code-423n4 / 2023-07-chainlink

Chainlink Cross-Chain Contract Administration: Multi-signature Contract, Timelock and Call Proxies audit details

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• Starts July 05, 2023 20:00 UTC
• Ends July 12, 2023 20:00 UTC

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Overview

CallProxy, ManyChainMultiSig, RBACTimelock

The CallProxy, ManyChainMultiSig, RBACTimelock contracts are all part of a system of owner contracts that is supposed to administer other contracts (henceforth referred to as OWNED). OWNED contracts represent any system of contracts that (1) have an owner or similar role (e.g. using OpenZeppelin's OwnableInterface) and that (2) are potentially deployed across multiple chains.

Here is a diagram of how we envision these contracts to interact:

Regular administration of the OWNED contracts is expected to happen through the RBACTimelock's Proposer/Executor/Canceller roles. The Bypasser role is expected to only become active in "break-glass" type emergency scenarios where waiting for RBACTimelock.minDelay would be harmful.

The set of OWNED contracts will comprise Chainlink's upcoming Cross-Chain Interoperability Protocol (CCIP) system.

Proposers can also cancel so that they may "undo" proposals with mistakes in them.

RBACTimelock Considerations

We expect to set RBACTimelock.minDelay and delay to ~ 24 hours, but in general values between 1 hour and 1 month should be supported. This enables anyone to inspect configuration changes to the OWNED contracts before they take effect. For example, a user that disagrees with a configuration change might choose to withdraw funds stored in OWNED contracts before they can be executed. (Though the exact mechanism and assumptions around how this would work are out of scope.)

We may use RBACTimelock.blockFunctionSelector to prevent specific functions on the OWNED contracts from being called through the regular propose-execute flow.

CallProxy Considerations

The CallProxy is intentionally callable by anyone. Offchain tooling used for generating configuration changes will make appropriate use of the RBACTimelock's support for predecessors to ensure that configuration changes are sequenced properly even if an adversary is executing them. Since the adversary can control the gas amount and gas price, callees are expected to not have gas-dependent behavior other than reverting if insufficient gas is supplied.

The CallProxy is not expected to be used with contracts that could SELFDESTRUCT. It thus has no EXTCODESIZE-check prior to making a call. We expect it to be configured correctly (i.e. pointing to a real RBACTimelock) on deployment.

ManyChainMultiSig Considerations

Unlike standard multi-sig contracts, ManyChainMultiSig supports signing many transactions targeting many chains with a single set of signatures. (We currently only target EVM chains and all EVM chains support the same ECDSA secp256k1 standard.) This is useful for administering systems of contracts spanning many chains without increasing signing overhead linearly with the number of supported chains. We expect to use the same set of EOA signers across many chains.Consequently, ManyChainMultiSig only supports EOAs as signers, not other smart contracts. Similar to the rest of the system, anyone who can furnish a correct Merkle proof is allowed to execute authorized calls on the ManyChainMultiSig, including a potential adversary. The adversary will be able to control the gas price and gas amount for the execution.

The proposer and canceller ManyChainMultiSig contracts are expected to be configured with a group structure like this, with different sets of signers for each (exact k-of-n parameters might differ):

          ┌──────────┐
          │Root Group│
      ┌──►│  6-of-8  │◄─────────┐
      │   └──────────┘          │
      │         ▲               │
      │         │               │
 ┌────┴───┐ ┌───┴────┐     ┌────┴───┐
 │signer 1│ │signer 2│ ... │signer 8│
 └────────┘ └────────┘     └────────┘

The bypasser ManyChainMultiSig contract is expected to be configured with a more complex group structure like this (exact structure might differ):

Subgroup 1 has the same signers as the canceller ManyChainMultiSig. No change can ever be enacted without approval of this group.

In practice, we expect the k-of-n configurations of groups to typically have 1<=k<=32 and 1<=n<=32 (where k<=n and we tolerate the overall limits on groups/signers set in ManyChainMultiSig code).

Propose-and-Execute Flow

The following steps need to be performed for a set of onchain maintenance operations on the OWNED contracts:

• [offchain, out of scope] Merkle tree generation & signing: A Merkle tree containing all the required ManyChainMultiSig ops (containing RBACTimelock.scheduleBatch calls) for the desired maintenance operations is generated by the proposers. A quorum of signers from the proposer ManyChainMultiSig must sign (offchain) the Merkle root.
• setRoot call on all relevant ManyChainMultiSig contracts across chains: The signed Merkle root is then sent to ManyChainMultiSigs. Anyone who has been given the root and the signatures offchain can send it to ManyChainMultiSigs.
• execute on ManyChainMultiSig: To propose an action to the RBACTimelock, a multi-sig op is executed by providing a Merkle proof for that specific op. Anyone who has been given the full Merkle tree offchain can propose the action.
• executeBatch on RBACTimelock: After the timelock wait period expires, the proposed actions in TimeLock can be executed. This assumes that the cancellers have not cancelled them in the meantime. Anyone can execute the actions because all the required information is available on the blockchain through event logs.

