`index()` allows out-of-bound read and `remove()` has off-by-one error

[Qwaz]

Hello, we have noticed a soundness issue and/or a potential security vulnerability in this crate while performing a security scan on crates.io.

simple-slab/src/lib.rs

Lines 160 to 165 in f1b18e1

	impl<T> Index<usize> for Slab<T> {
	type Output = T;
	fn index(&self, index: usize) -> &Self::Output {
	unsafe { &(*(self.mem.offset(index as isize))) }
	}
	}

simple-slab/src/lib.rs

Lines 82 to 103 in f1b18e1

	#[inline]
	pub fn remove(&mut self, offset: usize) -> T {
	assert!(offset < self.len, "Offset out of bounds");
	let elem: T;
	let last_elem: T;
	let elem_ptr: *mut T;
	let last_elem_ptr: *mut T;
	unsafe {
	elem_ptr = self.mem.offset(offset as isize);
	last_elem_ptr = self.mem.offset(self.len as isize);
	elem = ptr::read(elem_ptr);
	last_elem = ptr::read(last_elem_ptr);
	ptr::write(elem_ptr, last_elem);
	}
	self.len -= 1;
	return elem;
	}

Description

Slab::index() does not perform the boundary checking, which leads to out-of-bound read access. Slab::remove() copies an element from an invalid address due to off-by-one error, resulting in memory leakage and uninitialized memory drop.

Demonstration

• Crate: simple-slab
• Version: 0.3.2
• OS: Ubuntu 18.04.5 LTS
• Rust: rustc 1.46.0 (04488afe3 2020-08-24)

#![forbid(unsafe_code)]

mod boilerplate;

use simple_slab::Slab;

#[derive(Debug, PartialEq)]
struct DropDetector(u32);

impl Drop for DropDetector {
    fn drop(&mut self) {
        println!("Dropping {}", self.0);
    }
}
fn main() {
    boilerplate::init();

    let mut slab = Slab::with_capacity(2);
    slab.insert(DropDetector(123));
    slab.insert(DropDetector(456));

    // 1. No boundary checking leads to OOB read in `index()`
    println!("{:?}", slab[20]);

    // 2. Memory leak / uninitialized memory access in `remove()`
    // element should be copied from `len - 1`, not `len`
    assert_eq!(slab.remove(0).0, 123);
    assert_eq!(slab[0].0, 456); // copied from uninitialized region `slab[2]`
}

Output:

DropDetector(3090534592)
Dropping 123
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `456`', src/main.rs:49:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Dropping 0

Return Code: 101