On Fri, Sep 15, 2023 at 10:59:20AM +0000, Matteo Rizzo wrote:
> slab_free_freelist_hook tries to read a freelist pointer from the
> current object even when freeing a single object. This is invalid
> because single objects don't actually contain a freelist pointer when
> they're freed and the memory contains other data. This causes problems
> for checking the integrity of freelist in get_freepointer.
>
> Signed-off-by: Matteo Rizzo <matteori...@google.com>
> ---
> mm/slub.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index f7940048138c..a7dae207c2d2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1820,7 +1820,9 @@ static inline bool slab_free_freelist_hook(struct
> kmem_cache *s,
>
> do {
> object = next;
> - next = get_freepointer(s, object);
> + /* Single objects don't actually contain a freepointer */
> + if (object != old_tail)
> + next = get_freepointer(s, object);
>
> /* If object's reuse doesn't have to be delayed */
> if (!slab_free_hook(s, object, slab_want_init_on_free(s))) {
I find this loop's logic a bit weird, but, yes, this ends up being
correct and avoids needless work.
Reviewed-by: Kees Cook <keesc...@chromium.org>
--
Kees Cook
