Re: [PATCH v7 02/15] kasan: arm64: x86: Make special tags arch specific

[Will Deacon <will@kernel.org>]

On Wed, Dec 10, 2025 at 05:28:43PM +0000, Maciej Wieczor-Retman wrote:
> From: Samuel Holland <samuel.holland@sifive.com>
> 
> KASAN's tag-based mode defines multiple special tag values. They're
> reserved for:
> - Native kernel value. On arm64 it's 0xFF and it causes an early return
>   in the tag checking function.
> - Invalid value. 0xFE marks an area as freed / unallocated. It's also
>   the value that is used to initialize regions of shadow memory.
> - Min and max values. 0xFD is the highest value that can be randomly
>   generated for a new tag. 0 is the minimal value with the exception of
>   arm64's hardware mode where it is equal to 0xF0.
> 
> Metadata macro is also defined:
> - Tag width equal to 8.
> 
> Tag-based mode on x86 is going to use 4 bit wide tags so all the above
> values need to be changed accordingly.
> 
> Make tag width and native kernel tag arch specific for x86 and arm64.
> 
> Base the invalid tag value and the max value on the native kernel tag
> since they follow the same pattern on both mentioned architectures.
> 
> Also generalize KASAN_SHADOW_INIT and 0xff used in various
> page_kasan_tag* helpers.
> 
> Give KASAN_TAG_MIN the default value of zero, and move the special value
> for hw_tags arm64 to its arch specific kasan-tags.h.
> 
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
> Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> ---
> Changelog v7:
> - Reorder defines of arm64 tag width to prevent redefinition warnings.
> - Remove KASAN_TAG_MASK so it's only defined in mmzone.h (Andrey
>   Konovalov)
> - Merge the 'support tag widths less than 8 bits' with this patch since
>   they do similar things and overwrite each other. (Alexander)
> 
> Changelog v6:
> - Add hardware tags KASAN_TAG_WIDTH value to the arm64 arch file.
> - Keep KASAN_TAG_MASK in the mmzone.h.
> - Remove ifndef from KASAN_SHADOW_INIT.
> 
> Changelog v5:
> - Move KASAN_TAG_MIN to the arm64 kasan-tags.h for the hardware KASAN
>   mode case.
> 
> Changelog v4:
> - Move KASAN_TAG_MASK to kasan-tags.h.
> 
> Changelog v2:
> - Remove risc-v from the patch.
> 
>  MAINTAINERS                         |  2 +-
>  arch/arm64/include/asm/kasan-tags.h | 14 ++++++++++++++
>  arch/arm64/include/asm/kasan.h      |  2 --
>  arch/arm64/include/asm/uaccess.h    |  1 +
>  arch/x86/include/asm/kasan-tags.h   |  9 +++++++++
>  include/linux/kasan-tags.h          | 19 ++++++++++++++-----
>  include/linux/kasan.h               |  3 +--
>  include/linux/mm.h                  |  6 +++---
>  include/linux/page-flags-layout.h   |  9 +--------
>  9 files changed, 44 insertions(+), 21 deletions(-)
>  create mode 100644 arch/arm64/include/asm/kasan-tags.h
>  create mode 100644 arch/x86/include/asm/kasan-tags.h
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 7bf6385efe04..a591598cc4b5 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -13420,7 +13420,7 @@ L:	kasan-dev@googlegroups.com
>  S:	Maintained
>  B:	https://bugzilla.kernel.org/buglist.cgi?component=Sanitizers&product=Memory%20Management
>  F:	Documentation/dev-tools/kasan.rst
> -F:	arch/*/include/asm/*kasan.h
> +F:	arch/*/include/asm/*kasan*.h
>  F:	arch/*/mm/kasan_init*
>  F:	include/linux/kasan*.h
>  F:	lib/Kconfig.kasan
> diff --git a/arch/arm64/include/asm/kasan-tags.h b/arch/arm64/include/asm/kasan-tags.h
> new file mode 100644
> index 000000000000..259952677443
> --- /dev/null
> +++ b/arch/arm64/include/asm/kasan-tags.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_KASAN_TAGS_H
> +#define __ASM_KASAN_TAGS_H
> +
> +#define KASAN_TAG_KERNEL	0xFF /* native kernel pointers tag */
> +
> +#ifdef CONFIG_KASAN_HW_TAGS
> +#define KASAN_TAG_MIN		0xF0 /* minimum value for random tags */
> +#define KASAN_TAG_WIDTH		4
> +#else
> +#define KASAN_TAG_WIDTH		8
> +#endif

Shouldn't this be 0 when KASAN is not in use at all?

Will