[PATCH v12 01/15] kasan: sw_tags: Use arithmetic shift for shadow computation

[PATCH v12 01/15] kasan: sw_tags: Use arithmetic shift for shadow computation

[Maciej Wieczor-Retman]

From: Samuel Holland <samuel.holland@sifive.com>

Currently, kasan_mem_to_shadow() uses a logical right shift, which turns
canonical kernel addresses into non-canonical addresses by clearing the
high KASAN_SHADOW_SCALE_SHIFT bits. The value of KASAN_SHADOW_OFFSET is
then chosen so that the addition results in a canonical address for the
shadow memory.

For KASAN_GENERIC, this shift/add combination is ABI with the compiler,
because KASAN_SHADOW_OFFSET is used in compiler-generated inline tag
checks[1], which must only attempt to dereference canonical addresses.

However, for KASAN_SW_TAGS there is some freedom to change the algorithm
without breaking the ABI. Because TBI is enabled for kernel addresses,
the top bits of shadow memory addresses computed during tag checks are
irrelevant, and so likewise are the top bits of KASAN_SHADOW_OFFSET.
This is demonstrated by the fact that LLVM uses a logical right shift in
the tag check fast path[2] but a sbfx (signed bitfield extract)
instruction in the slow path[3] without causing any issues.

Use an arithmetic shift in kasan_mem_to_shadow() as it provides a number
of benefits:

1) The memory layout doesn't change but is easier to understand.
KASAN_SHADOW_OFFSET becomes a canonical memory address, and the shifted
pointer becomes a negative offset, so KASAN_SHADOW_OFFSET ==
KASAN_SHADOW_END regardless of the shift amount or the size of the
virtual address space.

2) KASAN_SHADOW_OFFSET becomes a simpler constant, requiring only one
instruction to load instead of two. Since it must be loaded in each
function with a tag check, this decreases kernel text size by 0.5%.

3) This shift and the sign extension from kasan_reset_tag() can be
combined into a single sbfx instruction. When this same algorithm change
is applied to the compiler, it removes an instruction from each inline
tag check, further reducing kernel text size by an additional 4.6%.

These benefits extend to other architectures as well. On RISC-V, where
the baseline ISA does not shifted addition or have an equivalent to the
sbfx instruction, loading KASAN_SHADOW_OFFSET is reduced from 3 to 2
instructions, and kasan_mem_to_shadow(kasan_reset_tag(addr)) similarly
combines two consecutive right shifts.

Link: https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp#L1316 [1]
Link: https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L895 [2]
Link: https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp#L669 [3]
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
---
Changelog v11: (Maciej)
- Remove the arch_kasan_non_canonical_hook() scheme in favor of Andrey
  Ryabinin's much nicer simple implementation.

Changelog v10: (Maciej)
- Update the Documentation/dev-tools/kasan.rst file with the changed
  kasan_mem_to_shadow().

Changelog v9: (Maciej)
- Take out the arm64 related code from mm/kasan/report.c and put it in
  the arch specific directory in a new file so the kasan_mem_to_shadow()
  function can be included.
- Reset addr tag bits in arm64's arch_kasan_non_canonical_hook() so the
  inline mode can also work with that function (Andrey Ryabinin).
- Fix incorrect number of zeros in a comment in mm/kasan/report.c.
- Remove Catalin's acked-by since changes were made.

Changelog v7: (Maciej)
- Change UL to ULL in report.c to fix some compilation warnings.

Changelog v6: (Maciej)
- Add Catalin's acked-by.
- Move x86 gdb snippet here from the last patch.

Changelog v5: (Maciej)
- (u64) -> (unsigned long) in report.c

Changelog v4: (Maciej)
- Revert x86 to signed mem_to_shadow mapping.
- Remove last two paragraphs since they were just poorer duplication of
  the comments in kasan_non_canonical_hook().

Changelog v3: (Maciej)
- Fix scripts/gdb/linux/kasan.py so the new signed mem_to_shadow() is
  reflected there.
- Fix Documentation/arch/arm64/kasan-offsets.sh to take new offsets into
  account.
- Made changes to the kasan_non_canonical_hook() according to upstream
  discussion. Settled on overflow on both ranges and separate checks for
  x86 and arm.

Changelog v2: (Maciej)
- Correct address range that's checked in kasan_non_canonical_hook().
  Adjust the comment inside.
- Remove part of comment from arch/arm64/include/asm/memory.h.
- Append patch message paragraph about the overflow in
  kasan_non_canonical_hook().

