Re: [PATCH v2] rust_binder: use a u64 stride when cleaning up the offsets array

[Alice Ryhl <aliceryhl@google.com>]

On Sun, May 31, 2026 at 10:29:24PM +0900, Hyunwoo Kim wrote:
> Allocation's Drop walks the offsets array (binder_size_t = u64 entries),
> cleaning up the objects, but it used usize instead of u64 for both the
> stride and the per-entry read.
> 
> On 64-bit kernels (usize == u64) this is harmless, but on 32-bit kernels
> it walks the 8-byte entries in 4-byte steps, iterating an N-entry array
> 2N times, and reads the always-zero high word as offset 0, cleaning up
> the object at offset 0 N extra times. As a result the referenced node or
> handle ends up with a lower reference count than it actually has (a
> refcount over-decrement), and binder's reference accounting is corrupted;
> for example, the owner can be notified of a strong reference release
> (BR_RELEASE) even though references still remain.
> 
> Change the stride to u64, and read each entry as a u64, narrowing it to
> usize with try_into().
> 
> On 32-bit ARM, when this over-decrement would drive a count below zero,
> the driver's existing refcount guard refuses it and fires:
> 
>   rust_binder: Failure: refcount underflow!
> 
> Cc: stable@vger.kernel.org
> Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>

Thanks!

Reviewed-by: Alice Ryhl <aliceryhl@google.com>