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

outer_cgi/src/fcgi.rs

Lines 183 to 196 in a981e7e

	let mut key_buffer = Vec::with_capacity(key_length);
	let mut value_buffer = Vec::with_capacity(value_length);
	unsafe {
	key_buffer.set_len(key_length);
	value_buffer.set_len(value_length);
	}
	match self.reader.read_exact(key_buffer.as_mut_slice()) {
	Ok(()) => (),
	Err(x) => return Some(Err(x)),
	}
	match self.reader.read_exact(value_buffer.as_mut_slice()) {
	Ok(()) => (),
	Err(x) => return Some(Err(x)),
	}

<fcgi::KeyValueReader<R> as std::iter::Iterator>::next() method creates an uninitialized buffer and passes it to user-provided Read implementation (2 such occurrences within the same function). 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

[SolraBizna]

Sorry it took so long. I've been swamped.

Simply zero-initializing all the buffers is definitely a good enough approach for this crate, so I've done that everywhere I had uninitialized memory before. The most performance-critical buffers are heavily reused, so the performance impact is minimal. This also gave me an excuse to go through and put in all the missing dyns and replace all my confusing uses of is_ok with the clearer let _ =...

Anyway, thanks very much for the report. dd59b30 and 9a7de8a should eliminate any possible issues from uninitialized memory.