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Message-ID: <97b033941d8e146a71827cf31d4d0a12303bcc41.1765386422.git.m.wieczorretman@pm.me>
Date: Wed, 10 Dec 2025 17:30:23 +0000
From: Maciej Wieczor-Retman <m.wieczorretman@...me>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>, Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: m.wieczorretman@...me, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, kasan-dev@...glegroups.com
Subject: [PATCH v7 15/15] x86/kasan: Make software tag-based kasan available
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Make CONFIG_KASAN_SW_TAGS available for x86 machines if they have
ADDRESS_MASKING enabled (LAM) as that works similarly to Top-Byte Ignore
(TBI) that allows the software tag-based mode on arm64 platform.
The value for sw_tags KASAN_SHADOW_OFFSET was calculated by rearranging
the formulas for KASAN_SHADOW_START and KASAN_SHADOW_END from
arch/x86/include/asm/kasan.h - the only prerequisites being
KASAN_SHADOW_SCALE_SHIFT of 4, and KASAN_SHADOW_END equal to the
one from KASAN generic mode.
Set scale macro based on KASAN mode: in software tag-based mode 16 bytes
of memory map to one shadow byte and 8 in generic mode.
Disable CONFIG_KASAN_INLINE and CONFIG_KASAN_STACK when
CONFIG_KASAN_SW_TAGS is enabled on x86 until the appropriate compiler
support is available.
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
---
Changelog v7:
- Add a paragraph to the patch message explaining how the various
addresses and the KASAN_SHADOW_OFFSET were calculated.
Changelog v6:
- Don't enable KASAN if LAM is not supported.
- Move kasan_init_tags() to kasan_init_64.c to not clutter the setup.c
file.
- Move the #ifdef for the KASAN scale shift here.
- Move the gdb code to patch "Use arithmetic shift for shadow
computation".
- Return "depends on KASAN" line to Kconfig.
- Add the defer kasan config option so KASAN can be disabled on hardware
that doesn't have LAM.
Changelog v4:
- Add x86 specific kasan_mem_to_shadow().
- Revert x86 to the older unsigned KASAN_SHADOW_OFFSET. Do the same to
KASAN_SHADOW_START/END.
- Modify scripts/gdb/linux/kasan.py to keep x86 using unsigned offset.
- Disable inline and stack support when software tags are enabled on
x86.
Changelog v3:
- Remove runtime_const from previous patch and merge the rest here.
- Move scale shift definition back to header file.
- Add new kasan offset for software tag based mode.
- Fix patch message typo 32 -> 16, and 16 -> 8.
- Update lib/Kconfig.kasan with x86 now having software tag-based
support.
Changelog v2:
- Remove KASAN dense code.
Documentation/arch/x86/x86_64/mm.rst | 6 ++++--
arch/x86/Kconfig | 4 ++++
arch/x86/boot/compressed/misc.h | 1 +
arch/x86/include/asm/kasan.h | 4 ++++
arch/x86/mm/kasan_init_64.c | 6 ++++++
lib/Kconfig.kasan | 3 ++-
6 files changed, 21 insertions(+), 3 deletions(-)
diff --git a/Documentation/arch/x86/x86_64/mm.rst b/Documentation/arch/x86/x86_64/mm.rst
index a6cf05d51bd8..ccbdbb4cda36 100644
--- a/Documentation/arch/x86/x86_64/mm.rst
+++ b/Documentation/arch/x86/x86_64/mm.rst
@@ -60,7 +60,8 @@ Complete virtual memory map with 4-level page tables
ffffe90000000000 | -23 TB | ffffe9ffffffffff | 1 TB | ... unused hole
ffffea0000000000 | -22 TB | ffffeaffffffffff | 1 TB | virtual memory map (vmemmap_base)
ffffeb0000000000 | -21 TB | ffffebffffffffff | 1 TB | ... unused hole
- ffffec0000000000 | -20 TB | fffffbffffffffff | 16 TB | KASAN shadow memory
+ ffffec0000000000 | -20 TB | fffffbffffffffff | 16 TB | KASAN shadow memory (generic mode)
+ fffff40000000000 | -8 TB | fffffbffffffffff | 8 TB | KASAN shadow memory (software tag-based mode)
__________________|____________|__________________|_________|____________________________________________________________
|
| Identical layout to the 56-bit one from here on:
