Guard against concurrent withdraws causing unexpected reverts

.gas-snapshot

@@ -1,27 +1,35 @@
-PostOpTest:test_RevertsIfPostOpFailed(uint256,uint256,uint256) (runs: 256, μ: 94981, ~: 97095)
+GetHashTest:test_returnsValidHash() (gas: 46531)
+IsValidWithdrawalSignature:test_returnsFalseWithInvalidSignature() (gas: 53845)
+IsValidWithdrawalSignature:test_returnsTrueWithValidSignature() (gas: 51016)
+OwnerWithdrawTest:test_revertsIfNotOwner() (gas: 15305)
+OwnerWithdrawTest:test_transfersERC20Successfully(uint256) (runs: 256, μ: 97271, ~: 99711)
+OwnerWithdrawTest:test_transfersETHSuccessfully(uint256) (runs: 256, μ: 51418, ~: 53415)
+PostOpTest:test_RevertsIfPostOpFailed(uint256,uint256,uint256) (runs: 256, μ: 97167, ~: 99371)
 PostOpTest:test_entryPointAddStake(uint112,uint32) (runs: 256, μ: 56233, ~: 56233)
-PostOpTest:test_entryPointDeposit(uint112) (runs: 256, μ: 32331, ~: 32962)
-PostOpTest:test_entryPointUnlockStake() (gas: 54910)
-PostOpTest:test_entryPointWithdraw(uint112) (runs: 256, μ: 63500, ~: 65300)
-PostOpTest:test_entryPointWithdrawStake() (gas: 71825)
-PostOpTest:test_paymasterPaysForOp() (gas: 208402)
-PostOpTest:test_transfersExcess(uint256,uint256,uint256,uint256) (runs: 256, μ: 110216, ~: 111227)
+PostOpTest:test_entryPointDeposit(uint112) (runs: 256, μ: 32516, ~: 32962)
+PostOpTest:test_entryPointUnlockStake() (gas: 54888)
+PostOpTest:test_entryPointWithdraw(uint112) (runs: 256, μ: 63200, ~: 65300)
+PostOpTest:test_entryPointWithdrawStake() (gas: 71803)
+PostOpTest:test_paymasterPaysForOp() (gas: 210663)
+PostOpTest:test_transfersExcess(uint256,uint256,uint256,uint256) (runs: 256, μ: 112465, ~: 113488)
 Simulate:test() (gas: 12114)
-ValidatePaymasterUserOpTest:test_emitsCorrectly(address,uint256,uint256) (runs: 256, μ: 102564, ~: 103835)
-ValidatePaymasterUserOpTest:test_recordsNonceUsed(uint256) (runs: 256, μ: 109936, ~: 111025)
-ValidatePaymasterUserOpTest:test_returns1sIfWrongSignature() (gas: 94076)
-ValidatePaymasterUserOpTest:test_returnsCorrectly() (gas: 88920)
+ValidatePaymasterUserOpTest:test_emitsCorrectly(address,uint256,uint256) (runs: 256, μ: 104847, ~: 106080)
+ValidatePaymasterUserOpTest:test_recordsNonceUsed(uint256) (runs: 256, μ: 112158, ~: 113247)
+ValidatePaymasterUserOpTest:test_returns1sIfWrongSignature() (gas: 96298)
+ValidatePaymasterUserOpTest:test_returnsCorrectly() (gas: 91185)
 ValidatePaymasterUserOpTest:test_revertsIfMaxCostMoreThanRequested() (gas: 39141)
-ValidatePaymasterUserOpTest:test_revertsIfNonceUsed() (gas: 101280)
-ValidatePaymasterUserOpTest:test_revertsIfWithdrawAssetNotZero() (gas: 55974)
+ValidatePaymasterUserOpTest:test_revertsIfNonceUsed() (gas: 103502)
+ValidatePaymasterUserOpTest:test_revertsIfWithdrawAssetNotZero() (gas: 55952)
+ValidatePaymasterUserOpTest:test_reverts_whenWithdrawExceedsMaxAllowed(uint256,uint256,uint256) (runs: 256, μ: 100811, ~: 100992)
 ValidateTest:test_receive() (gas: 14687)
-WithdrawGasExcess:test_RevertsIfNoExcess(uint256) (runs: 256, μ: 68914, ~: 69177)
-WithdrawGasExcess:test_transferExcess(uint256,uint256,uint256) (runs: 256, μ: 102074, ~: 102074)
-WithdrawTest:test_emitsCorrectly(address,uint256,uint256) (runs: 256, μ: 115868, ~: 119073)
-WithdrawTest:test_emitsCorrectlyERC20Withdraw(address,uint256,uint256) (runs: 256, μ: 158052, ~: 160513)
-WithdrawTest:test_recordsNonceUsed(uint256) (runs: 256, μ: 109078, ~: 110167)
-WithdrawTest:test_revertsIfExpired(uint48,uint256) (runs: 256, μ: 59403, ~: 59535)
-WithdrawTest:test_revertsIfNonceUsed() (gas: 93506)
-WithdrawTest:test_revertsIfWrongSignature() (gas: 61536)
-WithdrawTest:test_transfersERC20Successfully(uint256) (runs: 256, μ: 136385, ~: 138825)
-WithdrawTest:test_transfersETHSuccessfully(uint256) (runs: 256, μ: 89398, ~: 91395)
+WithdrawGasExcess:test_RevertsIfNoExcess(uint256) (runs: 256, μ: 71133, ~: 71396)
+WithdrawGasExcess:test_transferExcess(uint256,uint256,uint256) (runs: 256, μ: 104279, ~: 104290)
+WithdrawTest:test_emitsCorrectly(address,uint256,uint256) (runs: 256, μ: 118413, ~: 121368)
+WithdrawTest:test_emitsCorrectlyERC20Withdraw(address,uint256,uint256) (runs: 256, μ: 162446, ~: 164773)
+WithdrawTest:test_recordsNonceUsed(uint256) (runs: 256, μ: 111372, ~: 112461)
+WithdrawTest:test_revertsIfExpired(uint48,uint256) (runs: 256, μ: 61664, ~: 61785)
+WithdrawTest:test_revertsIfNonceUsed() (gas: 95844)
+WithdrawTest:test_revertsIfWrongSignature() (gas: 63830)
+WithdrawTest:test_reverts_whenWithdrawExceedsMaxAllowed(uint256,uint256,uint256) (runs: 256, μ: 85124, ~: 85096)
+WithdrawTest:test_transfersERC20Successfully(uint256) (runs: 256, μ: 140626, ~: 143066)
+WithdrawTest:test_transfersETHSuccessfully(uint256) (runs: 256, μ: 91692, ~: 93689)

