GitHub Security Advisory: GHSA-c827-hfw6-qwvm
Package: rustix (RUST)
Severity: MODERATE (CVSS 6.5)
CWE: CWE-400: Uncontrolled Resource Consumption
Vulnerable versions: >= 0.38.0, < 0.38.19
Patched in: 0.38.19

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

Details:
### 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 https://github.com/imsnif/bandwhich/issues/284. That post has lots of details of our investigation. See [this post](https://github.com/imsnif/bandwhich/issues/284#issuecomment[PII_REDACTED]) and the [Discord thread](https://discord.com/channels/[PII_REDACTED]10479360/[PII_REDACTED]95237556) 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.
2. 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

```rust
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 implementa