Re: [PATCH v5 2/2] slub: Introduce CONFIG_SLUB_RCU_DEBUG

[Andrey Konovalov <andreyknvl@gmail.com>]

On Fri, Aug 2, 2024 at 1:23 PM Jann Horn <jannh@google.com> wrote:
>
> On Fri, Aug 2, 2024 at 11:09 AM Jann Horn <jannh@google.com> wrote:
> > I guess I could also change the API to pass something different - like
> > a flag meaning "the object is guaranteed to no longer be in use".
> > There is already code in slab_free_hook() that computes this
> > expression, so we could easily pass that to KASAN and then avoid doing
> > the same logic in KASAN again... I think that would be the most
> > elegant approach?
>
> Regarding this, I think I'll add something like this on top of this patch in v6:
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index b63f5351c5f3..50bad011352e 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -201,16 +201,17 @@ bool __kasan_slab_free(struct kmem_cache *s,
> void *object, bool init,
>  /**
>   * kasan_slab_free - Possibly handle slab object freeing.
>   * @object: Object to free.
> + * @still_accessible: Whether the object contents are still accessible.
>   *
>   * This hook is called from the slab allocator to give KASAN a chance to take
>   * ownership of the object and handle its freeing.
>   * kasan_slab_pre_free() must have already been called on the same object.
>   *
>   * @Return true if KASAN took ownership of the object; false otherwise.
>   */
>  static __always_inline bool kasan_slab_free(struct kmem_cache *s,
>                                                 void *object, bool init,
> -                                               bool after_rcu_delay)
> +                                               bool still_accessible)
>  {
>         if (kasan_enabled())
>                 return __kasan_slab_free(s, object, init, after_rcu_delay);
> @@ -410,7 +411,7 @@ static inline bool kasan_slab_pre_free(struct
> kmem_cache *s, void *object)
>  }
>
>  static inline bool kasan_slab_free(struct kmem_cache *s, void *object,
> -                                  bool init, bool after_rcu_delay)
> +                                  bool init, bool still_accessible)
>  {
>         return false;
>  }
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 71a20818b122..ed4873e18c75 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -230,14 +230,14 @@ static bool check_slab_allocation(struct
> kmem_cache *cache, void *object,
>  }
>
>  static inline void poison_slab_object(struct kmem_cache *cache, void *object,
> -                                     bool init, bool after_rcu_delay)
> +                                     bool init, bool still_accessible)
>  {
>         void *tagged_object = object;
>
>         object = kasan_reset_tag(object);
>
>         /* RCU slabs could be legally used after free within the RCU period. */
> -       if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU) && !after_rcu_delay)
> +       if (unlikely(still_accessible))
>                 return;
>
>         kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
> @@ -256,12 +256,12 @@ bool __kasan_slab_pre_free(struct kmem_cache
> *cache, void *object,
>  }
>
>  bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
> -                      bool after_rcu_delay)
> +                      bool still_accessible)
>  {
>         if (!kasan_arch_is_ready() || is_kfence_address(object))
>                 return false;
>
> -       poison_slab_object(cache, object, init, after_rcu_delay);
> +       poison_slab_object(cache, object, init, still_accessible);
>
>         /*
>          * If the object is put into quarantine, do not let slab put the object
> diff --git a/mm/slub.c b/mm/slub.c
> index 49571d5ded75..a89f2006d46e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2221,31 +2221,34 @@ static __always_inline
>  bool slab_free_hook(struct kmem_cache *s, void *x, bool init,
>                     bool after_rcu_delay)
>  {
> +       /* Are the object contents still accessible? */
> +       bool still_accessible = (s->flags & SLAB_TYPESAFE_BY_RCU) &&
> !after_rcu_delay;
> +
>         kmemleak_free_recursive(x, s->flags);
>         kmsan_slab_free(s, x);
>
>         debug_check_no_locks_freed(x, s->object_size);
>
>         if (!(s->flags & SLAB_DEBUG_OBJECTS))
>                 debug_check_no_obj_freed(x, s->object_size);
>
>         /* Use KCSAN to help debug racy use-after-free. */
> -       if (!(s->flags & SLAB_TYPESAFE_BY_RCU) || after_rcu_delay)
> +       if (!still_accessible)
>                 __kcsan_check_access(x, s->object_size,
>                                      KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
>
>         if (kfence_free(x))
>                 return false;
>
>         /*
>          * Give KASAN a chance to notice an invalid free operation before we
>          * modify the object.
>          */
>         if (kasan_slab_pre_free(s, x))
>                 return false;
>
>  #ifdef CONFIG_SLUB_RCU_DEBUG
> -       if ((s->flags & SLAB_TYPESAFE_BY_RCU) && !after_rcu_delay) {
> +       if (still_accessible) {
>                 struct rcu_delayed_free *delayed_free;
>
>                 delayed_free = kmalloc(sizeof(*delayed_free), GFP_NOWAIT);
> @@ -2289,7 +2292,7 @@ bool slab_free_hook(struct kmem_cache *s, void
> *x, bool init,
>                        s->size - inuse - rsize);
>         }
>         /* KASAN might put x into memory quarantine, delaying its reuse. */
> -       return !kasan_slab_free(s, x, init, after_rcu_delay);
> +       return !kasan_slab_free(s, x, init, still_accessible);
>  }
>
>  static __fastpath_inline

Ok, let's do it like this.

Thank you!