rustix "memory explosion" vulnerability

[brandonmarzolf]

Summary

tl;dr: rustix memory vulnerability affects rustix version < 0.38.19, please update to latest rustix version via updating to the latest 2.2.xx version of async-io to resolve this vulnerability. This affects async-std v1.12.0 because it depends on async-io v1.13.0 which depends on the vulnerable rustix v0.37.xx. What follows is directly from GitHub's Dependabot

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. See imsnif/bandwhich#284 (comment) and the Discord thread for details.
Diagnosis

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

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.

Memory over-allocation

Dir::read_more incorrectly grows the read buffer unconditionally each time it is called, regardless of necessity.

Since

::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> {
// 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.

[sunfishcode]

Rustix maintainer here. I can confirm that async-io is not vulnerable to this bug, because none of async-io or its dependencies async-signal, async-process, or polling use the affectted rustix::fs::Dir API.