Re: [PATCH v3 1/2] kasan: catch invalid free before SLUB reinitializes the object

[Andrey Konovalov <andreyknvl@gmail.com>]

On Thu, Jul 25, 2024 at 5:32 PM Jann Horn <jannh@google.com> wrote:
>
> Currently, when KASAN is combined with init-on-free behavior, the
> initialization happens before KASAN's "invalid free" checks.
>
> More importantly, a subsequent commit will want to use the object metadata
> region to store an rcu_head, and we should let KASAN check that the object
> pointer is valid before that. (Otherwise that change will make the existing
> testcase kmem_cache_invalid_free fail.)

This is not the case since v3, right? Do we still need this patch?

If it's still needed, see the comment below.

Thank you!

> So add a new KASAN hook that allows KASAN to pre-validate a
> kmem_cache_free() operation before SLUB actually starts modifying the
> object or its metadata.
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz> #slub
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>  include/linux/kasan.h | 16 ++++++++++++++++
>  mm/kasan/common.c     | 51 +++++++++++++++++++++++++++++++++++++++------------
>  mm/slub.c             |  7 +++++++
>  3 files changed, 62 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 70d6a8f6e25d..ebd93c843e78 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -175,6 +175,16 @@ static __always_inline void * __must_check kasan_init_slab_obj(
>         return (void *)object;
>  }
>
> +bool __kasan_slab_pre_free(struct kmem_cache *s, void *object,
> +                       unsigned long ip);
> +static __always_inline bool kasan_slab_pre_free(struct kmem_cache *s,
> +                                               void *object)
> +{
> +       if (kasan_enabled())
> +               return __kasan_slab_pre_free(s, object, _RET_IP_);
> +       return false;
> +}

Please add a documentation comment for this new hook; something like
what we have for kasan_mempool_poison_pages() and some of the others.
(I've been meaning to add them for all of them, but still didn't get
around to that.)

> +
>  bool __kasan_slab_free(struct kmem_cache *s, void *object,
>                         unsigned long ip, bool init);
>  static __always_inline bool kasan_slab_free(struct kmem_cache *s,
> @@ -371,6 +381,12 @@ static inline void *kasan_init_slab_obj(struct kmem_cache *cache,
>  {
>         return (void *)object;
>  }
> +
> +static inline bool kasan_slab_pre_free(struct kmem_cache *s, void *object)
> +{
> +       return false;
> +}
> +
>  static inline bool kasan_slab_free(struct kmem_cache *s, void *object, bool init)
>  {
>         return false;
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 85e7c6b4575c..7c7fc6ce7eb7 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -208,31 +208,52 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache,
>         return (void *)object;
>  }
>
> -static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
> -                                     unsigned long ip, bool init)
> +enum free_validation_result {
> +       KASAN_FREE_IS_IGNORED,
> +       KASAN_FREE_IS_VALID,
> +       KASAN_FREE_IS_INVALID
> +};
> +
> +static enum free_validation_result check_slab_free(struct kmem_cache *cache,
> +                                               void *object, unsigned long ip)
>  {
> -       void *tagged_object;
> +       void *tagged_object = object;
>
> -       if (!kasan_arch_is_ready())
> -               return false;
> +       if (is_kfence_address(object) || !kasan_arch_is_ready())
> +               return KASAN_FREE_IS_IGNORED;
>
> -       tagged_object = object;
>         object = kasan_reset_tag(object);
>
>         if (unlikely(nearest_obj(cache, virt_to_slab(object), object) != object)) {
>                 kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_INVALID_FREE);
> -               return true;
> +               return KASAN_FREE_IS_INVALID;
>         }
>
> -       /* RCU slabs could be legally used after free within the RCU period. */
> -       if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
> -               return false;
> -
>         if (!kasan_byte_accessible(tagged_object)) {
>                 kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_DOUBLE_FREE);
> -               return true;
> +               return KASAN_FREE_IS_INVALID;
>         }
>
> +       return KASAN_FREE_IS_VALID;
> +}
> +
> +static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
> +                                     unsigned long ip, bool init)
> +{
> +       void *tagged_object = object;
> +       enum free_validation_result valid = check_slab_free(cache, object, ip);

I believe we don't need check_slab_free() here, as it was already done
in kasan_slab_pre_free()? Checking just kasan_arch_is_ready() and
is_kfence_address() should save a bit on performance impact.

Though if we remove check_slab_free() from here, we do need to add it
to __kasan_mempool_poison_object().

> +
> +       if (valid == KASAN_FREE_IS_IGNORED)
> +               return false;
> +       if (valid == KASAN_FREE_IS_INVALID)
> +               return true;
> +
> +       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))
> +               return false;

I vaguely recall there was some reason why this check was done before
the kasan_byte_accessible() check, but I might be wrong. Could you try
booting the kernel with only this patch applied to see if anything
breaks?




> +
>         kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
>                         KASAN_SLAB_FREE, init);
>
> @@ -242,6 +263,12 @@ static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
>         return false;
>  }
>
> +bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object,
> +                               unsigned long ip)
> +{
> +       return check_slab_free(cache, object, ip) == KASAN_FREE_IS_INVALID;
> +}
> +
>  bool __kasan_slab_free(struct kmem_cache *cache, void *object,
>                                 unsigned long ip, bool init)
>  {
> diff --git a/mm/slub.c b/mm/slub.c
> index 4927edec6a8c..34724704c52d 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2170,6 +2170,13 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
>         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;
> +
>         /*
>          * As memory initialization might be integrated into KASAN,
>          * kasan_slab_free and initialization memset's must be
>
> --
> 2.45.2.1089.g2a221341d9-goog
>