 Documentation/arch/arm64/kasan-offsets.sh |  8 ++++++--
 Documentation/dev-tools/kasan.rst         | 18 ++++++++++++------
 arch/arm64/Kconfig                        | 10 +++++-----
 arch/arm64/include/asm/memory.h           | 14 +++++++++++++-
 arch/arm64/mm/kasan_init.c                |  7 +++++--
 include/linux/kasan.h                     | 10 ++++++++--
 mm/kasan/report.c                         | 16 ++++++++++++----
 scripts/gdb/linux/kasan.py                |  5 ++++-
 scripts/gdb/linux/mm.py                   |  5 +++--
 9 files changed, 68 insertions(+), 25 deletions(-)

diff --git a/Documentation/arch/arm64/kasan-offsets.sh b/Documentation/arch/arm64/kasan-offsets.sh
index 2dc5f9e18039..ce777c7c7804 100644
--- a/Documentation/arch/arm64/kasan-offsets.sh
+++ b/Documentation/arch/arm64/kasan-offsets.sh
@@ -5,8 +5,12 @@
 
 print_kasan_offset () {
 	printf "%02d\t" $1
-	printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) \
-			- (1 << (64 - 32 - $2)) ))
+	if [[ $2 -ne 4 ]] then
+		printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) \
+				- (1 << (64 - 32 - $2)) ))
+	else
+		printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) ))
+	fi
 }
 
 echo KASAN_SHADOW_SCALE_SHIFT = 3
diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 4968b2aa60c8..b11c1be8dff4 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -315,13 +315,19 @@ translate a memory address to its corresponding shadow address.
 Here is the function which translates an address to its corresponding shadow
 address::
 
-    static inline void *kasan_mem_to_shadow(const void *addr)
-    {
-	return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
-		+ KASAN_SHADOW_OFFSET;
-    }
+        static inline void *kasan_mem_to_shadow(const void *addr)
+        {
+                void *scaled;
 
-where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
+                if (IS_ENABLED(CONFIG_KASAN_GENERIC))
+                        scaled = (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT);
+                else
+                        scaled = (void *)((long)addr >> KASAN_SHADOW_SCALE_SHIFT);
+
+                return KASAN_SHADOW_OFFSET + scaled;
+        }
+
+where for Generic KASAN ``KASAN_SHADOW_SCALE_SHIFT = 3``.
 
 Compile-time instrumentation is used to insert memory access checks. Compiler
 inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index bcd9f5bc66e2..87239396ed23 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -434,11 +434,11 @@ config KASAN_SHADOW_OFFSET
 	default 0xdffffe0000000000 if ARM64_VA_BITS_42 && !KASAN_SW_TAGS
 	default 0xdfffffc000000000 if ARM64_VA_BITS_39 && !KASAN_SW_TAGS
 	default 0xdffffff800000000 if ARM64_VA_BITS_36 && !KASAN_SW_TAGS
-	default 0xefff800000000000 if (ARM64_VA_BITS_48 || (ARM64_VA_BITS_52 && !ARM64_16K_PAGES)) && KASAN_SW_TAGS
-	default 0xefffc00000000000 if (ARM64_VA_BITS_47 || ARM64_VA_BITS_52) && ARM64_16K_PAGES && KASAN_SW_TAGS
-	default 0xeffffe0000000000 if ARM64_VA_BITS_42 && KASAN_SW_TAGS
-	default 0xefffffc000000000 if ARM64_VA_BITS_39 && KASAN_SW_TAGS
-	default 0xeffffff800000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS
+	default 0xffff800000000000 if (ARM64_VA_BITS_48 || (ARM64_VA_BITS_52 && !ARM64_16K_PAGES)) && KASAN_SW_TAGS
+	default 0xffffc00000000000 if (ARM64_VA_BITS_47 || ARM64_VA_BITS_52) && ARM64_16K_PAGES && KASAN_SW_TAGS
+	default 0xfffffe0000000000 if ARM64_VA_BITS_42 && KASAN_SW_TAGS
+	default 0xffffffc000000000 if ARM64_VA_BITS_39 && KASAN_SW_TAGS
+	default 0xfffffff800000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS
 	default 0xffffffffffffffff
 
