Semgrep Bug: Rust Metavariable Matching Incorrectly

by Sebastian Müller 52 views

Hey everyone, let's dive into a peculiar issue I've encountered while using Semgrep with Rust code. It seems like there's a slight hiccup in how metavariables are being matched, leading to some unexpected outputs. I'm running Semgrep version v1.130.0, and the problem manifests when using metavariable-regex within my rules. So let's get started and fix this bug.

The Rule and the Code

Here’s the Semgrep rule I've defined:

rules:
  - id: redacted
    languages:
      - rust
    severity: INFO
    message: a hash $ALG was detected
    metadata:
      copyright: redacted
    patterns:
      - metavariable-regex:
          metavariable: $REGEX
          regex: \A(?<ALG>Sha\d\d\d(_\d\d\d)?)\Z
      - pattern: sha2::$REGEX::$

This rule is designed to detect the usage of specific SHA hash algorithms in Rust code. The core of the rule lies in the metavariable-regex pattern. Let’s break it down:

  • metavariable: $REGEX – This declares $REGEX as a metavariable, which means it will capture the text matched by the regex.
  • regex: \A(?<ALG>Sha\d\d\d(_\d\d\d)?)\Z – This is the regular expression that defines what we're looking for. Let's dissect it further:
    • \A – Matches the start of the string.
    • (?<ALG> ...) – This is a named capture group. It captures the matched text and assigns it the name ALG. This allows us to reference the captured text later using $ALG in the message.
    • Sha\d\d\d – Matches "Sha" followed by three digits (e.g., Sha256, Sha512).
    • (_\d\d\d)? – Optionally matches an underscore followed by three digits (e.g., _256). This accounts for variants like Sha512_256.
    • \Z – Matches the end of the string.
  • pattern: sha2::$REGEX::$_ – This is the main pattern that uses the $REGEX metavariable. It looks for code that matches the structure sha2::$REGEX::$_. Here,
    • sha2:: – Matches the literal text "sha2::".
    • $REGEX – Represents the text captured by the $REGEX metavariable, which should be the name of the SHA algorithm.
    • ::$_ – Matches the scope resolution operator :: followed by any expression $__.

The idea is to capture the algorithm name (e.g., Sha512_256) using the regex and then use it in the overall pattern to detect the usage of that specific algorithm. Now, let’s look at the Rust code we're testing this against:

use sha2::{Sha512_256, Digest};
fn main() {
    let mut hasher = Sha512_256::new();11111111111111111111;
}

This is a simple Rust snippet that uses the Sha512_256 algorithm from the sha2 crate. We import the necessary modules, and inside the main function, we create a new hasher instance. The goal here is to make sure Semgrep correctly identifies the Sha512_256 part.

The Problem: Off-by-One Error

When I run Semgrep with the above rule and code, I expect it to output: "a hash Sha512_256 was detected". This is because the regex should correctly capture Sha512_256, and the overall pattern should match the line where the hasher is initialized. However, the actual output I'm getting is: "a hash new();1111 was detected". This is quite puzzling, guys!

It appears there’s an off-by-one error in how Semgrep is handling the metavariable match. Instead of starting the match at the correct namespace section (Sha512_256), it’s starting somewhere within the new() call. The length of the match seems correct, as it captures up to the semicolon, but the starting position is clearly wrong. This means Semgrep isn't correctly identifying the algorithm name, which defeats the purpose of the rule. It's like it's peeking at the right answer through a keyhole but misinterpreting what it sees.

This kind of issue can lead to false negatives, where Semgrep fails to detect the usage of specific algorithms, or false positives, where it flags incorrect code sections. In a security context, this can be particularly problematic, as it could lead to overlooking potential vulnerabilities. So, nailing down the root cause is super important.

Root Cause Analysis

To really understand what's happening, let's break down why Semgrep might be getting tripped up here. The issue seems to stem from the interaction between the metavariable-regex and the main pattern. Here’s a potential chain of events:

  1. Regex Matching: The regex \A(?<ALG>Sha\d\d\d(_\d\d\d)?)\Z is designed to match algorithm names like Sha512_256. It correctly identifies and captures this part.
  2. Metavariable Substitution: Semgrep then substitutes the captured text into the $REGEX metavariable within the main pattern.
  3. Pattern Matching: The main pattern sha2::$REGEX::$_ is where things go awry. Semgrep tries to match this pattern against the code. The problem might be in how Semgrep determines the starting position for this match. It seems to be off by a few characters, causing it to miss the correct starting point.

One possibility is that the internal representation of the code or the way Semgrep indexes it might be causing this offset. It could be related to how Semgrep tokenizes the code or handles namespaces and scopes. Another factor could be the interaction between the regex match and the subsequent pattern match. Semgrep might be miscalculating the starting position based on the regex match's boundaries.

Another way to think about it is that Semgrep's internal cursor, which it uses to walk through the code, isn't positioned correctly after the regex match. It might be skipping ahead or falling behind by a few characters. This is similar to a word processor's cursor jumping to the wrong spot after you highlight and copy some text.

Potential Workarounds and Next Steps

While we've identified the issue, let's think about some immediate steps and potential workarounds.

Workarounds

  1. Simplify the Pattern: A temporary fix could be to simplify the pattern to avoid the metavariable-regex altogether. For example, instead of using a regex, we could list out the specific algorithms we want to detect. This isn't ideal, as it's less flexible, but it could help in the short term.

    For instance, we could replace the metavariable-regex and pattern with multiple patterns, one for each algorithm:

    patterns:
  - pattern: sha2::Sha512_256::$
  - pattern: sha2::Sha256::$
  # Add more patterns for other algorithms

    This approach avoids the complexities of metavariable matching but requires more manual effort to maintain.

  2. Adjust the Regex: We could try tweaking the regex to see if it affects the matching behavior. For example, we might try adding or removing anchors (\A and \Z) or modifying the capture group. However, this is more of a shot in the dark, as the issue seems to be with how Semgrep handles the metavariable substitution rather than the regex itself.

  3. Target a Broader Pattern: Instead of focusing on the specific algorithm name, we could try matching a broader pattern that includes the entire line of code. This might help avoid the off-by-one error, but it would also capture more code than necessary, potentially leading to more false positives.

Next Steps

  1. Isolate the Problem: The next step is to try and isolate the problem further. We can create more test cases with different code structures and regex patterns to see if we can narrow down the specific conditions that trigger the bug. This will help us provide more detailed information to the Semgrep team.

  2. Report the Bug: The most important step is to report this bug to the Semgrep maintainers. We can provide them with the rule, the code snippet, and the observed behavior. The more information we can provide, the easier it will be for them to reproduce and fix the issue.

  3. Contribute a Test Case: If possible, we can also contribute a test case to the Semgrep repository. This will help ensure that the bug is fixed properly and that it doesn't reappear in future versions. Contributing a test case also helps the Semgrep team build a more robust and reliable tool.

Conclusion

So, there you have it, guys! We've uncovered a tricky little bug in Semgrep's metavariable matching for Rust code. It seems like there's an off-by-one error that causes the match to start at the wrong position. While we've explored some potential workarounds, the real solution lies in reporting this issue to the Semgrep team and helping them fix it. By working together, we can make Semgrep an even more powerful tool for code analysis and security.

Remember, even the best tools can have quirks, and it's our job as users and developers to identify and address them. Keep experimenting, keep reporting, and keep contributing! Happy coding!