script/DeployMagicSpend.s.sol

@@ -12,7 +12,7 @@ contract MagicSpendDeployScript is Script {
         address signerAddress = 0x3E0cd4Dc43811888efa242Ab17118FcE0035EFF7;
         uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY");
         vm.startBroadcast(deployerPrivateKey);
-        MagicSpend c = new MagicSpend{salt: "0x1"}(vm.addr(deployerPrivateKey));
+        MagicSpend c = new MagicSpend{salt: "0x1"}(vm.addr(deployerPrivateKey), 20);
         console2.log(address(c));
         c.entryPointDeposit{value: 0.01 ether}(0.01 ether);
         c.entryPointAddStake{value: 0x16345785d8a0000}(0x16345785d8a0000, 0x15180);

src/MagicSpend.sol

@@ -30,6 +30,10 @@ contract MagicSpend is Ownable, IPaymaster {
         uint48 expiry;
     }
 
+    /// @notice The maximum percentage of address.balance a single withdraw.
+    /// @dev Helps prevent withdraws in the same transaction leading to reverts and hurting paymaster reputation.
+    uint256 public maxWithdrawDenominator;
+
     /// @notice Track the ETH available to be withdrawn per user.
     mapping(address user => uint256 amount) internal _withdrawableETH;
 
@@ -44,6 +48,11 @@ contract MagicSpend is Ownable, IPaymaster {
     /// @param nonce   The request nonce.
     event MagicSpendWithdrawal(address indexed account, address indexed asset, uint256 amount, uint256 nonce);
 
+    /// @notice Emitted when the `maxWithdrawDenominator` is set.
+    ///
+    /// @param newDenominator The new maxWithdrawDenominator value.
+    event MaxWithdrawDenominatorSet(uint256 newDenominator);
+
     /// @notice Thrown when the withdraw request signature is invalid.
     ///
     /// @dev The withdraw request signature MUST be:
@@ -72,12 +81,11 @@ contract MagicSpend is Ownable, IPaymaster {
     /// @param asset The requested asset.
     error UnsupportedPaymasterAsset(address asset);
 
-    /// @notice Thrown during `UserOperation` validation when the current balance is insufficient to cover the
-    ///         requested amount (exluding the `maxGasCost` set by the Entrypoint).
+    /// @notice Thrown if WithdrawRequest.amount exceeds address(this).balance / maxWithdrawDenominator.
     ///
     /// @param requestedAmount The requested amount excluding gas.
-    /// @param balance         The current contract balance.
-    error InsufficientBalance(uint256 requestedAmount, uint256 balance);
+    /// @param maxAllowed      The current max allowed withdraw.
+    error WithdrawTooLarge(uint256 requestedAmount, uint256 maxAllowed);
 
