Welcome back to “Code of the Week,” where we uncover subtle yet impactful vulnerabilities in our codebases. This week, we’re diving into a tricky issue in Rust that revolves around object lifetimes and unsafe code.

The code

The task is to identify the vulnerability in the following Rust function, which signs data using a cryptographic function:

pub fn sign(data: Option<&[u8]>) {
    let p = match data {
        Some(data) => BioSlice::new(data).as_ptr(),
        None => std::ptr::null_mut(),
    };
    unsafe {
        let cms = cvt_p(CMS_sign(p));
        // Further processing...
    }
}

The bug

The bug here is a classic use-after-free (UAF) issue, and it comes from an example discussed in a study on Rust vulnerabilities by UC San Diego, which you can find here. In Rust, the object created by BioSlice::new(data) has its lifetime ended when the match statement resolves, but p is a raw pointer that outlives this scope. When p is used in the unsafe block, it points to freed memory, leading to undefined behavior.

Here’s a breakdown of the problem: 1) Temporary BioSlice object: The BioSlice::new(data) creates a temporary object. 2) Lifetime ends: This object’s lifetime ends at the end of the match statement. 3) Dangling pointer: p holds a pointer to this now-freed memory. 4) Unsafe use: The unsafe block uses p, leading to a use-after-free situation.

The fix

To fix this issue, we need to ensure that the BioSlice object outlives the use of its pointer. This can be done by storing the BioSlice in a variable that lives long enough.

pub fn sign(data: Option<&[u8]>) {
    let bio_slice;
    let p = match data {
        Some(data) => {
            bio_slice = BioSlice::new(data);
            bio_slice.as_ptr()
        }
        None => std::ptr::null_mut(),
    };
    unsafe {
        let cms = cvt_p(CMS_sign(p));
        // Further processing...
    }
}

The detection

Detecting use-after-free issues in Rust can be challenging, especially when using raw pointers and unsafe code. However, Rust’s borrow checker usually prevents such issues when using safe code. Here are some strategies to detect and prevent such issues: * Code reviews: Carefully review unsafe code to ensure proper lifetimes; * Static analysis: Use tools like Clippy and Miri to analyze code for potential issues; * Testing: Write extensive tests, including edge cases, to catch runtime issues.

The conclusion

In this sixth installment of “Code of the Week,” we explored a subtle use-after-free vulnerability in a Rust function. This bug highlights the importance of understanding lifetimes and the risks associated with unsafe code. By ensuring that objects outlive their raw pointers, we can prevent such issues and write safer Rust code.

Stay tuned for our next installment, where we’ll continue to uncover hidden flaws in our code and learn how to mitigate them effectively. Remember, the goal is not just to find bugs, but to ensure they never reappear in our codebases.