Summary

When usingrustix::fs::Dir using thelinux_raw backend, it's possible for the iterator to "get stuck" when an IO error is encountered. Combined with a memory over-allocation issue inrustix::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 inimsnif/bandwhich#284. That post has lots of details of our investigation. Seethis post and theDiscord thread for details.

Diagnosis

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

1. Stuck iterator

• Therustix::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 callsDir::read, which in turn callsDir::read_more, this means an IO error encountered during reading a directory can lead to rapid and unbounded growth of memory use.

PoC

fnmain() ->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 pathprintln!("{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 theDir 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 usingrustix::fs::Dir directly or indirectly (e.g. with theprocfs 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.

References

• GHSA-c827-hfw6-qwvm
• imsnif/bandwhich#284
• imsnif/bandwhich#284 (comment)
• bytecodealliance/rustix@31fd98c
• bytecodealliance/rustix@87481a9
• bytecodealliance/rustix@df3c3a1
• bytecodealliance/rustix@eecece4
• https://discord.com/channels/273534239310479360/1161137828395237556
• https://nvd.nist.gov/vuln/detail/CVE-2024-43806