Read on uninitialized buffer may cause UB ('tectonic_xdv' crate)

[JOE1994]

Hello 🦀,
we (Rust group @sslab-gatech) found a memory-safety/soundness issue in this crate while scanning Rust code on crates.io for potential vulnerabilities.

Issue Description

tectonic/crates/xdv/src/lib.rs

Lines 224 to 231 in 28a9b46

	let mut buf = Vec::with_capacity(BUF_SIZE);
	unsafe {
	buf.set_len(BUF_SIZE);
	}
	let mut n_saved_bytes = 0;
	loop {
	let n_read = stream.read(&mut buf[n_saved_bytes..])?;

XdvParser::<T>::process() method creates an uninitialized buffer and passes it to user-provided Read implementation. This is unsound, because it allows safe Rust code to exhibit an undefined behavior (read from uninitialized memory).

This part from the Read trait documentation explains the issue:

It is your responsibility to make sure that buf is initialized before calling read. Calling read with an uninitialized buf (of the kind one obtains via MaybeUninit<T>) is not safe, and can lead to undefined behavior.

How to fix the issue?

The Naive & safe way to fix the issue is to always zero-initialize a buffer before lending it to a user-provided Read implementation. Note that this approach will add runtime performance overhead of zero-initializing the buffer.

As of Feb 2021, there is not yet an ideal fix that works with no performance overhead. Below are links to relevant discussions & suggestions for the fix.

• Rust RFC 2930
• Discussion in Rust Internals Forum

[pkgw]

Thank you for filing this. I've read about this topic since I wrote this code but forgot that I had this implementation in here. I'm looking forward to the day that there's a nicer Rust solution for avoiding this unsoundness issue, but in the meantime, it is totally fine to fix this by zeroing out the buffer. I'll prepare a PR to do so.

[ammaraskar]

Has there been a release of tectonic_xdv with this fix incorporated? If not, could you make one please. Thank you!

[pkgw]

It took a long time, but now there is both a fixed version of tectonic_xdv release, and the new 0.5.0 release of tectonic is based on that version of the crate.