Re: [PATCH v5 05/19] kasan: arm64: x86: Make special tags arch specific

[Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>]

On 2025-09-06 at 19:18:33 +0200, Andrey Konovalov wrote:
>On Mon, Aug 25, 2025 at 10:27 PM Maciej Wieczor-Retman
><maciej.wieczor-retman@intel.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@intel.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@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..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?

I just checked and you're right, it's not needed. I think it might have been a
leftover of my dense mode code.

>
>> +#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.

I'll retest with a couple of configs but I removed this change and everything
compile fine. Thanks for noticing that

>
>>  #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.

Right, I'll correct that.

>
>
>> -#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
>>

-- 
Kind regards
Maciej Wieczór-Retman