Re: [PATCH] kasan: Fix tag for large allocations when using CONFIG_SLAB

[Andrey Konovalov]

On Fri, Oct 1, 2021 at 4:42 AM Matthew Wilcox (Oracle)
<willy@xxxxxxxxxxxxx> wrote:
>
> If an object is allocated on a tail page of a multi-page slab, kasan
> will get the wrong tagbecause page->s_mem is NULL for tail pages.

Interesting. Is this a known property of tail pages? Why does this
happen? I failed to find this exception in the code.

The tag value won't really be "wrong", just unexpected. But if s_mem
is indeed NULL for tail pages, your fix makes sense.

> I'm not quite sure what the user-visible effect of this might be.

Everything should work, as long as tag values are assigned
consistently based on the object address.

>
> Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  mm/kasan/common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 2baf121fb8c5..41779ad109cd 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -298,7 +298,7 @@ static inline u8 assign_tag(struct kmem_cache *cache,
>         /* For caches that either have a constructor or SLAB_TYPESAFE_BY_RCU: */
>  #ifdef CONFIG_SLAB
>         /* For SLAB assign tags based on the object index in the freelist. */
> -       return (u8)obj_to_index(cache, virt_to_page(object), (void *)object);
> +       return (u8)obj_to_index(cache, virt_to_head_page(object), (void *)object);
>  #else
>         /*
>          * For SLUB assign a random tag during slab creation, otherwise reuse
> --
> 2.32.0
>