Re: [PATCH v2 01/10] mm/kasan: implement kasan_poison_range

[Andrey Konovalov <andreyknvl@gmail.com>]

On Fri, Sep 19, 2025 at 4:58 PM Ethan Graham <ethan.w.s.graham@gmail.com> wrote:
>
> From: Ethan Graham <ethangraham@google.com>
>
> Introduce a new helper function, kasan_poison_range(), to encapsulate
> the logic for poisoning an arbitrary memory range of a given size, and
> expose it publically in <include/linux/kasan.h>.
>
> This is a preparatory change for the upcoming KFuzzTest patches, which
> requires the ability to poison the inter-region padding in its input
> buffers.
>
> No functional change to any other subsystem is intended by this commit.
>
> Signed-off-by: Ethan Graham <ethangraham@google.com>
> Reviewed-by: Alexander Potapenko <glider@google.com>
>
> ---
> PR v1:
> - Enforce KASAN_GRANULE_SIZE alignment for the end of the range in
>   kasan_poison_range(), and return -EINVAL when this isn't respected.
> ---
> ---
>  include/linux/kasan.h | 11 +++++++++++
>  mm/kasan/shadow.c     | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 890011071f2b..cd6cdf732378 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -102,6 +102,16 @@ static inline bool kasan_has_integrated_init(void)
>  }
>
>  #ifdef CONFIG_KASAN
> +
> +/**
> + * kasan_poison_range - poison the memory range [@addr, @addr + @size)
> + *
> + * The exact behavior is subject to alignment with KASAN_GRANULE_SIZE, defined
> + * in <mm/kasan/kasan.h>: if @start is unaligned, the initial partial granule
> + * at the beginning of the range is only poisoned if CONFIG_KASAN_GENERIC=y.
> + */
> +int kasan_poison_range(const void *addr, size_t size);
> +
>  void __kasan_unpoison_range(const void *addr, size_t size);
>  static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
>  {
> @@ -402,6 +412,7 @@ static __always_inline bool kasan_check_byte(const void *addr)
>
>  #else /* CONFIG_KASAN */
>
> +static inline int kasan_poison_range(const void *start, size_t size) { return 0; }
>  static inline void kasan_unpoison_range(const void *address, size_t size) {}
>  static inline void kasan_poison_pages(struct page *page, unsigned int order,
>                                       bool init) {}
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index d2c70cd2afb1..7faed02264f2 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -147,6 +147,40 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
>  }
>  EXPORT_SYMBOL_GPL(kasan_poison);
>
> +int kasan_poison_range(const void *addr, size_t size)

This should go into common.c, otherwise this won't be built with the
HW_TAGS mode enabled.

Also, you need a wrapper with a kasan_enabled() check; see how
kasan_unpoison_range() is defined.

> +{
> +       uintptr_t start_addr = (uintptr_t)addr;
> +       uintptr_t head_granule_start;
> +       uintptr_t poison_body_start;
> +       uintptr_t poison_body_end;
> +       size_t head_prefix_size;
> +       uintptr_t end_addr;
> +
> +       if ((start_addr + size) % KASAN_GRANULE_SIZE)
> +               return -EINVAL;

Other similar KASAN functions do a WARN_ON(bad alignment). I think
printing a warning is fair for this to force the caller to enforce
proper alignment.

> +
> +       end_addr = ALIGN_DOWN(start_addr + size, KASAN_GRANULE_SIZE);

I don't think we need to ALIGN_DOWN(): we already checked that
(start_addr + size) % KASAN_GRANULE_SIZE == 0.

> +       if (start_addr >= end_addr)
> +               return -EINVAL;

Can also do a WARN_ON().

> +
> +       head_granule_start = ALIGN_DOWN(start_addr, KASAN_GRANULE_SIZE);
> +       head_prefix_size = start_addr - head_granule_start;
> +
> +       if (IS_ENABLED(CONFIG_KASAN_GENERIC) && head_prefix_size > 0)
> +               kasan_poison_last_granule((void *)head_granule_start,
> +                                         head_prefix_size);

Let's rename kasan_poison_last_granule() to kasan_poison_granule()
then. Here the granule being poisoned is not the last one.


> +
> +       poison_body_start = ALIGN(start_addr, KASAN_GRANULE_SIZE);
> +       poison_body_end = ALIGN_DOWN(end_addr, KASAN_GRANULE_SIZE);

end_addr is already aligned.


> +
> +       if (poison_body_start < poison_body_end)
> +               kasan_poison((void *)poison_body_start,
> +                            poison_body_end - poison_body_start,
> +                            KASAN_SLAB_REDZONE, false);
> +       return 0;
> +}
> +EXPORT_SYMBOL(kasan_poison_range);
> +
>  #ifdef CONFIG_KASAN_GENERIC
>  void kasan_poison_last_granule(const void *addr, size_t size)
>  {
> --
> 2.51.0.470.ga7dc726c21-goog
>