Re: [PATCH v3] mm: use compare-exchange operation to set KASAN page tag

[Andrey Konovalov <andreyknvl@gmail.com>]

On Thu, Jan 20, 2022 at 3:02 AM Peter Collingbourne <pcc@google.com> wrote:
>
> It has been reported that the tag setting operation on newly-allocated
> pages can cause the page flags to be corrupted when performed
> concurrently with other flag updates as a result of the use of
> non-atomic operations. Fix the problem by using a compare-exchange
> loop to update the tag.
>
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/I456b24a2b9067d93968d43b4bb3351c0cec63101
> Fixes: 2813b9c02962 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
> Cc: stable@vger.kernel.org
> ---
> v3:
> - use try_cmpxchg() as suggested by Peter Zijlstra on another
>   patch
>
> v2:
> - use READ_ONCE()
>
>  include/linux/mm.h | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c768a7c81b0b..87473fe52c3f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1531,11 +1531,18 @@ static inline u8 page_kasan_tag(const struct page *page)
>
>  static inline void page_kasan_tag_set(struct page *page, u8 tag)
>  {
> -       if (kasan_enabled()) {
> -               tag ^= 0xff;
> -               page->flags &= ~(KASAN_TAG_MASK << KASAN_TAG_PGSHIFT);
> -               page->flags |= (tag & KASAN_TAG_MASK) << KASAN_TAG_PGSHIFT;
> -       }
> +       unsigned long old_flags, flags;
> +
> +       if (!kasan_enabled())
> +               return;
> +
> +       tag ^= 0xff;
> +       old_flags = READ_ONCE(page->flags);
> +       do {
> +               flags = old_flags;
> +               flags &= ~(KASAN_TAG_MASK << KASAN_TAG_PGSHIFT);
> +               flags |= (tag & KASAN_TAG_MASK) << KASAN_TAG_PGSHIFT;
> +       } while (unlikely(!try_cmpxchg(&page->flags, &old_flags, flags)));
>  }
>
>  static inline void page_kasan_tag_reset(struct page *page)
> --
> 2.34.1.703.g22d0c6ccf7-goog
>

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

FWIW, try_cmpxchg() doesn't seem to be doing annotated atomic accesses
when accessing old_flags, so using READ_ONCE() in page_kasan_tag_set()
seems pointless after all.