 config UNWIND_TABLES
diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index a2b7a33966ff..875c0bd0d85a 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -89,7 +89,15 @@
  *
  * KASAN_SHADOW_END is defined first as the shadow address that corresponds to
  * the upper bound of possible virtual kernel memory addresses UL(1) << 64
- * according to the mapping formula.
+ * according to the mapping formula. For Generic KASAN, the address in the
+ * mapping formula is treated as unsigned (part of the compiler's ABI), so the
+ * end of the shadow memory region is at a large positive offset from
+ * KASAN_SHADOW_OFFSET. For Software Tag-Based KASAN, the address in the
+ * formula is treated as signed. Since all kernel addresses are negative, they
+ * map to shadow memory below KASAN_SHADOW_OFFSET, making KASAN_SHADOW_OFFSET
+ * itself the end of the shadow memory region. (User pointers are positive and
+ * would map to shadow memory above KASAN_SHADOW_OFFSET, but shadow memory is
+ * not allocated for them.)
  *
  * KASAN_SHADOW_START is defined second based on KASAN_SHADOW_END. The shadow
  * memory start must map to the lowest possible kernel virtual memory address
@@ -100,7 +108,11 @@
  */
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 #define KASAN_SHADOW_OFFSET	_AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
+#ifdef CONFIG_KASAN_GENERIC
 #define KASAN_SHADOW_END	((UL(1) << (64 - KASAN_SHADOW_SCALE_SHIFT)) + KASAN_SHADOW_OFFSET)
+#else
+#define KASAN_SHADOW_END	KASAN_SHADOW_OFFSET
+#endif
 #define _KASAN_SHADOW_START(va)	(KASAN_SHADOW_END - (UL(1) << ((va) - KASAN_SHADOW_SCALE_SHIFT)))
 #define KASAN_SHADOW_START	_KASAN_SHADOW_START(vabits_actual)
 #define PAGE_END		KASAN_SHADOW_START
diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index abeb81bf6ebd..937f6eb8115b 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -198,8 +198,11 @@ static bool __init root_level_aligned(u64 addr)
 /* The early shadow maps everything to a single page of zeroes */
 asmlinkage void __init kasan_early_init(void)
 {
-	BUILD_BUG_ON(KASAN_SHADOW_OFFSET !=
-		KASAN_SHADOW_END - (1UL << (64 - KASAN_SHADOW_SCALE_SHIFT)));
+	if (IS_ENABLED(CONFIG_KASAN_GENERIC))
+		BUILD_BUG_ON(KASAN_SHADOW_OFFSET !=
+			KASAN_SHADOW_END - (1UL << (64 - KASAN_SHADOW_SCALE_SHIFT)));
+	else
+		BUILD_BUG_ON(KASAN_SHADOW_OFFSET != KASAN_SHADOW_END);
 	BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS), SHADOW_ALIGN));
 	BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS_MIN), SHADOW_ALIGN));
 	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, SHADOW_ALIGN));
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index bf233bde68c7..fbff1b759c85 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -62,8 +62,14 @@ int kasan_populate_early_shadow(const void *shadow_start,
 #ifndef kasan_mem_to_shadow
 static inline void *kasan_mem_to_shadow(const void *addr)
 {
-	return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
-		+ KASAN_SHADOW_OFFSET;
+	void *scaled;
+
+	if (IS_ENABLED(CONFIG_KASAN_GENERIC))
+		scaled = (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT);
+	else
+		scaled = (void *)((long)addr >> KASAN_SHADOW_SCALE_SHIFT);
+
+	return KASAN_SHADOW_OFFSET + scaled;
 }
 #endif
 
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index e804b1e1f886..1e4521b5ef14 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -640,12 +640,20 @@ void kasan_non_canonical_hook(unsigned long addr)
 {
 	unsigned long orig_addr, user_orig_addr;
 	const char *bug_type;
+	void *tagged_null = set_tag(NULL, KASAN_TAG_KERNEL);
+	void *tagged_addr = set_tag((void *)addr, KASAN_TAG_KERNEL);
 
 	/*
-	 * All addresses that came as a result of the memory-to-shadow mapping
-	 * (even for bogus pointers) must be >= KASAN_SHADOW_OFFSET.
+	 * Filter out addresses that cannot be shadow memory accesses generated
+	 * by the compiler.
+	 *
+	 * In SW_TAGS mode, when computing a shadow address, the compiler always
+	 * sets the kernel tag (some top bits) on the pointer *before* computing
+	 * the memory-to-shadow mapping. As a result, valid shadow addresses
+	 * are derived from tagged kernel pointers.
 	 */
-	if (addr < KASAN_SHADOW_OFFSET)
+	if (tagged_addr < kasan_mem_to_shadow(tagged_null) ||
+	    tagged_addr > kasan_mem_to_shadow((void *)(~0ULL)))
 		return;
 
