Read on uninitialized buffer may cause UB

[JOE1994]

Hello fellow Rustacean,
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

slice/src/lib.rs

Lines 196 to 201 in 3e4505e

	unsafe {
	let pointer = buffer.as_mut_ptr().add(length);
	let slice = std::slice::from_raw_parts_mut(pointer, remaining);
	self.underlying.read_exact(slice)?;
	buffer.set_len(length + remaining);

<IoSlice<T> as std::io::Read>::read_to_end() 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