Re: [PATCH v15 2/4] kasan: allow architectures to provide an outline readiness check

From: Andrey Konovalov
Date: Sun Jun 20 2021 - 07:16:43 EST


On Thu, Jun 17, 2021 at 12:30 PM 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@xxxxxxxxxxxxx>
> Suggested-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
>
> --
>
> 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
>
> Build tested on arm64 with SW_TAGS and x86 with INLINE: the error fires
> if I add a kasan_arch_is_ready define.
> ---
> mm/kasan/common.c | 4 ++++
> mm/kasan/generic.c | 3 +++
> mm/kasan/kasan.h | 6 ++++++
> mm/kasan/shadow.c | 8 ++++++++
> 4 files changed, 21 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 */

This comment brings no value. The fact that we bail is clear from the
following line. The comment should explain why we bail.

> + if (!kasan_arch_is_ready())
> + return false;

Have you considered including these checks into the high-level
wrappers in include/linux/kasan.h? Would that work?


> +
> 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..4dbc8def64f4 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -449,6 +449,12 @@ 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; }
> +#elif !defined(CONFIG_KASAN_GENERIC) || !defined(CONFIG_KASAN_OUTLINE)
> +#error kasan_arch_is_ready only works in KASAN generic outline mode!
> +#endif
> +
> /*
> * 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
>