 	orig_addr = (unsigned long)kasan_shadow_to_mem((void *)addr);
@@ -670,7 +678,7 @@ void kasan_non_canonical_hook(unsigned long addr)
 	} else if (user_orig_addr < TASK_SIZE) {
 		bug_type = "probably user-memory-access";
 		orig_addr = user_orig_addr;
-	} else if (addr_in_shadow((void *)addr))
+	} else if (addr_in_shadow(tagged_addr))
 		bug_type = "probably wild-memory-access";
 	else
 		bug_type = "maybe wild-memory-access";
diff --git a/scripts/gdb/linux/kasan.py b/scripts/gdb/linux/kasan.py
index 56730b3fde0b..4b86202b155f 100644
--- a/scripts/gdb/linux/kasan.py
+++ b/scripts/gdb/linux/kasan.py
@@ -7,7 +7,8 @@
 #
 
 import gdb
-from linux import constants, mm
+from linux import constants, utils, mm
+from ctypes import c_int64 as s64
 
 def help():
     t = """Usage: lx-kasan_mem_to_shadow [Hex memory addr]
@@ -39,6 +40,8 @@ class KasanMemToShadow(gdb.Command):
         else:
             help()
     def kasan_mem_to_shadow(self, addr):
+        if constants.CONFIG_KASAN_SW_TAGS and not utils.is_target_arch('x86'):
+            addr = s64(addr)
         return (addr >> self.p_ops.KASAN_SHADOW_SCALE_SHIFT) + self.p_ops.KASAN_SHADOW_OFFSET
 
 KasanMemToShadow()
diff --git a/scripts/gdb/linux/mm.py b/scripts/gdb/linux/mm.py
index d78908f6664d..d4ab341d89c5 100644
--- a/scripts/gdb/linux/mm.py
+++ b/scripts/gdb/linux/mm.py
@@ -281,12 +281,13 @@ class aarch64_page_ops():
         self.KERNEL_END = gdb.parse_and_eval("_end")
 
         if constants.LX_CONFIG_KASAN_GENERIC or constants.LX_CONFIG_KASAN_SW_TAGS:
+            self.KASAN_SHADOW_OFFSET = constants.LX_CONFIG_KASAN_SHADOW_OFFSET
             if constants.LX_CONFIG_KASAN_GENERIC:
                 self.KASAN_SHADOW_SCALE_SHIFT = 3
+                self.KASAN_SHADOW_END = (1 << (64 - self.KASAN_SHADOW_SCALE_SHIFT)) + self.KASAN_SHADOW_OFFSET
             else:
                 self.KASAN_SHADOW_SCALE_SHIFT = 4
-            self.KASAN_SHADOW_OFFSET = constants.LX_CONFIG_KASAN_SHADOW_OFFSET
-            self.KASAN_SHADOW_END = (1 << (64 - self.KASAN_SHADOW_SCALE_SHIFT)) + self.KASAN_SHADOW_OFFSET
+                self.KASAN_SHADOW_END = self.KASAN_SHADOW_OFFSET
             self.PAGE_END = self.KASAN_SHADOW_END - (1 << (self.vabits_actual - self.KASAN_SHADOW_SCALE_SHIFT))
         else:
             self.PAGE_END = self._PAGE_END(self.VA_BITS_MIN)
-- 
2.53.0

Re: [PATCH v12 01/15] kasan: sw_tags: Use arithmetic shift for shadow computation

[Catalin Marinas]

On Mon, Mar 30, 2026 at 02:33:05PM +0000, Maciej Wieczor-Retman wrote:
> diff --git a/Documentation/arch/arm64/kasan-offsets.sh b/Documentation/arch/arm64/kasan-offsets.sh
> index 2dc5f9e18039..ce777c7c7804 100644
> --- a/Documentation/arch/arm64/kasan-offsets.sh
> +++ b/Documentation/arch/arm64/kasan-offsets.sh
> @@ -5,8 +5,12 @@
>  
>  print_kasan_offset () {
>  	printf "%02d\t" $1
> -	printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) \
> -			- (1 << (64 - 32 - $2)) ))
> +	if [[ $2 -ne 4 ]] then

Nitpick: does this need a semicolon before 'then'?

