Re: [PATCH v13 2/3] kasan: allow architectures to provide an outline readiness check

[Marco Elver]

On Wed, 16 Jun 2021 at 10:02, 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>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

but also check if an assertion that this is only used with
KASAN_GENERIC might make sense (below). Depends on how much we want to
make sure kasan_arch_is_ready() could be useful for other modes (which
I don't think it makes sense).

> --
>
> 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
> ---
>  mm/kasan/common.c  | 4 ++++
>  mm/kasan/generic.c | 3 +++
>  mm/kasan/kasan.h   | 4 ++++
>  mm/kasan/shadow.c  | 8 ++++++++
>  4 files changed, 19 insertions(+)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 10177cc26d06..0ad615f3801d 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -331,6 +331,10 @@ static inline bool ____kasan_slab_free(struct kmem_cache *cache, void *object,
>         u8 tag;
>         void *tagged_object;
>
> +       /* Bail if the arch isn't ready */
> +       if (!kasan_arch_is_ready())
> +               return false;
> +
>         tag = get_tag(object);
>         tagged_object = object;
>         object = kasan_reset_tag(object);
> 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/kasan.h b/mm/kasan/kasan.h
> index 8f450bc28045..19323a3d5975 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -449,6 +449,10 @@ static inline void kasan_poison_last_granule(const void *address, size_t size) {
>
>  #endif /* CONFIG_KASAN_GENERIC */
>
> +#ifndef kasan_arch_is_ready
> +static inline bool kasan_arch_is_ready(void)   { return true; }
> +#endif
> +

I've been trying to think of a way to make it clear this is only for
KASAN_GENERIC mode, and not the others. An arch can always define this
function, but of course it might not be used. One way would be to add
an '#ifndef CONFIG_KASAN_GENERIC' in the #else case and #error if it's
not generic mode.

I think trying to make this do anything useful for SW_TAGS or HW_TAGS
modes does not make sense (at least right now).

>  /*
>   * Exported functions for interfaces called from assembly or from generated
>   * code. Declarations here to avoid warning about missing declarations.
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index 082ee5b6d9a1..3c7f7efe6f68 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -73,6 +73,10 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
>  {
>         void *shadow_start, *shadow_end;
>
> +       /* Don't touch the shadow memory if arch isn't ready */
> +       if (!kasan_arch_is_ready())
> +               return;
> +
>         /*
>          * Perform shadow offset calculation based on untagged address, as
>          * some of the callers (e.g. kasan_poison_object_data) pass tagged
> @@ -99,6 +103,10 @@ EXPORT_SYMBOL(kasan_poison);
>  #ifdef CONFIG_KASAN_GENERIC
>  void kasan_poison_last_granule(const void *addr, size_t size)
>  {
> +       /* Don't touch the shadow memory if arch isn't ready */
> +       if (!kasan_arch_is_ready())
> +               return;
> +
>         if (size & KASAN_GRANULE_MASK) {
>                 u8 *shadow = (u8 *)kasan_mem_to_shadow(addr + size);
>                 *shadow = size & KASAN_GRANULE_MASK;
> --
> 2.30.2
>
> --
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