Re: [PATCH v6 2/2] kasan: Add memzero int for unaligned size at DEBUG

[Marco Elver <elver@google.com>]

On Mon, 5 Jul 2021 at 12:33, <yee.lee@mediatek.com> wrote:
> From: Yee Lee <yee.lee@mediatek.com>
>
> Issue: when SLUB debug is on, hwtag kasan_unpoison() would overwrite
> the redzone of object with unaligned size.
>
> An additional memzero_explicit() path is added to replacing init by
> hwtag instruction for those unaligned size at SLUB debug mode.
>
> The penalty is acceptable since they are only enabled in debug mode,
> not production builds. A block of comment is added for explanation.
>
> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Suggested-by: Marco Elver <elver@google.com>
> Suggested-by: Andrey Konovalov <andreyknvl@gmail.com>
> Signed-off-by: Yee Lee <yee.lee@mediatek.com>

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

Thank you!

> ---
>  mm/kasan/kasan.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 98e3059bfea4..d739cdd1621a 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -9,6 +9,7 @@
>  #ifdef CONFIG_KASAN_HW_TAGS
>
>  #include <linux/static_key.h>
> +#include "../slab.h"
>
>  DECLARE_STATIC_KEY_FALSE(kasan_flag_stacktrace);
>  extern bool kasan_flag_async __ro_after_init;
> @@ -387,6 +388,17 @@ static inline void kasan_unpoison(const void *addr, size_t size, bool init)
>
>         if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
>                 return;
> +       /*
> +        * Explicitly initialize the memory with the precise object size to
> +        * avoid overwriting the SLAB redzone. This disables initialization in
> +        * the arch code and may thus lead to performance penalty. The penalty
> +        * is accepted since SLAB redzones aren't enabled in production builds.
> +        */
> +       if (__slub_debug_enabled() &&
> +           init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
> +               init = false;
> +               memzero_explicit((void *)addr, size);
> +       }
>         size = round_up(size, KASAN_GRANULE_SIZE);
>
>         hw_set_mem_tag_range((void *)addr, size, tag, init);
> --
> 2.18.0