Add ERC2771Forwarder as an enhanced successor to MinimalForwarder (…

…OpenZeppelin#4346)

Co-authored-by: Francisco <fg@frang.io>

.changeset/blue-horses-do.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': major
+---
+
+`ERC2771Forwarder`: Added `deadline` for expiring transactions, batching, and more secure handling of `msg.value`.

.github/workflows/checks.yml

@@ -100,6 +100,7 @@ jobs:
       - uses: crytic/slither-action@v0.3.0
         with:
           node-version: 18.15
+          slither-version: 0.9.3
 
   codespell:
     runs-on: ubuntu-latest

contracts/metatx/ERC2771Forwarder.sol

@@ -0,0 +1,289 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (metatx/ERC2771Forwarder.sol)
+
+pragma solidity ^0.8.19;
+
+import "../utils/cryptography/ECDSA.sol";
+import "../utils/cryptography/EIP712.sol";
+import "../utils/Nonces.sol";
+import "../utils/Address.sol";
+
+/**
+ * @dev A forwarder compatible with ERC2771 contracts. See {ERC2771Context}.
+ *
+ * This forwarder operates on forward requests that include:
+ *
+ * * `from`: An address to operate on behalf of. It is required to be equal to the request signer.
+ * * `to`: The address that should be called.
+ * * `value`: The amount of native token to attach with the requested call.
+ * * `gas`: The amount of gas limit that will be forwarded with the requested call.
+ * * `nonce`: A unique transaction ordering identifier to avoid replayability and request invalidation.
+ * * `deadline`: A timestamp after which the request is not executable anymore.
+ * * `data`: Encoded `msg.data` to send with the requested call.
+ */
+contract ERC2771Forwarder is EIP712, Nonces {
+    using ECDSA for bytes32;
+
+    struct ForwardRequestData {
+        address from;
+        address to;
+        uint256 value;
+        uint256 gas;
+        uint48 deadline;
+        bytes data;
+        bytes signature;
+    }
+
+    bytes32 private constant _FORWARD_REQUEST_TYPEHASH =
+        keccak256(
+            "ForwardRequest(address from,address to,uint256 value,uint256 gas,uint256 nonce,uint48 deadline,bytes data)"
+        );
+
+    /**
+     * @dev Emitted when a `ForwardRequest` is executed.
+     *
+     * NOTE: An unsuccessful forward request could be due to an invalid signature, an expired deadline,
+     * or simply a revert in the requested call. The contract guarantees that the relayer is not able to force
+     * the requested call to run out of gas.
+     */
+    event ExecutedForwardRequest(address indexed signer, uint256 nonce, bool success);
+
+    /**
+     * @dev The request `from` doesn't match with the recovered `signer`.
+     */
+    error ERC2771ForwarderInvalidSigner(address signer, address from);
+
+    /**
+     * @dev The `requestedValue` doesn't match with the available `msgValue`.
+     */
+    error ERC2771ForwarderMismatchedValue(uint256 requestedValue, uint256 msgValue);
+
+    /**
+     * @dev The request `deadline` has expired.
+     */
+    error ERC2771ForwarderExpiredRequest(uint48 deadline);
+
+    /**
+     * @dev See {EIP712-constructor}.
+     */
+    constructor(string memory name) EIP712(name, "1") {}
+
+    /**
+     * @dev Returns `true` if a request is valid for a provided `signature` at the current block timestamp.
+     *
+     * A transaction is considered valid when it hasn't expired (deadline is not met), and the signer
+     * matches the `from` parameter of the signed request.
+     *
+     * NOTE: A request may return false here but it won't cause {executeBatch} to revert if a refund
+     * receiver is provided.
+     */
+    function verify(ForwardRequestData calldata request) public view virtual returns (bool) {
+        (bool alive, bool signerMatch, ) = _validate(request);
+        return alive && signerMatch;
+    }
+
+    /**
+     * @dev Executes a `request` on behalf of `signature`'s signer using the ERC-2771 protocol. The gas
+     * provided to the requested call may not be exactly the amount requested, but the call will not run
+     * out of gas. Will revert if the request is invalid or the call reverts, in this case the nonce is not consumed.
+     *
+     * Requirements:
+     *
+     * - The request value should be equal to the provided `msg.value`.
+     * - The request should be valid according to {verify}.
+     */
+    function execute(ForwardRequestData calldata request) public payable virtual {
+        // We make sure that msg.value and request.value match exactly.
+        // If the request is invalid or the call reverts, this whole function