     /// @notice Thrown when trying to withdraw funds but nothing is available.
     error NoExcess();
@@ -97,9 +105,10 @@ contract MagicSpend is Ownable, IPaymaster {
 
     /// @notice Deploy the contract and set its initial owner.
     ///
-    /// @param _owner The initial owner of this contract.
-    constructor(address _owner) {
-        Ownable._initializeOwner(_owner);
+    /// @param owner_ The initial owner of this contract.
+    constructor(address owner_, uint256 maxWithdrawDenominator_) {
+        Ownable._initializeOwner(owner_);
+        _setMaxWithdrawDenominator(maxWithdrawDenominator_);
     }
 
     /// @notice Receive function allowing ETH to be deposited in this contract.
@@ -127,13 +136,6 @@ contract MagicSpend is Ownable, IPaymaster {
         bool sigFailed = !isValidWithdrawSignature(userOp.sender, withdrawRequest);
         validationData = (sigFailed ? 1 : 0) | (uint256(withdrawRequest.expiry) << 160);
 
-        // Ensure at validation that the contract has enough balance to cover the requested funds.
-        // NOTE: This check is necessary to enforce that the contract will be able to transfer the remaining funds
-        //       when `postOp()` is called back after the `UserOperation` has been executed.
-        if (address(this).balance < withdrawAmount) {
-            revert InsufficientBalance(withdrawAmount, address(this).balance);
-        }
-
         // NOTE: Do not include the gas part in withdrawable funds as it will be handled in `postOp()`.
         _withdrawableETH[userOp.sender] += withdrawAmount - maxCost;
         context = abi.encode(maxCost, userOp.sender);
@@ -144,10 +146,14 @@ contract MagicSpend is Ownable, IPaymaster {
         external
         onlyEntryPoint
     {
-        // `PostOpMode.postOpReverted` should be impossible.
-        // Only possible cause would be if this contract does not own enough ETH to transfer
-        // but this is checked at the validation step.
-        assert(mode != PostOpMode.postOpReverted);
+        // `PostOpMode.postOpReverted` should never happen.
+        // The flow here can only revert if there are > maxWithdrawDenominator
+        // withdraws in the same transaction, which should be highly unlikely.
+        // If the ETH transfer fails, the entire bundle will revert due an issue in the EntryPoint
+        // https://github.com/eth-infinitism/account-abstraction/pull/293
+        if (mode == PostOpMode.postOpReverted) {
+            revert UnexpectedPostOpRevertedMode();
+        }
 
         (uint256 maxGasCost, address account) = abi.decode(context, (uint256, address));
 
@@ -249,6 +255,10 @@ contract MagicSpend is Ownable, IPaymaster {
         IEntryPoint(entryPoint()).withdrawStake(to);
     }
 
+    function setMaxWithdrawDenominator(uint256 newDenominator) external onlyOwner {
+        _setMaxWithdrawDenominator(newDenominator);
+    }
+
     /// @notice Returns whether the `withdrawRequest` signature is valid for the given `account`.
     ///
     /// @dev Does not validate nonce or expiry.
@@ -305,6 +315,12 @@ contract MagicSpend is Ownable, IPaymaster {
         return 0x5FF137D4b0FDCD49DcA30c7CF57E578a026d2789;
     }
 
+    function _setMaxWithdrawDenominator(uint256 newDenominator) internal {
+        maxWithdrawDenominator = newDenominator;
+
+        emit MaxWithdrawDenominatorSet(newDenominator);
+    }
+
     /// @notice Validate the `withdrawRequest` against the given `account`.
     ///
     /// @dev Runs all non-signature validation checks.
@@ -317,6 +333,11 @@ contract MagicSpend is Ownable, IPaymaster {
             revert InvalidNonce(withdrawRequest.nonce);
         }
 
+        uint256 maxAllowed = address(this).balance / maxWithdrawDenominator;
+        if (withdrawRequest.asset == address(0) && withdrawRequest.amount > maxAllowed) {
+            revert WithdrawTooLarge(withdrawRequest.amount, maxAllowed);
+        }
+
         _nonceUsed[withdrawRequest.nonce][account] = true;
 
         // This is emitted ahead of fund transfer, but allows a consolidated code path

test/MagicSpend.t.sol

@@ -10,7 +10,7 @@ contract MagicSpendTest is Test {
     address withdrawer = address(0xb0b);
     uint256 ownerPrivateKey = 0xa11ce;
     address owner = vm.addr(ownerPrivateKey);
-    MagicSpend magic = new MagicSpend(owner);
+    MagicSpend magic = new MagicSpend(owner, 1);
 
