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`rustix::fs::Dir` iterator with the `linux_raw` backend can cause memory explosion

Moderate
sunfishcode published GHSA-c827-hfw6-qwvm Oct 17, 2023

Package

cargo rustix (Rust)

Affected versions

>0.35.11

Patched versions

~0.35.15, ~0.36.16, ~0.37.25, ~0.38.19

Description

Summary

When using rustix::fs::Dir using the linux_raw backend, it's possible for the iterator to "get stuck" when an IO error is encountered. Combined with a memory over-allocation issue in rustix::fs::Dir::read_more, this can cause quick and unbounded memory explosion (gigabytes in a few seconds if used on a hot path) and eventually lead to an OOM crash of the application.

Details

Discovery

The symptoms were initially discovered in imsnif/bandwhich#284. That post has lots of details of our investigation. cyqsimon was able to track the bug to rustix, and konnorandrews figured out the offending code. See his post and the Discord thread for details.

Diagnosis

This issue is caused by the combination of two independent bugs:

  1. Stuck iterator
  • The rustix::fs::Dir iterator can fail to halt after encountering an IO error, causing the caller to be stuck in an infinite loop.
  1. Memory over-allocation
  • Dir::read_more incorrectly grows the read buffer unconditionally each time it is called, regardless of necessity.

Since <Dir as Iterator>::next calls Dir::read, which in turn calls Dir::read_more, this means an IO error encountered during reading a directory can lead to rapid and unbounded growth of memory use.

PoC

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // create a directory, get a FD to it, then unlink the directory but keep the FD
    std::fs::create_dir("tmp_dir")?;
    let dir_fd = rustix::fs::openat(
        rustix::fs::CWD,
        rustix::cstr!("tmp_dir"),
        rustix::fs::OFlags::RDONLY | rustix::fs::OFlags::CLOEXEC,
        rustix::fs::Mode::empty(),
    )?;
    std::fs::remove_dir("tmp_dir")?;

    // iterator gets stuck in infinite loop and memory explodes
    rustix::fs::Dir::read_from(dir_fd)?
        // the iterator keeps returning `Some(Err(_))`, but never halts by returning `None`
        // therefore if the implementation ignores the error (or otherwise continues
        // after seeing the error instead of breaking), the loop will not halt
        .filter_map(|dirent_maybe_error| dirent_maybe_error.ok())
        .for_each(|dirent| {
            // your happy path
            println!("{dirent:?}");
        });

    Ok(())
}

Impact

If a program tries to access a directory with its file descriptor after the file has been unlinked (or any other action that leaves the Dir iterator in the stuck state), and the implementation does not break after seeing an error, it can cause a memory explosion.

As an example, Linux's various virtual file systems (e.g. /proc, /sys) can contain directories that spontaneously pop in and out of existence. Attempting to iterate over them using rustix::fs::Dir directly or indirectly (e.g. with the procfs crate) can trigger this fault condition if the implementation decides to continue on errors.

An attacker knowledgeable about the implementation details of a vulnerable target can therefore try to trigger this fault condition via any one or a combination of several available APIs. If successful, the application host will quickly run out of memory, after which the application will likely be terminated by an OOM killer, leading to denial of service.

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CVE ID

CVE-2024-43806

Weaknesses

No CWEs

Credits