+        // will revert, ensuring value isn't stuck.
+        if (msg.value != request.value) {
+            revert ERC2771ForwarderMismatchedValue(request.value, msg.value);
+        }
+
+        if (!_execute(request, true)) {
+            revert Address.FailedInnerCall();
+        }
+    }
+
+    /**
+     * @dev Batch version of {execute} with optional refunding and atomic execution.
+     *
+     * In case a batch contains at least one invalid request (see {verify}), the
+     * request will be skipped and the `refundReceiver` parameter will receive back the
+     * unused requested value at the end of the execution. This is done to prevent reverting
+     * the entire batch when a request is invalid or has already been submitted.
+     *
+     * If the `refundReceiver` is the `address(0)`, this function will revert when at least
+     * one of the requests was not valid instead of skipping it. This could be useful if
+     * a batch is required to get executed atomically (at least at the top-level). For example,
+     * refunding (and thus atomicity) can be opt-out if the relayer is using a service that avoids
+     * including reverted transactions.
+     *
+     * Requirements:
+     *
+     * - The sum of the requests' values should be equal to the provided `msg.value`.
+     * - All of the requests should be valid (see {verify}) when `refundReceiver` is the zero address.
+     *
+     * NOTE: Setting a zero `refundReceiver` guarantees an all-or-nothing requests execution only for
+     * the first-level forwarded calls. In case a forwarded request calls to a contract with another
+     * subcall, the second-level call may revert without the top-level call reverting.
+     */
+    function executeBatch(
+        ForwardRequestData[] calldata requests,
+        address payable refundReceiver
+    ) public payable virtual {
+        bool atomic = refundReceiver == address(0);
+
+        uint256 requestsValue;
+        uint256 refundValue;
+
+        for (uint256 i; i < requests.length; ++i) {
+            requestsValue += requests[i].value;
+            bool success = _execute(requests[i], atomic);
+            if (!success) {
+                refundValue += requests[i].value;
+            }
+        }
+
+        // The batch should revert if there's a mismatched msg.value provided
+        // to avoid request value tampering
+        if (requestsValue != msg.value) {
+            revert ERC2771ForwarderMismatchedValue(requestsValue, msg.value);
+        }
+
+        // Some requests with value were invalid (possibly due to frontrunning).
+        // To avoid leaving ETH in the contract this value is refunded.
+        if (refundValue != 0) {
+            // We know refundReceiver != address(0) && requestsValue == msg.value
+            // meaning we can ensure refundValue is not taken from the original contract's balance
+            // and refundReceiver is a known account.
+            Address.sendValue(refundReceiver, refundValue);
+        }
+    }
+
+    /**
+     * @dev Validates if the provided request can be executed at current block timestamp with
+     * the given `request.signature` on behalf of `request.signer`.
+     */
+    function _validate(
+        ForwardRequestData calldata request
+    ) internal view virtual returns (bool alive, bool signerMatch, address signer) {
+        signer = _recoverForwardRequestSigner(request);
+        return (request.deadline >= block.timestamp, signer == request.from, signer);
+    }
+
+    /**
+     * @dev Recovers the signer of an EIP712 message hash for a forward `request` and its corresponding `signature`.
+     * See {ECDSA-recover}.
+     */
+    function _recoverForwardRequestSigner(ForwardRequestData calldata request) internal view virtual returns (address) {
+        return
+            _hashTypedDataV4(
+                keccak256(
+                    abi.encode(
+                        _FORWARD_REQUEST_TYPEHASH,
+                        request.from,
+                        request.to,
+                        request.value,
+                        request.gas,
+                        nonces(request.from),
+                        request.deadline,
+                        keccak256(request.data)
+                    )
+                )
+            ).recover(request.signature);
+    }
+
+    /**
+     * @dev Validates and executes a signed request returning the request call `success` value.
+     *
+     * Internal function without msg.value validation.
