Linea Rollup Update

1 Executive Summary

This report presents the results of our engagement with Linea to review Linea Rollup Update.

The review was conducted over two weeks, from July 3, 2024 to July 17, 2024, by Rai Yang and Vlad Yaroshuk. A total of 20 person-days were spent.

We reviewed the updates implemented between Linea Alpha v2 and Alpha v3.2. The key updates include:

• Leveraging EIP-4844 blob for blob submission (Alpha v3.0).

• Smart contract gas optimizations and message service updates (Alpha v3.2).

• Minor changes to the token bridge.

2 Scope

Our review focused on the commit hash b17e7c79b5647e47c175c6367dea30c3f1c66738 and it’s has been verified to match the code in scope in the monorepo. The list of files in scope can be found in the Appendix.

2.1 Objectives

Together with the Linea team, we identified the following priorities for our review:

1. Correctness of the implementation, consistent with the intended functionality and without unintended edge cases.
2. Identify known vulnerabilities particular to smart contract systems, as outlined in our Smart Contract Best Practices, and the Smart Contract Weakness Classification Registry.

3 Security Specification

This section describes, from a security perspective, the expected behavior of the system under audit. It is not a substitute for documentation. The purpose of this section is to identify specific security properties that were validated by the audit team.

The code update reviewed in this audit doesn’t change from the previous security properties.

3.1 Actors

The relevant actors with their respective abilities that are changed are listed below :

• Operator: Submits block data either by blobs or compressed data to L1 contract depending on the gas cost

3.2 Trust Model

In any system, it’s important to identify what trust is expected/required between various actors. For this audit, we established the trust model remains unchanged from the previous versions

3.3 Security Properties

The following is a non-exhaustive list of security properties that were reviewed in this audit:

• EIP-4844 blob submission, validation and finalization is correct and sound
• Proof verification is sound (public input generation)
• Storage layout is not broken
• Changes to the Shnarf for data submission cardinality is correct and sound
• The replacement of the rolling hash, timestamp and message number storage variables with last finalized state is correct and sound
• Efficient hashing for data submission, public input, Shnarf and last finalized state is correct

4 Findings

Each issue has an assigned severity:

• Minor issues are subjective in nature. They are typically suggestions around best practices or readability. Code maintainers should use their own judgment as to whether to address such issues.
• Medium issues are objective in nature but are not security vulnerabilities. These should be addressed unless there is a clear reason not to.
• Major issues are security vulnerabilities that may not be directly exploitable or may require certain conditions in order to be exploited. All major issues should be addressed.
• Critical issues are directly exploitable security vulnerabilities that need to be fixed.

function initializeParentShnarfsAndFinalizedState(
  bytes32[] calldata _shnarfs,
  uint256[] calldata _finalBlockNumbers
) external reinitializer(5) {
  if (_shnarfs.length != _finalBlockNumbers.length) {
    revert ShnarfAndFinalBlockNumberLengthsMismatched(_shnarfs.length, _finalBlockNumbers.length);
  }

  for (uint256 i; i < _shnarfs.length; i++) {
    shnarfFinalBlockNumbers[_shnarfs[i]] = _finalBlockNumbers[i];
  }

  currentFinalizedState = _computeLastFinalizedState(
    currentL2StoredL1MessageNumber,
    currentL2StoredL1RollingHash,
    currentTimestamp
  );
}

if (_finalizationData.lastFinalizedShnarf != lastFinalizedShnarf) {
  revert LastFinalizedShnarfWrong(lastFinalizedShnarf, _finalizationData.lastFinalizedShnarf);
}

if (shnarfFinalBlockNumbers[finalShnarf] != _finalizationData.finalBlockInData) {
  revert FinalBlockDoesNotMatchShnarfFinalBlock(
    _finalizationData.finalBlockInData,
    shnarfFinalBlockNumbers[finalShnarf]
  );
}

uint256 publicInput = _computePublicInput(
  _finalizationData,
  lastFinalizedShnarf,
  finalShnarf,
  lastFinalizedBlockNumber
);

Appendix 1 - Files in Scope

This audit covered the following files:

Appendix 2 - Disclosure

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A.2.1 Purpose of Reports

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