[vm] Fail closed on aliased RefCell borrows in Move values

[sausagee]

Description

Clean-room rewrite (not a cherry-pick or copy of aptos-labs sources). Intent only: aliased paired RefCell::borrow_mut must not abort the interpreter thread (aptos-labs#20439).

Container::swap_contents and VectorRef::move_range took two exclusive borrow_muts. Move's borrow checker promises the cells are distinct; if that invariant is broken, stacked borrow_mut panicked the VM thread.

This change acquires the pair through a new helper, exclusive_cell_pair:

1. Reject Rc::ptr_eq immediately.
2. Otherwise try_borrow_mut each cell.
3. On either failure, return UNKNOWN_INVARIANT_VIOLATION_ERROR.

Verified distinct cells still swap / move as before. No other crate in the monorepo is touched.

Invariant: a broken aliasing (or already-borrowed) pair on these paths surfaces as a catchable VM status, never a Rust panic.

Follows SECURITY.md / RUST_SECURE_CODING.md: Result instead of panic, documented safety invariant, no unsafe.

How Has This Been Tested?

• cargo test -p move-vm-types — 70 passed, 0 failed
• container_self_swap_returns_status_instead_of_panicking — specialized vector, generic struct, generic vector; self-swap is UNKNOWN_INVARIANT_VIOLATION_ERROR and leaves contents unchanged
• move_range_on_aliased_vector_refs_returns_status_instead_of_panicking — distinct vectors still transfer; aliased refs return the same status
• Existing test_mem_swap covers the happy path for distinct container refs

Key Areas to Review

• exclusive_cell_pair in third_party/move/move-vm/types/src/values/values_impl.rs — fail-closed helper used by both hardened paths
• Confirm this is an independent rewrite (helper + ptr_eq first, not a port of the upstream patch)

Type of Change

• Bug fix

Which Components or Systems Does This Change Impact?

• Move/Aptos Virtual Machine

Checklist

• I have read and followed the CONTRIBUTING doc
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I identified and added all stakeholders and component owners affected by this change as reviewers
• I tested both happy and unhappy path of the functionality
• I have made corresponding changes to the documentation

  

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