@@ -130,7 +131,8 @@ Complete virtual memory map with 5-level page tables
ffd2000000000000 | -11.5 PB | ffd3ffffffffffff | 0.5 PB | ... unused hole
ffd4000000000000 | -11 PB | ffd5ffffffffffff | 0.5 PB | virtual memory map (vmemmap_base)
ffd6000000000000 | -10.5 PB | ffdeffffffffffff | 2.25 PB | ... unused hole
- ffdf000000000000 | -8.25 PB | fffffbffffffffff | ~8 PB | KASAN shadow memory
+ ffdf000000000000 | -8.25 PB | fffffbffffffffff | ~8 PB | KASAN shadow memory (generic mode)
+ ffeffc0000000000 | -6 PB | fffffbffffffffff | 4 PB | KASAN shadow memory (software tag-based mode)
__________________|____________|__________________|_________|____________________________________________________________
|
| Identical layout to the 47-bit one from here on:
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index a26dc3bad804..b5275e322061 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -67,6 +67,7 @@ config X86
select ARCH_CLOCKSOURCE_INIT
select ARCH_CONFIGURES_CPU_MITIGATIONS
select ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
+ select ARCH_DISABLE_KASAN_INLINE if X86_64 && KASAN_SW_TAGS
select ARCH_ENABLE_HUGEPAGE_MIGRATION if X86_64 && HUGETLB_PAGE && MIGRATION
select ARCH_ENABLE_MEMORY_HOTPLUG if X86_64
select ARCH_ENABLE_MEMORY_HOTREMOVE if MEMORY_HOTPLUG
@@ -196,6 +197,8 @@ config X86
select HAVE_ARCH_JUMP_LABEL_RELATIVE
select HAVE_ARCH_KASAN if X86_64
select HAVE_ARCH_KASAN_VMALLOC if X86_64
+ select HAVE_ARCH_KASAN_SW_TAGS if ADDRESS_MASKING
+ select ARCH_NEEDS_DEFER_KASAN if ADDRESS_MASKING
select HAVE_ARCH_KFENCE
select HAVE_ARCH_KMSAN if X86_64
select HAVE_ARCH_KGDB
@@ -408,6 +411,7 @@ config AUDIT_ARCH
config KASAN_SHADOW_OFFSET
hex
depends on KASAN
+ default 0xeffffc0000000000 if KASAN_SW_TAGS
default 0xdffffc0000000000
config HAVE_INTEL_TXT
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index db1048621ea2..ded92b439ada 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -13,6 +13,7 @@
#undef CONFIG_PARAVIRT_SPINLOCKS
#undef CONFIG_KASAN
#undef CONFIG_KASAN_GENERIC
+#undef CONFIG_KASAN_SW_TAGS
#define __NO_FORTIFY
diff --git a/arch/x86/include/asm/kasan.h b/arch/x86/include/asm/kasan.h
index 395e133d551d..3fa63036c93c 100644
--- a/arch/x86/include/asm/kasan.h
+++ b/arch/x86/include/asm/kasan.h
@@ -7,6 +7,7 @@
#include <linux/types.h>
#define KASAN_SHADOW_OFFSET _AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
+#ifdef CONFIG_KASAN_SW_TAGS
/*
* LLVM ABI for reporting tag mismatches in inline KASAN mode.
* On x86 the UD1 instruction is used to carry metadata in the ECX register
@@ -24,7 +25,10 @@
#define KASAN_ECX_WRITE 0x10
#define KASAN_ECX_SIZE_MASK 0x0f
#define KASAN_ECX_SIZE(ecx) (1 << ((ecx) & KASAN_ECX_SIZE_MASK))
+#define KASAN_SHADOW_SCALE_SHIFT 4
+#else
#define KASAN_SHADOW_SCALE_SHIFT 3
+#endif
/*
* Compiler uses shadow offset assuming that addresses start
diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index 7f5c11328ec1..3a5577341805 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -465,4 +465,10 @@ void __init kasan_init(void)
init_task.kasan_depth = 0;
kasan_init_generic();
+ pr_info("KernelAddressSanitizer initialized\n");
+
+ if (boot_cpu_has(X86_FEATURE_LAM))
+ kasan_init_sw_tags();
+ else
+ pr_info("KernelAddressSanitizer not initialized (sw-tags): hardware doesn't support LAM\n");
}
diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index a4bb610a7a6f..d13ea8da7bfd 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -112,7 +112,8 @@ config KASAN_SW_TAGS
Requires GCC 11+ or Clang.
- Supported only on arm64 CPUs and relies on Top Byte Ignore.
+ Supported on arm64 CPUs that support Top Byte Ignore and on x86 CPUs
+ that support Linear Address Masking.
Consumes about 1/16th of available memory at kernel start and
add an overhead of ~20% for dynamic allocations.
--
2.52.0
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