Re: [PATCH v6 4/9] mm, kfence: insert KFENCE hooks for SLAB

[Jann Horn <jannh@google.com>]

On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@google.com> wrote:
> Inserts KFENCE hooks into the SLAB allocator.
[...]
> diff --git a/mm/slab.c b/mm/slab.c
[...]
> @@ -3416,6 +3427,11 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
>  static __always_inline void __cache_free(struct kmem_cache *cachep, void *objp,
>                                          unsigned long caller)
>  {
> +       if (kfence_free(objp)) {
> +               kmemleak_free_recursive(objp, cachep->flags);
> +               return;
> +       }

This looks dodgy. Normally kmemleak is told that an object is being
freed *before* the object is actually released. I think that if this
races really badly, we'll make kmemleak stumble over this bit in
create_object():

kmemleak_stop("Cannot insert 0x%lx into the object search tree
(overlaps existing)\n",
      ptr);


> +
>         /* Put the object into the quarantine, don't touch it for now. */
>         if (kasan_slab_free(cachep, objp, _RET_IP_))
>                 return;