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Message-ID: <mjuxddfbpdvjpjetctqbs5kg4iz4ngsnwl5ovvv63k2gc3f2ir@n67fqqlnyuz6>
Date: Mon, 8 Sep 2025 12:12:35 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Andrey Konovalov <andreyknvl@...il.com>
CC: <sohil.mehta@...el.com>, <baohua@...nel.org>, <david@...hat.com>,
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Subject: Re: [PATCH v5 05/19] kasan: arm64: x86: Make special tags arch
specific
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@...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?
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
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