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Message-ID: <CA+fCnZecdx5QsYcwn6ZyNoOkQRqmonUmSmfiihYTF8Ws_0O9KA@mail.gmail.com>
Date: Sat, 6 Sep 2025 19:18:33 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Cc: sohil.mehta@...el.com, baohua@...nel.org, david@...hat.com,
kbingham@...nel.org, weixugc@...gle.com, Liam.Howlett@...cle.com,
alexandre.chartre@...cle.com, kas@...nel.org, mark.rutland@....com,
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Subject: Re: [PATCH v5 05/19] kasan: arm64: x86: Make special tags arch specific
On Mon, Aug 25, 2025 at 10:27 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@...el.com> wrote:
>
> 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.
> - Max value. 0xFD is the highest value that can be randomly generated
> for a new tag.
>
> 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 native kernel tag arch specific for x86 and arm64.
>
> Replace hardcoded kernel tag value and tag width with macros in KASAN's
> non-arch specific code.
>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
> ---
> 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 | 13 +++++++++++++
> arch/arm64/include/asm/kasan.h | 4 ----
> arch/x86/include/asm/kasan-tags.h | 9 +++++++++
> include/linux/kasan-tags.h | 10 +++++++++-
> include/linux/kasan.h | 4 +++-
> include/linux/mm.h | 6 +++---
> include/linux/mmzone.h | 1 -
> include/linux/page-flags-layout.h | 9 +--------
> 9 files changed, 39 insertions(+), 19 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 fed6cd812d79..788532771832 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -13176,7 +13176,7 @@ L: kasan-dev@...glegroups.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..152465d03508
> --- /dev/null
> +++ b/arch/arm64/include/asm/kasan-tags.h
> @@ -0,0 +1,13 @@
> +/* 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 */
> +
> +#define KASAN_TAG_WIDTH 8
> +
> +#ifdef CONFIG_KASAN_HW_TAGS
> +#define KASAN_TAG_MIN 0xF0 /* minimum value for random tags */
> +#endif
> +
> +#endif /* ASM_KASAN_TAGS_H */
> diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
> index 4ab419df8b93..d2841e0fb908 100644
> --- a/arch/arm64/include/asm/kasan.h
> +++ b/arch/arm64/include/asm/kasan.h
> @@ -7,10 +7,6 @@
> #include <linux/linkage.h>
> #include <asm/memory.h>
>
> -#ifdef CONFIG_KASAN_HW_TAGS
> -#define KASAN_TAG_MIN 0xF0 /* minimum value for random tags */
> -#endif
> -
> #define arch_kasan_set_tag(addr, tag) __tag_set(addr, tag)
> #define arch_kasan_reset_tag(addr) __tag_reset(addr)
> #define arch_kasan_get_tag(addr) __tag_get(addr)
> diff --git a/arch/x86/include/asm/kasan-tags.h b/arch/x86/include/asm/kasan-tags.h
> new file mode 100644
> index 000000000000..68ba385bc75c
> --- /dev/null
> +++ b/arch/x86/include/asm/kasan-tags.h
> @@ -0,0 +1,9 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_KASAN_TAGS_H
> +#define __ASM_KASAN_TAGS_H
> +
> +#define KASAN_TAG_KERNEL 0xF /* native kernel pointers tag */
> +
> +#define KASAN_TAG_WIDTH 4
> +
> +#endif /* ASM_KASAN_TAGS_H */
> diff --git a/include/linux/kasan-tags.h b/include/linux/kasan-tags.h
> index e07c896f95d3..fe80fa8f3315 100644
> --- a/include/linux/kasan-tags.h
> +++ b/include/linux/kasan-tags.h
> @@ -2,7 +2,15 @@
> #ifndef _LINUX_KASAN_TAGS_H
> #define _LINUX_KASAN_TAGS_H
>
> -#include <asm/kasan.h>
> +#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
> +#include <asm/kasan-tags.h>
> +#endif
> +
> +#ifndef KASAN_TAG_WIDTH
> +#define KASAN_TAG_WIDTH 0
> +#endif
> +
> +#define KASAN_TAG_MASK ((1UL << KASAN_TAG_WIDTH) - 1)
>
> #ifndef KASAN_TAG_KERNEL
> #define KASAN_TAG_KERNEL 0xFF /* native kernel pointers tag */
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index b396feca714f..54481f8c30c5 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -40,7 +40,9 @@ typedef unsigned int __bitwise kasan_vmalloc_flags_t;
>
> #ifdef CONFIG_KASAN_SW_TAGS
> /* This matches KASAN_TAG_INVALID. */
> -#define KASAN_SHADOW_INIT 0xFE
> +#ifndef KASAN_SHADOW_INIT
Do we need this ifndef?
