Read on uninitialized buffer may cause UB (2 functions)

[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

livesplit-core/src/run/parser/llanfair.rs

Lines 131 to 145 in d263170

	fn read_string<R: Read>(
	mut source: R,
	buf: &mut Vec<u8>,
	max_length: u64,
	) -> StdResult<&str, StringError> {
	let str_length = source.read_u16::<BE>().context(ReadLength)? as usize;
	if str_length as u64 > max_length {
	return Err(StringError::LengthOutOfBounds);
	}
	buf.clear();
	buf.reserve(str_length);
	unsafe { buf.set_len(str_length) };
	source.read_exact(buf).context(ReadData)?;
	from_utf8(buf).context(Validate)
	}

livesplit-core/src/run/parser/llanfair.rs

Lines 243 to 246 in d263170

	buf.clear();
	buf.reserve(len);
	unsafe { buf.set_len(len) };
	source.read_exact(&mut buf).context(ReadImageData)?;

llanfair::read_string() & llanfair::parse() 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 Jan 2021, there is not yet an ideal fix that works in stable Rust with no performance overhead. Below are links to relevant discussions & suggestions for the fix.

• Well-written document regarding the issue
• Rust RFC 2930
• nightly feature std::io::Initializer
• Discussion in Rust Internals Forum

[CryZe]

Thanks, I was aware of this issue, but wasn't sure if I should wait for the safe solution or fix it. It's pretty unlikely that any Read impl actually reads from the buffer anyway. Anyways, the code isn't all that hot anyway, so it should be fine to zero initialize for now and later switch to the safe abstraction. Thanks for opening this, a PR is now up.

[JOE1994]

@CryZe Thank you so much for your quick fix!
Would you also mind publishing a new release to crates.io? (maybe v0.11.1)

[CryZe]

0.11 is very outdated. I think I want to mostly finish #386 (basically done, just needs to be documented) and #372 and then publish v0.12.