+     *
+     * Requirements:
+     *
+     * - The caller must have provided enough gas to forward with the call.
+     * - The request must be valid (see {verify}) if the `requireValidRequest` is true.
+     *
+     * Emits an {ExecutedForwardRequest} event.
+     *
+     * IMPORTANT: Using this function doesn't check that all the `msg.value` was sent, potentially
+     * leaving value stuck in the contract.
+     */
+    function _execute(
+        ForwardRequestData calldata request,
+        bool requireValidRequest
+    ) internal virtual returns (bool success) {
+        (bool alive, bool signerMatch, address signer) = _validate(request);
+
+        // Need to explicitly specify if a revert is required since non-reverting is default for
+        // batches and reversion is opt-in since it could be useful in some scenarios
+        if (requireValidRequest) {
+            if (!alive) {
+                revert ERC2771ForwarderExpiredRequest(request.deadline);
+            }
+
+            if (!signerMatch) {
+                revert ERC2771ForwarderInvalidSigner(signer, request.from);
+            }
+        }
+
+        // Ignore an invalid request because requireValidRequest = false
+        if (signerMatch && alive) {
+            // Nonce should be used before the call to prevent reusing by reentrancy
+            uint256 currentNonce = _useNonce(signer);
+
+            (success, ) = request.to.call{gas: request.gas, value: request.value}(
+                abi.encodePacked(request.data, request.from)
+            );
+
+            _checkForwardedGas(request);
+
+            emit ExecutedForwardRequest(signer, currentNonce, success);
+        }
+    }
+
+    /**
+     * @dev Checks if the requested gas was correctly forwarded to the callee.
+     *
+     * As a consequence of https://eips.ethereum.org/EIPS/eip-150[EIP-150]:
+     * - At most `gasleft() - floor(gasleft() / 64)` is forwarded to the callee.
+     * - At least `floor(gasleft() / 64)` is kept in the caller.
+     *
+     * It reverts consuming all the available gas if the forwarded gas is not the requested gas.
+     *
+     * IMPORTANT: This function should be called exactly the end of the forwarded call. Any gas consumed
+     * in between will make room for bypassing this check.
+     */
+    function _checkForwardedGas(ForwardRequestData calldata request) private view {
+        // To avoid insufficient gas griefing attacks, as referenced in https://ronan.eth.limo/blog/ethereum-gas-dangers/
+        //
+        // A malicious relayer can attempt to shrink the gas forwarded so that the underlying call reverts out-of-gas
+        // but the forwarding itself still succeeds. In order to make sure that the subcall received sufficient gas,
+        // we will inspect gasleft() after the forwarding.
+        //
+        // Let X be the gas available before the subcall, such that the subcall gets at most X * 63 / 64.
+        // We can't know X after CALL dynamic costs, but we want it to be such that X * 63 / 64 >= req.gas.
+        // Let Y be the gas used in the subcall. gasleft() measured immediately after the subcall will be gasleft() = X - Y.
+        // If the subcall ran out of gas, then Y = X * 63 / 64 and gasleft() = X - Y = X / 64.
+        // Under this assumption req.gas / 63 > gasleft() is true is true if and only if
+        // req.gas / 63 > X / 64, or equivalently req.gas > X * 63 / 64.
+        // This means that if the subcall runs out of gas we are able to detect that insufficient gas was passed.
+        //
+        // We will now also see that req.gas / 63 > gasleft() implies that req.gas >= X * 63 / 64.
+        // The contract guarantees Y <= req.gas, thus gasleft() = X - Y >= X - req.gas.
+        // -    req.gas / 63 > gasleft()
+        // -    req.gas / 63 >= X - req.gas
+        // -    req.gas >= X * 63 / 64
+        // In other words if req.gas < X * 63 / 64 then req.gas / 63 <= gasleft(), thus if the relayer behaves honestly
+        // the forwarding does not revert.
+        if (gasleft() < request.gas / 63) {
+            // We explicitly trigger invalid opcode to consume all gas and bubble-up the effects, since
+            // neither revert or assert consume all gas since Solidity 0.8.0
+            // https://docs.soliditylang.org/en/v0.8.0/control-structures.html#panic-via-assert-and-error-via-require
+            /// @solidity memory-safe-assembly
+            assembly {
+                invalid()
+            }
+        }
+    }
+}