Side effect handling in specialized zip implementation causes buffer overflow

[Qwaz]

rust/library/core/src/iter/adapters/zip.rs

Lines 200 to 208 in 0148b97

	} else if A::may_have_side_effect() && self.index < self.a.size() {
	let i = self.index;
	self.index += 1;
	// match the base implementation's potential side effects
	// SAFETY: we just checked that `i` < `self.a.len()`
	unsafe {
	self.a.__iterator_get_unchecked(i);
	}
	None

rust/library/core/src/iter/adapters/zip.rs

Lines 214 to 218 in 0148b97

	#[inline]
	fn size_hint(&self) -> (usize, Option<usize>) {
	let len = self.len - self.index;
	(len, Some(len))
	}

self.index can be set to a value greater than self.len in this branch. This causes integer overflow in size_hint() and lead to a buffer overflow.

Playground Link that demonstrates segfault with safe Rust code.

[Qwaz]

For the context, this causes a buffer overflow by violating the safety requirement of TrustedRandomAccess trait.

rust/library/core/src/iter/adapters/zip.rs

Lines 384 to 406 in 0148b97

	/// An iterator whose items are random-accessible efficiently
	///
	/// # Safety
	///
	/// The iterator's `size_hint` must be exact and cheap to call.
	///
	/// `size` may not be overridden.
	///
	/// `<Self as Iterator>::__iterator_get_unchecked` must be safe to call
	/// provided the following conditions are met.
	///
	/// 1. `0 <= idx` and `idx < self.size()`.
	/// 2. If `self: !Clone`, then `get_unchecked` is never called with the same
	/// index on `self` more than once.
	/// 3. After `self.get_unchecked(idx)` has been called then `next_back` will
	/// only be called at most `self.size() - idx - 1` times.
	/// 4. After `get_unchecked` is called, then only the following methods will be
	/// called on `self`:
	/// * `std::clone::Clone::clone()`
	/// * `std::iter::Iterator::size_hint()`
	/// * `std::iter::Iterator::next_back()`
	/// * `std::iter::Iterator::__iterator_get_unchecked()`
	/// * `std::iter::TrustedRandomAccess::size()`

[hameerabbasi]

Assigning P-critical as part of the WG-prioritization discussion on Zulip.