Read on uninitialized buffer may cause UB (4 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

messagepack-rs/messagepack-rs/src/deserializable.rs

Lines 59 to 71 in ceaf013

	fn deserialize_binary<R: Read>(size: usize, buf_reader: &mut R) -> Result<Self, DeserializeError> {
	let mut buf = Vec::with_capacity(size);
	unsafe { buf.set_len(size); }
	buf_reader.read_exact(&mut buf[..]).or(Err(DeserializeError::InvalidValue))?;
	Ok(From::from(Binary(buf)))
	}
	fn deserialize_string<R: Read>(size: usize, buf_reader: &mut R) -> Result<Self, DeserializeError> {
	let mut buf = Vec::with_capacity(size);
	unsafe { buf.set_len(size); }
	buf_reader.read_exact(&mut buf).or(Err(DeserializeError::InvalidValue))?;
	Ok(From::from(String::from_utf8(buf).or(Err(DeserializeError::InvalidValue))?))
	}

messagepack-rs/messagepack-rs/src/deserializable.rs

Lines 90 to 92 in ceaf013

	unsafe { buf.set_len(buf.capacity()); }
	buf_reader.read_exact(&mut buf[..]).or(Err(DeserializeError::InvalidValue))?;
	String::from_utf8(buf).or(Err(DeserializeError::InvalidValue))

messagepack-rs/messagepack-rs/src/deserializable.rs

Lines 90 to 92 in ceaf013

	unsafe { buf.set_len(buf.capacity()); }
	buf_reader.read_exact(&mut buf[..]).or(Err(DeserializeError::InvalidValue))?;
	String::from_utf8(buf).or(Err(DeserializeError::InvalidValue))

4 functions (deserializable::Deserializable::{deserialize_string, deserialize_extension_others, deserialize_map::deserialize_string_primitive}) create 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

[otake84]

Sorry for the late reply.
I have responded with the following PR.
#3