[PATCH v10 13/13] x86/kasan: Make software tag-based kasan available

Maciej Wieczor-Retman
2026-02-04T19:20:49Z

From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.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.

Lock software tag KASAN behind CC_IS_CLANG due to lack of proper support
by gcc resulting in kernel booting issues.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
---
Changelog v10:
- Update Documentation/dev-tools/kasan.rst with x86 related
informations.

Changelog v9:
- Lock HAVE_ARCH_KASAN_HAS_SW_TAGS behind CC_IS_CLANG due to lack of
support from gcc.
- Remove pr_info() from KASAN initialization since it's now done by the
generic init helper.
- Add paragraph to the mm.rst to explain the mutual exclusive nature of
the KASAN address ranges.
- Use cpu_feature_enabled() instead of boot_cpu_has() in
kasan_init_64.c.

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 | 10 ++++++++--
Documentation/dev-tools/kasan.rst | 28 ++++++++++++++++------------
arch/x86/Kconfig | 4 ++++
arch/x86/boot/compressed/misc.h | 1 +
arch/x86/include/asm/kasan.h | 5 +++++
arch/x86/mm/kasan_init_64.c | 5 +++++
lib/Kconfig.kasan | 3 ++-
7 files changed, 41 insertions(+), 15 deletions(-)

diff --git a/Documentation/arch/x86/x86_64/mm.rst b/Documentation/arch/x86/x86_64/mm.rst
index a6cf05d51bd8..7e2e4c5fa661 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:
@@ -176,5 +178,9 @@ Be very careful vs. KASLR when changing anything here. The KASLR address
range must not overlap with anything except the KASAN shadow area, which is
correct as KASAN disables KASLR.

+The 'KASAN shadow memory (generic mode)/(software tag-based mode)' ranges are
+mutually exclusive and depend on which KASAN setting is chosen:
+CONFIG_KASAN_GENERIC or CONFIG_KASAN_SW_TAGS.
+
For both 4- and 5-level layouts, the KSTACK_ERASE_POISON value in the last 2MB
hole: ffffffffffff4111
diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 64dbf8b308bd..03b508ebe673 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -22,8 +22,8 @@ architectures, but it has significant performance and memory overheads.

Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS,
can be used for both debugging and dogfood testing, similar to userspace HWASan.
-This mode is only supported for arm64, but its moderate memory overhead allows
-using it for testing on memory-restricted devices with real workloads.
+This mode is only supported for arm64 and x86, but its moderate memory overhead
+allows using it for testing on memory-restricted devices with real workloads.

Hardware Tag-Based KASAN or HW_TAGS KASAN, enabled with CONFIG_KASAN_HW_TAGS,
is the mode intended to be used as an in-field memory bug detector or as a
@@ -351,10 +351,12 @@ Software Tag-Based KASAN
Software Tag-Based KASAN uses a software memory tagging approach to checking
access validity. It is currently only implemented for the arm64 architecture.

-Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
-to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
-to store memory tags associated with each 16-byte memory cell (therefore, it
-dedicates 1/16th of the kernel memory for shadow memory).
+Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs to
+store a pointer tag in the top byte of kernel pointers. Analogously to TBI on
+x86 CPUs Linear Address Masking (LAM) feature is used and the pointer tag is
+stored in four bits of the kernel pointer's top byte. Software Tag-Based mode
+uses shadow memory to store memory tags associated with each 16-byte memory cell
+(therefore, it dedicates 1/16th of the kernel memory for shadow memory).

On each memory allocation, Software Tag-Based KASAN generates a random tag, tags
the allocated memory with this tag, and embeds the same tag into the returned
@@ -370,12 +372,14 @@ Software Tag-Based KASAN also has two instrumentation modes (outline, which
emits callbacks to check memory accesses; and inline, which performs the shadow
memory checks inline). With outline instrumentation mode, a bug report is
printed from the function that performs the access check. With inline
-instrumentation, a ``brk`` instruction is emitted by the compiler, and a
-dedicated ``brk`` handler is used to print bug reports.
-
-Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
-pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
-reserved to tag freed memory regions.
+instrumentation, arm64's implementation uses the ``brk`` instruction emitted by
+the compiler, and a dedicated ``brk`` handler is used to print bug reports. On
+x86 inline mode doesn't work yet due to missing compiler support.
+
+For arm64 Software Tag-Based KASAN uses 0xFF as a match-all pointer tag
+(accesses through pointers with the 0xFF pointer tag are not checked). The value
+0xFE is currently reserved to tag freed memory regions. On x86 the same tags
+take on 0xF and 0xE respectively.

Hardware Tag-Based KASAN
~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 80527299f859..877668cd5deb 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 && CC_IS_CLANG
+ select ARCH_NEEDS_DEFER_KASAN if ADDRESS_MASKING
select HAVE_ARCH_KFENCE
select HAVE_ARCH_KMSAN if X86_64
select HAVE_ARCH_KGDB
@@ -410,6 +413,7 @@ 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 fd855e32c9b9..ba70036c2abd 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 90c18e30848f..53ab7de16517 100644
--- a/arch/x86/include/asm/kasan.h
+++ b/arch/x86/include/asm/kasan.h
@@ -6,7 +6,12 @@
#include <linux/kasan-tags.h>
#include <linux/types.h>
#define KASAN_SHADOW_OFFSET _AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
+
+#ifdef CONFIG_KASAN_SW_TAGS
+#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..8cbb8ec32061 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -465,4 +465,9 @@ void __init kasan_init(void)

init_task.kasan_depth = 0;
kasan_init_generic();
+
+ if (cpu_feature_enabled(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.
--
2.53.0