which will artificially increase the totalSupply of rToken.
Vulnerability Details
The LendingPool could accumulate Dust amount overTime which will be removed by owner of the pool via calling transferAccruedDust.
/contracts/core/pools/LendingPool/LendingPool.sol:784
784: function transferAccruedDust(address recipient, uint256 amount) external onlyOwner {
785:
786: ReserveLibrary.updateReserveState(reserve, rateData);
787: require(recipient != address(0), "LendingPool: Recipient cannot be zero address");
788: IRToken(reserve.reserveRTokenAddress).transferAccruedDust(recipient, amount);
789: }
First we get the Dust Amount available in the pool.
function calculateDustAmount() public view returns (uint256) {
uint256 contractBalance = IERC20(_assetAddress).balanceOf(address(this)).rayDiv(ILendingPool(_reservePool).getNormalizedIncome());
@--> uint256 currentTotalSupply = totalSupply();
uint256 totalRealBalance = currentTotalSupply.rayMul(ILendingPool(_reservePool).getNormalizedIncome());
return contractBalance <= totalRealBalance ? 0 : contractBalance - totalRealBalance;
}
The logic is that we check if there is any assets in the pool which is not backed by rToken than we transfer those assets out from pool.
Following step will help us to find the unbacked assets:
We fetch the assets balance of rToken.
Fetch the totalSupply of rToken.
Than we apply liquidityIndex on totalSupply again.
Than we check if there is assets without rToken backing than we withdraw extra assets.
The Issue in above flow is that when we fetch totalSupply it applies rayMul and return scaled balance.
function totalSupply() public view override(ERC20, IERC20) returns (uint256) {
return super.totalSupply().rayMul(ILendingPool(_reservePool).getNormalizedIncome());
}
and calculateDustAmount function we again apply rayMul which return wrong totalSupply:
totalSupply unscaled : 99999999996353916006
liquidityIndex : 1000000000036460839945676932
totalSupply scaled : 100000000000000000000
# as we apply it twice so
totalSupply scaled : 100000000003646083995
As it can be seen from above example that the totalSupply is artificially increased by 3646083995.
Add Following POC to LendingPool.test.js in describe("Full sequence", section:
it.only("apply rayMul on total supply twice", async function () {
const MockVault = await ethers.getContractFactory("MockUsdVault");
await crvusd.connect(user1).approve(lendingPool.target, ethers.parseEther("100"));
await lendingPool.connect(user1).deposit(ethers.parseEther("100"));
await crvusd.connect(owner).mint(owner.address, ethers.parseEther("1000"));
await crvusd.connect(owner).transfer(rToken.target, ethers.parseEther("1"));
await rToken.connect(owner).setReservePool(lendingPool.target);
let index = await lendingPool.getNormalizedIncome();
console.log("index" , index);
const dustAmount = await rToken.calculateDustAmount();
console.log("Dust amount:", dustAmount);
});
Logs:
Dust amount: 2717422734
Impact
Due To applying rayMul twice the dust amount return is not correct , which will not allow to remove the full dust from pool.
Tools Used
Unit Testing, Manual Review
Recommendations
One Recommended Fix could be :
@@ -318,16 +319,10 @@ contract RToken is ERC20, ERC20Permit, IRToken, Ownable {
function calculateDustAmount() public view returns (uint256) {
// Calculate the actual balance of the underlying asset held by this contract
uint256 contractBalance = IERC20(_assetAddress).balanceOf(address(this)).rayDiv(ILendingPool(_reservePool).getNormalizedIncome()); // @audit : this calculation is not correct as the as
sets will be deposit into crvVault
- console.log("actual contract bal" , contractBalance);
// contractBalance +=ILendingPool(_reservePool).getTotalVaultDeposits().rayDiv(ILendingPool(_reservePool).getNormalizedIncome());
// Calculate the total real obligations to the token holders
uint256 currentTotalSupply = totalSupply(); // @audit : it is already Mul with the index but in next call we again do it.
// Calculate the total real balance equivalent to the total supply
- // uint256 totalRealBalance = currentTotalSupply.rayMul(ILendingPool(_reservePool).getNormalizedIncome());
// All balance, that is not tied to rToken are dust (can be donated or is the rest of exponential vs linear)
- return contractBalance <= totalRealBalance ? 0 : contractBalance - totalRealBalance;
+ return contractBalance <= currentTotalSupply ? 0 : contractBalance - currentTotalSupply;
}
After runing again newDust amount is :
Logs:
Dust amount: 6363506729