     // signature params
     address asset;

test/PostOp.t.sol

@@ -32,7 +32,7 @@ contract PostOpTest is PaymasterMagicSpendBaseTest {
         vm.deal(address(magic), amount);
         (bytes memory context,) = magic.validatePaymasterUserOp(_getUserOp(), userOpHash, maxCost_);
         uint256 expectedBalance = 0;
-        vm.expectRevert();
+        vm.expectRevert(MagicSpend.UnexpectedPostOpRevertedMode.selector);
         magic.postOp(IPaymaster.PostOpMode.postOpReverted, context, actualCost);
         assertEq(withdrawer.balance, expectedBalance);
     }

test/Validate.t.sol

@@ -6,6 +6,8 @@ import {Test, console2} from "forge-std/Test.sol";
 import "./MagicSpend.t.sol";
 
 abstract contract ValidateTest is MagicSpendTest {
+    address invoker;
+
     function test_recordsNonceUsed(uint256 nonce_) public {
         nonce = nonce_;
         assertFalse(magic.nonceUsed(withdrawer, nonce));
@@ -36,5 +38,31 @@ abstract contract ValidateTest is MagicSpendTest {
         _validateInvokingCall();
     }
 
+    function test_reverts_whenWithdrawExceedsMaxAllowed(
+        uint256 accountBalance,
+        uint256 withdraw,
+        uint256 maxWithdrawDenominator
+    ) public virtual {
+        vm.assume(maxWithdrawDenominator > 0);
+        accountBalance = bound(accountBalance, 0, type(uint256).max - 1);
+        withdraw = bound(withdraw, accountBalance / maxWithdrawDenominator + 1, type(uint256).max);
+        amount = withdraw;
+
+        vm.stopPrank();
+        vm.startPrank(owner);
+        magic.setMaxWithdrawDenominator(maxWithdrawDenominator);
+        magic.ownerWithdraw(address(0), address(1), address(magic).balance);
+        vm.stopPrank();
+
+        vm.deal(address(magic), accountBalance);
+        vm.expectRevert(
+            abi.encodeWithSelector(
+                MagicSpend.WithdrawTooLarge.selector, withdraw, accountBalance / maxWithdrawDenominator
+            )
+        );
+        vm.startPrank(invoker);
+        _validateInvokingCall();
+    }
+
     function _validateInvokingCall() internal virtual;
 }

test/ValidatePaymasterUserOp.t.sol

@@ -8,6 +8,7 @@ contract ValidatePaymasterUserOpTest is PaymasterMagicSpendBaseTest, ValidateTes
     function setUp() public override {
         super.setUp();
         vm.startPrank(magic.entryPoint());
+        invoker = magic.entryPoint();
     }
 
     function test_revertsIfMaxCostMoreThanRequested() public {
@@ -16,12 +17,6 @@ contract ValidatePaymasterUserOpTest is PaymasterMagicSpendBaseTest, ValidateTes
         magic.validatePaymasterUserOp(_getUserOp(), userOpHash, maxCost);
     }
 
-    function test_revertsIfWithdrawalExceedsBalance() public {
-        vm.deal(address(magic), 0);
-        vm.expectRevert(abi.encodeWithSelector(MagicSpend.InsufficientBalance.selector, amount, 0));
-        magic.validatePaymasterUserOp(_getUserOp(), userOpHash, maxCost);
-    }
-
     function test_returnsCorrectly() public {
         (bytes memory context, uint256 validationData) =
             magic.validatePaymasterUserOp(_getUserOp(), userOpHash, maxCost);
@@ -51,6 +46,15 @@ contract ValidatePaymasterUserOpTest is PaymasterMagicSpendBaseTest, ValidateTes
         super.test_emitsCorrectly(magic.entryPoint(), amount_, nonce_);
     }
 
+    function test_reverts_whenWithdrawExceedsMaxAllowed(
+        uint256 accountBalance,
+        uint256 withdraw,
+        uint256 maxWithdrawDenominator
+    ) public override {
+        maxCost = withdraw;
+        super.test_reverts_whenWithdrawExceedsMaxAllowed(accountBalance, withdraw, maxWithdrawDenominator);
+    }
+
     function _validateInvokingCall() internal override {
         magic.validatePaymasterUserOp(_getUserOp(), userOpHash, maxCost);
     }

test/Withdraw.t.sol

@@ -11,6 +11,7 @@ contract WithdrawTest is MagicSpendTest, ValidateTest {
     function setUp() public override {
         super.setUp();
         vm.startPrank(withdrawer);
+        invoker = withdrawer;
     }
 
     function test_transfersETHSuccessfully(uint256 amount_) public {

test/echidna/FuzzSetup.sol

@@ -22,7 +22,7 @@ contract FuzzSetup is FuzzBase {
     uint256 totalWithdrawn;
 
     constructor() payable {
-        magic = new MagicSpend(OWNER);
+        magic = new MagicSpend(OWNER, 1);
         address(magic).call{value: PAYMASTER_STARTING_BALANCE}("");
     }