Re: [PATCH 2/3] kasan: record and print the free track

[Dmitry Vyukov]

On Wed, May 6, 2020 at 7:22 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
>
> We add new KASAN_RCU_STACK_RECORD configuration option. It will move
> free track from slub meta-data (struct kasan_alloc_meta) into freed object.
> Because we hope this options doesn't enlarge slub meta-data size.
>
> This option doesn't enlarge struct kasan_alloc_meta size.
> - add two call_rcu() call stack into kasan_alloc_meta, size is 8 bytes.
> - remove free track from kasan_alloc_meta, size is 8 bytes.
>
> This option is only suitable for generic KASAN, because we move free track
> into the freed object, so free track is valid information only when it
> exists in quarantine. If the object is in-use state, then the KASAN report
> doesn't print call_rcu() free track information.
>
> [1]https://bugzilla.kernel.org/show_bug.cgi?id=198437
>
> Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
>  mm/kasan/common.c | 10 +++++++++-
>  mm/kasan/report.c | 24 +++++++++++++++++++++---
>  2 files changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 32d422bdf127..13ec03e225a7 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -321,8 +321,15 @@ void kasan_record_callrcu(void *addr)
>                 /* record last call_rcu() call stack */
>                 alloc_info->rcu_free_stack[1] = save_stack(GFP_NOWAIT);
>  }
> -#endif
>
> +static void kasan_set_free_info(struct kmem_cache *cache,
> +               void *object, u8 tag)
> +{
> +       /* store free track into freed object */
> +       set_track((struct kasan_track *)(object + BYTES_PER_WORD), GFP_NOWAIT);
> +}
> +
> +#else
>  static void kasan_set_free_info(struct kmem_cache *cache,
>                 void *object, u8 tag)
>  {
> @@ -339,6 +346,7 @@ static void kasan_set_free_info(struct kmem_cache *cache,
>
>         set_track(&alloc_meta->free_track[idx], GFP_NOWAIT);
>  }
> +#endif
>
>  void kasan_poison_slab(struct page *page)
>  {
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 7aaccc70b65b..f2b0c6b9dffa 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -175,8 +175,23 @@ static void kasan_print_rcu_free_stack(struct kasan_alloc_meta *alloc_info)
>         print_track(&free_track, "Last call_rcu() call stack", true);
>         pr_err("\n");
>  }
> -#endif
>
> +static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> +               void *object, u8 tag, const void *addr)
> +{
> +       u8 *shadow_addr = (u8 *)kasan_mem_to_shadow(addr);
> +
> +       /*
> +        * Only the freed object can get free track,
> +        * because free track information is stored to freed object.
> +        */
> +       if (*shadow_addr == KASAN_KMALLOC_FREE)
> +               return (struct kasan_track *)(object + BYTES_PER_WORD);

Humm... the other patch defines BYTES_PER_WORD as 4... I would assume
seeing 8 (or sizeof(long)) here. Why 4?
Have you tested all 4 modes (RCU/no-RCU x SLAB/SLUB)? As far as I
remember one of the allocators stored something in the object.

Also, does this work with objects with ctors and slabs destroyed by
rcu? kasan_track may smash other things in these cases.
Have you looked at the KASAN implementation when free_track was
removed? That may have useful details :)


> +       else
> +               return NULL;
> +}
> +
> +#else
>  static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
>                 void *object, u8 tag, const void *addr)
>  {
> @@ -196,6 +211,7 @@ static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
>
>         return &alloc_meta->free_track[i];
>  }
> +#endif
>
>  static void describe_object(struct kmem_cache *cache, void *object,
>                                 const void *addr, u8 tag)
> @@ -208,8 +224,10 @@ static void describe_object(struct kmem_cache *cache, void *object,
>                 print_track(&alloc_info->alloc_track, "Allocated", false);
>                 pr_err("\n");
>                 free_track = kasan_get_free_track(cache, object, tag, addr);
> -               print_track(free_track, "Freed", false);
> -               pr_err("\n");
> +               if (free_track) {
> +                       print_track(free_track, "Freed", false);
> +                       pr_err("\n");
> +               }
>  #ifdef CONFIG_KASAN_RCU_STACK_RECORD
>                 kasan_print_rcu_free_stack(alloc_info);
>  #endif
> --
> 2.18.0
>
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