I can see Sashiko raised it here:

https://sashiko.dev/#/patchset/cover.1774872838.git.m.wieczorretman@pm.me

> +		printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) \
> +				- (1 << (64 - 32 - $2)) ))
> +	else
> +		printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) ))
> +	fi
>  }
>  
>  echo KASAN_SHADOW_SCALE_SHIFT = 3
[...]
> diff --git a/scripts/gdb/linux/kasan.py b/scripts/gdb/linux/kasan.py
> index 56730b3fde0b..4b86202b155f 100644
> --- a/scripts/gdb/linux/kasan.py
> +++ b/scripts/gdb/linux/kasan.py
> @@ -7,7 +7,8 @@
>  #
>  
>  import gdb
> -from linux import constants, mm
> +from linux import constants, utils, mm
> +from ctypes import c_int64 as s64
>  
>  def help():
>      t = """Usage: lx-kasan_mem_to_shadow [Hex memory addr]
> @@ -39,6 +40,8 @@ class KasanMemToShadow(gdb.Command):
>          else:
>              help()
>      def kasan_mem_to_shadow(self, addr):
> +        if constants.CONFIG_KASAN_SW_TAGS and not utils.is_target_arch('x86'):

Does this need to be constants.LX_CONFIG_KASAN_SW_TAGS? I don't claim I
fully understand this script but the other constants.* use LX_*.

> +            addr = s64(addr)
>          return (addr >> self.p_ops.KASAN_SHADOW_SCALE_SHIFT) + self.p_ops.KASAN_SHADOW_OFFSET

And, again, Sashiko mentions that the bitwise right shift here will fail
after the cast to c_int64. I just tried this in python:

>>> from ctypes import c_int64 as s64
>>> s64(0xffff000008eca008) >> 4
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    s64(0xffff000008eca008) >> 4
    ~~~~~~~~~~~~~~~~~~~~~~~~^^~~
TypeError: unsupported operand type(s) for >>: 'c_long' and 'int'

I guess it's hidden by the wrong check on
constants.CONFIG_KASAN_SW_TAGS.

Otherwise I think the changes are fine. If you fix the above, feel free
to add:

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

[PATCH v12 02/15] kasan: arm64: x86: Make special tags arch specific

[Maciej Wieczor-Retman]

From: Samuel Holland <samuel.holland@sifive.com>

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.
- Min and max values. 0xFD is the highest value that can be randomly
  generated for a new tag. 0 is the minimal value with the exception of
  arm64's hardware mode where it is equal to 0xF0.

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 tag width and native kernel tag arch specific for x86 and arm64.

Base the invalid tag value and the max value on the native kernel tag
since they follow the same pattern on both mentioned architectures.

Also generalize KASAN_SHADOW_INIT and 0xff used in various
page_kasan_tag* helpers.

Give KASAN_TAG_MIN the default value of zero, and move the special value
for hw_tags arm64 to its arch specific kasan-tags.h.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Acked-by: Will Deacon <will@kernel.org> (for the arm part)
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
---
Changelog v9:
- Add Andrey Ryabinin's Reviewed-by tag.
- Add Andrey Konovalov's Reviewed-by tag.

Changelog v8:
- Add Will's Acked-by tag.

Changelog v7:
- Reorder defines of arm64 tag width to prevent redefinition warnings.
- Remove KASAN_TAG_MASK so it's only defined in mmzone.h (Andrey
  Konovalov)
- Merge the 'support tag widths less than 8 bits' with this patch since
  they do similar things and overwrite each other. (Alexander)

Changelog v6:
- Add hardware tags KASAN_TAG_WIDTH value to the arm64 arch file.
- Keep KASAN_TAG_MASK in the mmzone.h.
- Remove ifndef from KASAN_SHADOW_INIT.

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 | 14 ++++++++++++++
 arch/arm64/include/asm/kasan.h      |  2 --
 arch/arm64/include/asm/uaccess.h    |  1 +
 arch/x86/include/asm/kasan-tags.h   |  9 +++++++++
 include/linux/kasan-tags.h          | 19 ++++++++++++++-----
 include/linux/kasan.h               |  3 +--
 include/linux/mm.h                  |  6 +++---
 include/linux/page-flags-layout.h   |  9 +--------
 9 files changed, 44 insertions(+), 21 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 16874c32e288..897210732d30 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -13735,7 +13735,7 @@ 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..259952677443
--- /dev/null
+++ b/arch/arm64/include/asm/kasan-tags.h
@@ -0,0 +1,14 @@
+/* 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 */
+
+#ifdef CONFIG_KASAN_HW_TAGS
+#define KASAN_TAG_MIN		0xF0 /* minimum value for random tags */
+#define KASAN_TAG_WIDTH		4
+#else
+#define KASAN_TAG_WIDTH		8
+#endif
+
+#endif /* ASM_KASAN_TAGS_H */
diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
index b167e9d3da91..fd4a8557d736 100644
--- a/arch/arm64/include/asm/kasan.h
+++ b/arch/arm64/include/asm/kasan.h
@@ -6,8 +6,6 @@
 
