[PATCH v2 1/2] kasan: initialize shadow to TAG_INVALID for SW_TAGS

[Andrey Konovalov]

On Fri, Feb 19, 2021 at 1:22 AM Andrey Konovalov <andreyknvl at google.com> wrote:
>
> Currently, KASAN_SW_TAGS uses 0xFF as the default tag value for
> unallocated memory. The underlying idea is that since that memory
> hasn't been allocated yet, it's only supposed to be dereferenced
> through a pointer with the native 0xFF tag.
>
> While this is a good idea in terms on consistency, practically it
> doesn't bring any benefit. Since the 0xFF pointer tag is a match-all
> tag, it doesn't matter what tag the accessed memory has. No accesses
> through 0xFF-tagged pointers are considered buggy by KASAN.
>
> This patch changes the default tag value for unallocated memory to 0xFE,
> which is the tag KASAN uses for inaccessible memory. This doesn't affect
> accesses through 0xFF-tagged pointer to this memory, but this allows
> KASAN to detect wild and large out-of-bounds invalid memory accesses
> through otherwise-tagged pointers.
>
> This is a prepatory patch for the next one, which changes the tag-based
> KASAN modes to not poison the boot memory.
>
> Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
> ---
>  include/linux/kasan.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 14f72ec96492..44c147dae7e3 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -30,7 +30,8 @@ struct kunit_kasan_expectation {
>  /* Software KASAN implementations use shadow memory. */
>
>  #ifdef CONFIG_KASAN_SW_TAGS
> -#define KASAN_SHADOW_INIT 0xFF
> +/* This matches KASAN_TAG_INVALID. */
> +#define KASAN_SHADOW_INIT 0xFE
>  #else
>  #define KASAN_SHADOW_INIT 0
>  #endif
> --
> 2.30.0.617.g56c4b15f3c-goog
>

Hi Andrew,

Could you pick up this series into mm?

The discussion on v1 of this series was hijacked discussing an unrelated issue.

Thanks!