Re: [PATCH v11 2/6] kasan: allow architectures to provide an outline readiness check

[Marco Elver]

On Fri, 19 Mar 2021 at 15:41, Daniel Axtens <dja@xxxxxxxxxx> wrote:
> Allow architectures to define a kasan_arch_is_ready() hook that bails
> out of any function that's about to touch the shadow unless the arch
> says that it is ready for the memory to be accessed. This is fairly
> uninvasive and should have a negligible performance penalty.
>
> This will only work in outline mode, so an arch must specify
> ARCH_DISABLE_KASAN_INLINE if it requires this.
>
> Cc: Balbir Singh <bsingharora@xxxxxxxxx>
> Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Suggested-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
>
> --
>
> I discuss the justfication for this later in the series. Also,
> both previous RFCs for ppc64 - by 2 different people - have
> needed this trick! See:
>  - https://lore.kernel.org/patchwork/patch/592820/ # ppc64 hash series
>  - https://patchwork.ozlabs.org/patch/795211/      # ppc radix series
> ---
>  include/linux/kasan.h | 4 ++++
>  mm/kasan/common.c     | 4 ++++
>  mm/kasan/generic.c    | 3 +++
>  mm/kasan/shadow.c     | 4 ++++
>  4 files changed, 15 insertions(+)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 8b3b99d659b7..6bd8343f0033 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h

Does kasan_arch_is_ready() need to be defined in the public interface
of KASAN? Could it instead be moved to mm/kasan/kasan.h?

> @@ -23,6 +23,10 @@ struct kunit_kasan_expectation {
>
>  #endif
>
> +#ifndef kasan_arch_is_ready
> +static inline bool kasan_arch_is_ready(void)   { return true; }
> +#endif
> +
>  #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
>
>  #include <linux/pgtable.h>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 6bb87f2acd4e..f23a9e2dce9f 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -345,6 +345,10 @@ static inline bool ____kasan_slab_free(struct kmem_cache *cache, void *object,
>         if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
>                 return false;
>
> +       /* We can't read the shadow byte if the arch isn't ready */
> +       if (!kasan_arch_is_ready())
> +               return false;
> +

While it probably doesn't matter much, it seems this check could be
moved up, rather than having it in the middle here.


>         if (!kasan_byte_accessible(tagged_object)) {
>                 kasan_report_invalid_free(tagged_object, ip);
>                 return true;
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index 53cbf28859b5..c3f5ba7a294a 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -163,6 +163,9 @@ static __always_inline bool check_region_inline(unsigned long addr,
>                                                 size_t size, bool write,
>                                                 unsigned long ret_ip)
>  {
> +       if (!kasan_arch_is_ready())
> +               return true;
> +
>         if (unlikely(size == 0))
>                 return true;
>
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index 727ad4629173..1f650c521037 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -80,6 +80,10 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
>          */
>         addr = kasan_reset_tag(addr);
>
> +       /* Don't touch the shadow memory if arch isn't ready */
> +       if (!kasan_arch_is_ready())
> +               return;
> +
>         /* Skip KFENCE memory if called explicitly outside of sl*b. */
>         if (is_kfence_address(addr))
>                 return;
> --
> 2.27.0
>
> --
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