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

[Marco Elver <elver@google.com>]

On Fri, 1 Oct 2021 at 04:42, Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
> If an object is allocated on a tail page of a multi-page slab, kasan
> will get the wrong tag because page->s_mem is NULL for tail pages.
> I'm not quite sure what the user-visible effect of this might be.
>
> Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Acked-by: Marco Elver <elver@google.com>

Indeed this looks wrong. I don't know how much this code is even
tested, because it depends on CONFIG_KASAN_SW_TAGS && CONFIG_SLAB, and
the cache having a constructor or SLAB_TYPESAFE_BY_RCU. HW_TAGS isn't
affected because it doesn't work with SLAB.

And to run SW_TAGS, one needs an arm64 CPU with TBI. And the instances
of KASAN_SW_TAGS I'm aware of use SLUB.

With eventual availability of Intel LAM, I expect KASAN_SW_TAGS to
become more widely used though, including its SLAB support.

> ---
>  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