Re: [PATCH v8 13/14] x86/kasan: Logical bit shift for kasan_mem_to_shadow

[Andrey Konovalov <andreyknvl@gmail.com>]

On Thu, Jan 15, 2026 at 5:43 PM Maciej Wieczor-Retman
<m.wieczorretman@pm.me> wrote:
>
> x86 was easy to do because the kasan_mem_to_shadow() was already in the
> asm/kasan.h. arm64 took a bit more changes since I had to write the
> arch_kasan_non_canonical_hook in a separate file that would import the
> linux/kasan.h header in order to use kasan_mem_to_shadow(). Anyway below are the
> relevant bits from the patch - does that look okay? Or would you prefer some
> different names/placements?

One comment below, otherwise looks fine to me, thanks!

>
> diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
> index b167e9d3da91..16b1f2ca3ea8 100644
> --- a/arch/arm64/include/asm/kasan.h
> +++ b/arch/arm64/include/asm/kasan.h
> @@ -17,6 +17,8 @@
>
>  asmlinkage void kasan_early_init(void);
>  void kasan_init(void);
> +bool __arch_kasan_non_canonical_hook(unsigned long addr);
> +#define arch_kasan_non_canonical_hook(addr) __arch_kasan_non_canonical_hook(addr)
>
>  #else
>  static inline void kasan_init(void) { }
>
> diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
> index c26489cf96cd..a122ea67eced 100644
> --- a/arch/arm64/mm/Makefile
> +++ b/arch/arm64/mm/Makefile
> @@ -15,4 +15,6 @@ obj-$(CONFIG_ARM64_GCS)               += gcs.o
>  KASAN_SANITIZE_physaddr.o      += n
>
>  obj-$(CONFIG_KASAN)            += kasan_init.o
> +obj-$(CONFIG_KASAN)            += kasan.o
>  KASAN_SANITIZE_kasan_init.o    := n
> +KASAN_SANITIZE_kasan.o         := n
> diff --git a/arch/arm64/mm/kasan.c b/arch/arm64/mm/kasan.c
> new file mode 100644
> index 000000000000..b94d5fb480ca
> --- /dev/null
> +++ b/arch/arm64/mm/kasan.c
> @@ -0,0 +1,31 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * This file contains ARM64 specific KASAN code.
> + */
> +
> +#include <linux/kasan.h>
> +
> +bool __arch_kasan_non_canonical_hook(unsigned long addr) {
> +       /*
> +        * For Software Tag-Based KASAN, kasan_mem_to_shadow() uses the
> +        * arithmetic shift. Normally, this would make checking for a possible
> +        * shadow address complicated, as the shadow address computation
> +        * operation would overflow only for some memory addresses. However, due
> +        * to the chosen KASAN_SHADOW_OFFSET values and the fact the
> +        * kasan_mem_to_shadow() only operates on pointers with the tag reset,
> +        * the overflow always happens.
> +        *
> +        * For arm64, the top byte of the pointer gets reset to 0xFF. Thus, the
> +        * possible shadow addresses belong to a region that is the result of
> +        * kasan_mem_to_shadow() applied to the memory range
> +        * [0xFF000000000000, 0xFFFFFFFFFFFFFFFF]. Despite the overflow, the
> +        * resulting possible shadow region is contiguous, as the overflow
> +        * happens for both 0xFF000000000000 and 0xFFFFFFFFFFFFFFFF.
> +        */
> +       if (IS_ENABLED(CONFIG_KASAN_SW_TAGS)) {
> +               if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0xFFULL << 56)) ||
> +                   addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
> +                       return true;
> +       }
> +       return false;
> +}
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 9c6ac4b62eb9..146eecae4e9c 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> ...
> @@ -403,6 +409,13 @@ static __always_inline bool kasan_check_byte(const void *addr)
>         return true;
>  }
>
> +#ifndef arch_kasan_non_canonical_hook
> +static inline bool arch_kasan_non_canonical_hook(unsigned long addr)
> +{
> +       return false;
> +}
> +#endif

Let's put this next to kasan_non_canonical_hook declaration.

> +
>  #else /* CONFIG_KASAN */
>
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 62c01b4527eb..1c4893729ff6 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -642,10 +642,19 @@ void kasan_non_canonical_hook(unsigned long addr)
>         const char *bug_type;
>
>         /*
> -        * All addresses that came as a result of the memory-to-shadow mapping
> -        * (even for bogus pointers) must be >= KASAN_SHADOW_OFFSET.
> +        * For Generic KASAN, kasan_mem_to_shadow() uses the logical right shift
> +        * and never overflows with the chosen KASAN_SHADOW_OFFSET values. Thus,
> +        * the possible shadow addresses (even for bogus pointers) belong to a
> +        * single contiguous region that is the result of kasan_mem_to_shadow()
> +        * applied to the whole address space.
>          */
> -       if (addr < KASAN_SHADOW_OFFSET)
> +       if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
> +               if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0ULL)) ||
> +                   addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
> +                       return;
> +       }
> +
> +       if(arch_kasan_non_canonical_hook(addr))
>                 return;
>
> --
> Kind regards
> Maciej Wieczór-Retman
>