 #include <linux/linkage.h>
 #include <asm/memory.h>
-#include <asm/mte-kasan.h>
-#include <asm/pgtable-types.h>
 
 #define arch_kasan_set_tag(addr, tag)	__tag_set(addr, tag)
 #define arch_kasan_reset_tag(addr)	__tag_reset(addr)
diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 9810106a3f66..5465bc97ccdd 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -22,6 +22,7 @@
 #include <asm/cpufeature.h>
 #include <asm/mmu.h>
 #include <asm/mte.h>
+#include <asm/mte-kasan.h>
 #include <asm/ptrace.h>
 #include <asm/memory.h>
 #include <asm/extable.h>
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 4f85f562512c..ad5c11950233 100644
--- a/include/linux/kasan-tags.h
+++ b/include/linux/kasan-tags.h
@@ -2,13 +2,22 @@
 #ifndef _LINUX_KASAN_TAGS_H
 #define _LINUX_KASAN_TAGS_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
+
+#ifndef KASAN_TAG_KERNEL
 #define KASAN_TAG_KERNEL	0xFF /* native kernel pointers tag */
-#define KASAN_TAG_INVALID	0xFE /* inaccessible memory tag */
-#define KASAN_TAG_MAX		0xFD /* maximum value for random tags */
+#endif
+
+#define KASAN_TAG_INVALID	(KASAN_TAG_KERNEL - 1) /* inaccessible memory tag */
+#define KASAN_TAG_MAX		(KASAN_TAG_KERNEL - 2) /* maximum value for random tags */
 
-#ifdef CONFIG_KASAN_HW_TAGS
-#define KASAN_TAG_MIN		0xF0 /* minimum value for random tags */
-#else
+#ifndef KASAN_TAG_MIN
 #define KASAN_TAG_MIN		0x00 /* minimum value for random tags */
 #endif
 
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index fbff1b759c85..e18908f3ad6e 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -40,8 +40,7 @@ typedef unsigned int __bitwise kasan_vmalloc_flags_t;
 /* Software KASAN implementations use shadow memory. */
 
 #ifdef CONFIG_KASAN_SW_TAGS
-/* This matches KASAN_TAG_INVALID. */
-#define KASAN_SHADOW_INIT 0xFE
+#define KASAN_SHADOW_INIT KASAN_TAG_INVALID
 #else
 #define KASAN_SHADOW_INIT 0
 #endif
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 633bbf9a184a..09044934dda8 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2440,7 +2440,7 @@ static inline u8 page_kasan_tag(const struct page *page)
 
 	if (kasan_enabled()) {
 		tag = (page->flags.f >> KASAN_TAG_PGSHIFT) & KASAN_TAG_MASK;
-		tag ^= 0xff;
+		tag ^= KASAN_TAG_KERNEL;
 	}
 
 	return tag;
@@ -2453,7 +2453,7 @@ 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.f);
 	do {
 		flags = old_flags;
@@ -2472,7 +2472,7 @@ 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/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
-#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)
-- 
2.53.0

Re: [PATCH v12 02/15] kasan: arm64: x86: Make special tags arch specific

[Catalin Marinas]

On Mon, Mar 30, 2026 at 02:33:16PM +0000, Maciej Wieczor-Retman wrote:
> From: Samuel Holland <samuel.holland@sifive.com>
> 
> 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.
> - Min and max values. 0xFD is the highest value that can be randomly
>   generated for a new tag. 0 is the minimal value with the exception of
>   arm64's hardware mode where it is equal to 0xF0.
> 
> 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 tag width and native kernel tag arch specific for x86 and arm64.
> 
> Base the invalid tag value and the max value on the native kernel tag
> since they follow the same pattern on both mentioned architectures.
> 
> Also generalize KASAN_SHADOW_INIT and 0xff used in various
> page_kasan_tag* helpers.
> 
> Give KASAN_TAG_MIN the default value of zero, and move the special value
> for hw_tags arm64 to its arch specific kasan-tags.h.
> 
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
> Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> Acked-by: Will Deacon <will@kernel.org> (for the arm part)
> Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
> Reviewed-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

[PATCH v12 06/15] kasan: arm64: x86: Make page_to_virt() KASAN aware

[Maciej Wieczor-Retman]

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

Special page_to_virt() implementation is needed if an architecture wants
to enable KASAN software tag-based mode.

