Aliasing violations in splice, mutable iteration and cursors

Overview

Four code paths create a pointer whose permission is later revoked, then write through it. Miri reports each as undefined behaviour under both the Tree Borrows and Stacked Borrows aliasing models (CWE-758). All four are reachable from safe code with standard Ord types.

1. Splice back-pointer. split_off, split_off_index and append stored the first spliced node's prev pointer from a reference to the head, then wrote through that same reference, invalidating the stored pointer. A later pop_first or pop_front writes through it. This affects all four collections.
2. SkipMap::range_mut back pointer. With an Included or Excluded upper bound, the back pointer came from a shared reference. next_back hands out &mut V through it, and writing to that value is undefined behaviour.
3. IterMut::next_back reads a yielded node. In SkipList and SkipMap, next_back read the predecessor node even after the front end had yielded it as &mut. The read freezes the caller's reference, and the caller's next write through it is undefined behaviour. This affects iter_mut, values_mut and range_mut used from both ends.
4. Cursor list pointer. lower_bound_mut and upper_bound_mut on OrderedSkipList and SkipSet (cursor feature) took the cursor's list pointer, then searched through &self. A comparator with interior-mutable state, such as a Cell call counter, wrote to the list's memory through that shared borrow and invalidated the cursor's pointer. The comparator can be a fully consistent order. SkipMap cursors are not affected.

No miscompilation has been observed in an optimised build. A future compiler may still optimise on the violated aliasing assumptions.

The flaws were corrected in 1.3.0. Each fix derives the pointer used for mutation from the collection's long-lived allocation pointers, and each has a Miri regression test.

Affected packages

• skiplist >= 0.0.0-0, < 1.3.0

Remediation

Upgrade to a patched release:

• skiplist 1.3.0