> +#define KASAN_SHADOW_INIT KASAN_TAG_INVALID
> +#endif
> #else
> #define KASAN_SHADOW_INIT 0
> #endif
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1ae97a0b8ec7..bb494cb1d5af 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1692,7 +1692,7 @@ static inline u8 page_kasan_tag(const struct page *page)
>
> if (kasan_enabled()) {
> tag = (page->flags >> KASAN_TAG_PGSHIFT) & KASAN_TAG_MASK;
> - tag ^= 0xff;
> + tag ^= KASAN_TAG_KERNEL;
> }
>
> return tag;
> @@ -1705,7 +1705,7 @@ static inline void page_kasan_tag_set(struct page *page, u8 tag)
> if (!kasan_enabled())
> return;
>
> - tag ^= 0xff;
> + tag ^= KASAN_TAG_KERNEL;
> old_flags = READ_ONCE(page->flags);
> do {
> flags = old_flags;
> @@ -1724,7 +1724,7 @@ static inline void page_kasan_tag_reset(struct page *page)
>
> static inline u8 page_kasan_tag(const struct page *page)
> {
> - return 0xff;
> + return KASAN_TAG_KERNEL;
> }
>
> static inline void page_kasan_tag_set(struct page *page, u8 tag) { }
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 0c5da9141983..c139fb3d862d 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1166,7 +1166,6 @@ static inline bool zone_is_empty(struct zone *zone)
> #define NODES_MASK ((1UL << NODES_WIDTH) - 1)
> #define SECTIONS_MASK ((1UL << SECTIONS_WIDTH) - 1)
> #define LAST_CPUPID_MASK ((1UL << LAST_CPUPID_SHIFT) - 1)
> -#define KASAN_TAG_MASK ((1UL << KASAN_TAG_WIDTH) - 1)
So we cannot define this here because of include dependencies? Having
this value defined here would look cleaner.
Otherwise, let's add a comment here with a reference to where this
value is defined.
> #define ZONEID_MASK ((1UL << ZONEID_SHIFT) - 1)
>
> static inline enum zone_type page_zonenum(const struct page *page)
> diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
> index 760006b1c480..b2cc4cb870e0 100644
> --- a/include/linux/page-flags-layout.h
> +++ b/include/linux/page-flags-layout.h
> @@ -3,6 +3,7 @@
> #define PAGE_FLAGS_LAYOUT_H
>
> #include <linux/numa.h>
> +#include <linux/kasan-tags.h>
> #include <generated/bounds.h>
>
> /*
> @@ -72,14 +73,6 @@
> #define NODE_NOT_IN_PAGE_FLAGS 1
> #endif
>
> -#if defined(CONFIG_KASAN_SW_TAGS)
> -#define KASAN_TAG_WIDTH 8
> -#elif defined(CONFIG_KASAN_HW_TAGS)
> -#define KASAN_TAG_WIDTH 4
This case is removed here but not added to arch/arm64/include/asm/kasan-tags.h.
> -#else
> -#define KASAN_TAG_WIDTH 0
> -#endif
> -
> #ifdef CONFIG_NUMA_BALANCING
> #define LAST__PID_SHIFT 8
> #define LAST__PID_MASK ((1 << LAST__PID_SHIFT)-1)
> --
> 2.50.1
>
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