Make page_to_virt() KASAN aware in arch-independent code so
architectures implementing the software tag-based mode don't have to
define their own implementations anymore. When KASAN is disabled or for
architectures that don't implement the software tag-based mode
page_to_virt() will be optimized to it's previous form.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
---
Changelog v11:
- Redo the patch to work on the page_to_virt macro. Split off changes
  about virt to phys conversion to an earlier patch.
- Remove Alexander's acked-by due to bigger changes.

Changelog v7:
- Add Alexander's Acked-by tag.

Changelog v5:
- Move __tag_reset() calls into __phys_addr_nodebug() and
  __virt_addr_valid() instead of calling it on the arguments of higher
  level functions.

Changelog v4:
- Simplify page_to_virt() by removing pointless casts.
- Remove change in __is_canonical_address() because it's taken care of
  in a later patch due to a LAM compatible definition of canonical.

 arch/arm64/include/asm/memory.h |  5 -----
 include/linux/kasan.h           | 10 ++++++++++
 include/linux/mm.h              |  5 ++++-
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 875c0bd0d85a..39dd0071d3ec 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -411,11 +411,6 @@ static inline unsigned long virt_to_pfn(const void *kaddr)
  */
 
 #if defined(CONFIG_DEBUG_VIRTUAL)
-#define page_to_virt(x)	({						\
-	__typeof__(x) __page = x;					\
-	void *__addr = __va(page_to_phys(__page));			\
-	(void *)__tag_set((const void *)__addr, page_kasan_tag(__page));\
-})
 #define virt_to_page(x)		pfn_to_page(virt_to_pfn(x))
 #else
 #define page_to_virt(x)	({						\
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index e18908f3ad6e..271c59e9f422 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -527,6 +527,11 @@ static inline void *kasan_reset_tag(const void *addr)
 	return (void *)arch_kasan_reset_tag(addr);
 }
 
+static inline void *kasan_set_tag(const void *addr, u8 tag)
+{
+	return (void *)arch_kasan_set_tag(addr, tag);
+}
+
 /**
  * kasan_report - print a report about a bad memory access detected by KASAN
  * @addr: address of the bad access
@@ -544,6 +549,11 @@ static inline void *kasan_reset_tag(const void *addr)
 	return (void *)addr;
 }
 
+static inline void *kasan_set_tag(const void *addr, u8 tag)
+{
+	return (void *)addr;
+}
+
 #endif /* CONFIG_KASAN_SW_TAGS || CONFIG_KASAN_HW_TAGS*/
 
 #ifdef CONFIG_KASAN_HW_TAGS
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 09044934dda8..f234650a4edf 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -117,7 +117,10 @@ extern int mmap_rnd_compat_bits __read_mostly;
 #endif
 
 #ifndef page_to_virt
-#define page_to_virt(x)	__va(PFN_PHYS(page_to_pfn(x)))
+#define page_to_virt(x) ({							\
+	void *__addr = __va(PFN_PHYS(page_to_pfn((struct page *)x)));		\
+	kasan_set_tag(__addr, page_kasan_tag(x));				\
+})
 #endif
 
 #ifndef lm_alias
-- 
2.53.0

Re: [PATCH v12 06/15] kasan: arm64: x86: Make page_to_virt() KASAN aware

[Will Deacon]

On Mon, Mar 30, 2026 at 02:33:43PM +0000, Maciej Wieczor-Retman wrote:
> From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> 
> Special page_to_virt() implementation is needed if an architecture wants
> to enable KASAN software tag-based mode.
> 
> Make page_to_virt() KASAN aware in arch-independent code so
> architectures implementing the software tag-based mode don't have to
> define their own implementations anymore. When KASAN is disabled or for
> architectures that don't implement the software tag-based mode
> page_to_virt() will be optimized to it's previous form.
> 
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> ---
> Changelog v11:
> - Redo the patch to work on the page_to_virt macro. Split off changes
>   about virt to phys conversion to an earlier patch.
> - Remove Alexander's acked-by due to bigger changes.
> 
> Changelog v7:
> - Add Alexander's Acked-by tag.
> 
> Changelog v5:
> - Move __tag_reset() calls into __phys_addr_nodebug() and
>   __virt_addr_valid() instead of calling it on the arguments of higher
>   level functions.
> 
> Changelog v4:
> - Simplify page_to_virt() by removing pointless casts.
> - Remove change in __is_canonical_address() because it's taken care of
>   in a later patch due to a LAM compatible definition of canonical.
> 
>  arch/arm64/include/asm/memory.h |  5 -----
>  include/linux/kasan.h           | 10 ++++++++++
>  include/linux/mm.h              |  5 ++++-
>  3 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 875c0bd0d85a..39dd0071d3ec 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -411,11 +411,6 @@ static inline unsigned long virt_to_pfn(const void *kaddr)
>   */
>  
>  #if defined(CONFIG_DEBUG_VIRTUAL)
> -#define page_to_virt(x)	({						\
> -	__typeof__(x) __page = x;					\
> -	void *__addr = __va(page_to_phys(__page));			\
> -	(void *)__tag_set((const void *)__addr, page_kasan_tag(__page));\
> -})

