Re: [PATCH v7 08/12] mm: slub: add kernel address sanitizer support for slub allocator

[Andrey Ryabinin]

On 11/25/2014 03:17 PM, Dmitry Chernenkov wrote:
> FYI, when I backported Kasan to 3.14, in kasan_mark_slab_padding()
> sometimes a negative size of padding was generated.

I don't see how this could happen if pointers passed to kasan_mark_slab_padding() are correct.

Negative padding would mean that (object + s->size) is crossing slab page boundary.
This is either slub allocator bug (very unlikely), or some pointers passed to kasan_mark_slab_padding()
not correct.

Or maybe I'm missing something?

> This started
> working when the patch below was applied:
> 
> @@ -262,12 +264,11 @@ void kasan_free_pages(struct page *page,
> unsigned int order)
>  void kasan_mark_slab_padding(struct kmem_cache *s, void *object,
>   struct page *page)
>  {
> - unsigned long object_end = (unsigned long)object + s->size;
> - unsigned long padding_start = round_up(object_end,
> - KASAN_SHADOW_SCALE_SIZE);
> - unsigned long padding_end = (unsigned long)page_address(page) +
> - (PAGE_SIZE << compound_order(page));
> - size_t size = padding_end - padding_start;
> + unsigned long page_start = (unsigned long) page_address(page);
> + unsigned long page_end = page_start + (PAGE_SIZE << compound_order(page));
> + unsigned long padding_start = round_up(page_end - s->reserved,
> + KASAN_SHADOW_SCALE_SIZE);
> + size_t size = page_end - padding_start;
> 
>   kasan_poison_shadow((void *)padding_start, size, KASAN_SLAB_PADDING);
>  }
> 
> Also, in kasan_slab_free you poison the shadow with FREE not just the
> object space, but also redzones. This is inefficient and will mistake
> right out-of-bounds error for the next object with use-after-free.
> This is fixed here
> https://github.com/google/kasan/commit/4b3238be392ba0bc56bbc934ac545df3ff840782
> , please patch.
> 

Makes sense.


> 
> LGTM
> 



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