Canceller Flow

This can be thought of as an optional step of the propose-and-execute flow. If a quorum of cancellers disapproves of an action pending on the RBACTimelock, they can create a set of ManyChainMultiSig.Ops that calls RBACTimelock.cancel on all relevant RBACTimelocks.

Bypasser Flow

This is completely independent of the propose-and-execute flow. Bypassers create a set of ManyChainMultiSig.Ops that calls RBACTimelock.bypasserExecuteBatch on all relevant RBACTimelocks.

ARMProxy

The ARMProxy enables an owner (using RBACTimelock) to upgrade an underlying ARM contract. When the owner wants to upgrade, they call ARMProxy.setARM(new ARM address). We expect the ARMProxy to transparently pass through any function calls except those to functions defined by ARMProxy and the contracts inherits from.

For more information on the ARM contract, see https://github.com/code-423n4/2023-05-chainlink#arm-contract. Note that the ARM contract and its functionality itself are not in scope. You should be able to generically think of the ARM contract as a contract that exposes some view functions, calls to which are proxied via ARMProxy for upgradeability.

Deployments are expected to look like this:

Initially, the "ARM implementation contract" will implement the IARM interface. As time goes by, we may add more functions to the IARM interface. By using a fallback function and assembly, we are future-proof against such updates.

Scope

Contract	SLOC	Purpose	Libraries used
src/ARMProxy.sol	36	ARM proxy contract	src/*Owner*.sol
src/CallProxy.sol	17	Call proxy contract callable by anyone	None
src/ManyChainMultiSig.sol	275	Cross-chain multi-sig	@openzeppelin/*
src/RBACTimelock.sol	216	Timelock with role-based access control	@openzeppelin/*

Out of scope

We intentionally use solidity version 0.8.19 and not 0.8.20.

We intentionally use the old require syntax (and some other old techniques) in RBACTimelock to keep the diff vs the original OZ contract smaller.

Gas cost isn't particularly important for these contracts because they're not expected to be called often. Correctness matters much more.

The fact that "anyone can execute" on the ManyChainMultiSig (only ops authorized through setRoot ofc!), the CallProxy, and the ARMProxy is intentional and not in scope. Consequently, so is the fact that the untrusted executor can choose the gas limit and gas price.

Return data bombs for CallProxy or ARMProxy are out of scope since both contracts are expected to be deployed pointing at trusted contracts.

Additional Context

No curve logic or math models. The ManyChainMultiSig has interesting schemes for configuring a tree of subgroups, Merkle trees of transactions, and chain-specific metadata. See the contract docs for details.

Scoping Details

- If you have a public code repo, please share it here:  No public repo
- How many contracts are in scope?: 4 contracts, see above
- Total SLoC for these contracts?: See above, roughly 550 lines
- How many external imports are there?: Just OpenZeppelin
- How many separate interfaces and struct definitions are there for the contracts within scope?: 5 interfaces, 8 struct definitions
- Does most of your code generally use composition or inheritance?: Mostly composition, but there is a little inheritance
- How many external calls?: Each contract in scope performs calls to external contracts from one location per contract
- What is the overall line coverage percentage provided by your tests?:  > 95%
- Is there a need to understand a separate part of the codebase / get context in order to audit this part of the protocol?: No
- Please describe required context: N/A
- Does it use an oracle?: No
- Does the token conform to the ERC20 standard?: No token
- Are there any novel or unique curve logic or mathematical models?: No
- Does it use a timelock function?: Yes
- Is it an NFT?: No
- Does it have an AMM?: No
- Is it a fork of a popular project?: Partly. RBACTimelock.sol is derived from OpenZeppelin code
- Does it use rollups?: It can run on roll-ups or regular L1s
- Is it multi-chain?: Yes
- Does it use a side-chain?: It could run on a side chain. It can run on any EVM chain

Tests

Our tests use foundry. They rely on some ffi code written in Go 1.18 (see testCommands/). See the official Go docs for installation instructions. Once you have Go running, forge test --ffi should do the trick.