So here we're using '__page' to avoid multiple evaluations of 'x'...

>  #define virt_to_page(x)		pfn_to_page(virt_to_pfn(x))
>  #else
>  #define page_to_virt(x)	({						\
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index e18908f3ad6e..271c59e9f422 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -527,6 +527,11 @@ static inline void *kasan_reset_tag(const void *addr)
>  	return (void *)arch_kasan_reset_tag(addr);
>  }
>  
> +static inline void *kasan_set_tag(const void *addr, u8 tag)
> +{
> +	return (void *)arch_kasan_set_tag(addr, tag);
> +}
> +
>  /**
>   * kasan_report - print a report about a bad memory access detected by KASAN
>   * @addr: address of the bad access
> @@ -544,6 +549,11 @@ static inline void *kasan_reset_tag(const void *addr)
>  	return (void *)addr;
>  }
>  
> +static inline void *kasan_set_tag(const void *addr, u8 tag)
> +{
> +	return (void *)addr;
> +}
> +
>  #endif /* CONFIG_KASAN_SW_TAGS || CONFIG_KASAN_HW_TAGS*/
>  
>  #ifdef CONFIG_KASAN_HW_TAGS
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 09044934dda8..f234650a4edf 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -117,7 +117,10 @@ extern int mmap_rnd_compat_bits __read_mostly;
>  #endif
>  
>  #ifndef page_to_virt
> -#define page_to_virt(x)	__va(PFN_PHYS(page_to_pfn(x)))
> +#define page_to_virt(x) ({							\
> +	void *__addr = __va(PFN_PHYS(page_to_pfn((struct page *)x)));		\
> +	kasan_set_tag(__addr, page_kasan_tag(x));				\
> +})

... but in the new code you're proposing, you evaluate 'x' twice.

Is there a reason not to take the arm64 code as-is?

Will

Re: [PATCH v12 06/15] kasan: arm64: x86: Make page_to_virt() KASAN aware

[Maciej Wieczor-Retman]

On 2026-05-18 at 12:56:51 +0100, Will Deacon wrote:
>On Mon, Mar 30, 2026 at 02:33:43PM +0000, Maciej Wieczor-Retman wrote:
>> From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
>> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
>> index 875c0bd0d85a..39dd0071d3ec 100644
>> --- a/arch/arm64/include/asm/memory.h
>> +++ b/arch/arm64/include/asm/memory.h
>> @@ -411,11 +411,6 @@ static inline unsigned long virt_to_pfn(const void *kaddr)
>>   */
>>  
>>  #if defined(CONFIG_DEBUG_VIRTUAL)
>> -#define page_to_virt(x)	({						\
>> -	__typeof__(x) __page = x;					\
>> -	void *__addr = __va(page_to_phys(__page));			\
>> -	(void *)__tag_set((const void *)__addr, page_kasan_tag(__page));\
>> -})
>
>So here we're using '__page' to avoid multiple evaluations of 'x'...
>
...
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 09044934dda8..f234650a4edf 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -117,7 +117,10 @@ extern int mmap_rnd_compat_bits __read_mostly;
>>  #endif
>>  
>>  #ifndef page_to_virt
>> -#define page_to_virt(x)	__va(PFN_PHYS(page_to_pfn(x)))
>> +#define page_to_virt(x) ({							\
>> +	void *__addr = __va(PFN_PHYS(page_to_pfn((struct page *)x)));		\
>> +	kasan_set_tag(__addr, page_kasan_tag(x));				\
>> +})
>
>... but in the new code you're proposing, you evaluate 'x' twice.
>
>Is there a reason not to take the arm64 code as-is?
>
>Will

I was going for less lines in this macro when I initially wrote it. But you're
right, the arm64 version is better, I'll switch to it.

Thanks!

-- 
Kind regards
Maciej Wieczór-Retman