reNFT Smart Contracts

Generalised collateral-free and permissionless rentals built on top of Gnosis Safe and Seaport.

Testing

Unit tests

Running all unit tests can be done with Foundry:

forge test -vvv

Static Analysis

We use slither to run static analysis. Make sure it is installed via:

pip3 install slither-analyzer

To run the static analysis on the contracts:

slither .

Invariants

Recipient of ERC721 / ERC1155 tokens is always the reNFT smart contract renter wallet
Recipient of ERC20 tokens is always the Payment Escrow Module
Stored token balance of the src/modules/PaymentEscrow.sol contract should never be less than the true token balance of the contract
Rental safes can never make a call to setGuard()
Rental safes can never make a call to enableModule() or disableModule unless the target has been whitelisted by src/policies/Admin.sol
Rental safes can never make a delegatecall unless the target has been whitelisted by src/policies/Admin.sol
ERC721 / ERC1155 tokens cannot leave a rental wallet via approve(), setApprovalForAll(), safeTransferFrom(), transferFrom(), or safeBatchTransferFrom()
Hooks can be specified for ERC721 and ERC1155 items
Only one hook can act as middleware to a target contract at one time. But, there is no limit on the amount of hooks that can execute during rental start or stop.
When control flow is passed to hook contracts, the rental concerning the hook will be active and a record of it will be stored in src/modules/Storage.sol

Generalized Rental Guards

Overview

When signing a rental order, the lender can decide to include an array of Hook structs along with it. These are bespoke restrictions or added functionality that can be applied to the rented token within the wallet. This protocol allows for flexibility in how these hooks are implemented and what they restrict. A common use-case for a hook is to prevent a call to a specific function selector on a contract when renting a particular token ID from an ERC721/ERC1155 collection.

Adding a Hook

Adding a hook contract to the protocol is an admin-permissioned action on the src/policies/Guard.sol contract which is done via:

updateHookStatus() which enables a hook for use within the protocol.

updateHookPath() which specifies the contract which the rental wallet interacts with that will activate the hook.

Specifying hooks as a lender

When creating a rental, a OrderMetadata struct will be added to the order which specifies extra parameters to pass along with the rentals:

struct OrderMetadata {
    // the type of order being created
    OrderType orderType;
    // the duration of the rental in seconds
    uint256 rentDuration;
    // the hooks that will act as middleware for the items in the order
    Hook[] hooks;
    // any extra data to be emitted upon order fulfillment
    bytes emittedExtraData;
}

Hooks can be added here to specify the unique functionality placed upon tokens in the order. Only hooks which have been enabled by the admin will be valid when passed to the address target field.

struct Hook {
    // the hook contract to target
    address target;
    // index of the item in the order to apply the hook to
    uint256 itemIndex;
    // any extra data that the hook will need. This will most likely
    // be some type of bitmap scheme
    bytes extraData;
}

Routing a call to the proper hook

After a renter has successfully rented an ERC721/ERC1155, the src/policies/Guard.sol contract will be invoked each time a transaction originates from the wallet. The contract will check its mapping for any hooks in the path of the interacting address.

If a hook exists, the control flow will be handed over to the hook contract for further processing.

If no hook address was found, the rental guard contract contains basic restrictions that prevents the usage of ERC721/ERC1155 state-changing functions.

Implementing a hook

Example implementations of hooks can be found in the src/examples/restricted-selector/ folder.

Per each erc721 GameToken ID, this hook uses a bitmap which tracks any function selectors that are restricted for that token ID only. Bitmaps allow support for up to 256 function selectors on a single contract.

Using a token ID -> bitmap mapping allows the property that 2 or more tokens from the same collection can be restricted in different ways based on how their lenders defined the permissions.

Extending a hook

Hooks are extendable. An allowlisted hook for a collection can be expanded even further to allow for multiple child hooks that are routed to based on logic defined in the parent hook. This pattern enables granular control flow of the transaction execution for any requirements or restrictions that a rental may have.

Known Limitations

Disabled Delegate call

Transactions that leverage delegate call on the safe contract are not allowed. This invariant helps protect the safe from a few attack vectors. If delegate call were allowed, a transaction could be crafted that would allow an approve() call on a rented NFT even though the guard contract disables calls to approve(). See test/adversarial/DelegateCallApproveAttack for more info on this. Also, allowing delegate call would introduce another attack surface via the re-initialization of the safe. Because the safe uses delegate call on instantiation to enable the module and guard contract, the initializeRentalSafe() function could be delegate called again after instantiation and overwrite the active guard or enable a new module.

To mitigate this attack vector while still allowing delegate call, the contracts use a whitelist to allow delegate call to specific addresses that are deemed safe. For example, a contract dedicated to updating a gnosis safe module to a newer version could be whitelisted by the protocol to allow a rental safe to call it.

Dishonest ERC721/ERC1155 Implementations

The src/policies/Guard.sol contract can only protect against the transfer of tokens that faithfully implement the ERC721/ERC1155 spec. A dishonest implementation that adds an additional function to transfer the token to another wallet cannot be prevented by the protocol.

Rebasing or Fee-On-Transfer ERC20 Implementations

The protocol contracts do not expect to be interacting with any ERC20 token balances that can change during transfer due to a fee, or change balance while owned by the src/modules/PaymentEscrow.sol contract.

chiefpunk/rental-contracts