[PATCH v8 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

[PATCH v8 01/14] 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.

Using an arithmetic shift in kasan_mem_to_shadow() 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>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
---
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 +++--
 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                         | 36 ++++++++++++++++++++---
 scripts/gdb/linux/kasan.py                |  5 +++-
 scripts/gdb/linux/mm.py                   |  5 ++--
 8 files changed, 76 insertions(+), 19 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/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 93173f0a09c7..c1b7261cdb96 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 9d54b2ea49d6..f127fbf691ac 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 9c6ac4b62eb9..0f65e88cc3f6 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 62c01b4527eb..b5beb1b10bd2 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -642,11 +642,39 @@ void kasan_non_canonical_hook(unsigned long addr)
 	const char *bug_type;
 
 	/*
-	 * All addresses that came as a result of the memory-to-shadow mapping
-	 * (even for bogus pointers) must be >= KASAN_SHADOW_OFFSET.
+	 * For Generic KASAN, kasan_mem_to_shadow() uses the logical right shift
+	 * and never overflows with the chosen KASAN_SHADOW_OFFSET values (on
+	 * both x86 and arm64). Thus, the possible shadow addresses (even for
+	 * bogus pointers) belong to a single contiguous region that is the
+	 * result of kasan_mem_to_shadow() applied to the whole address space.
 	 */
-	if (addr < KASAN_SHADOW_OFFSET)
-		return;
+	if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
+		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0ULL)) ||
+		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
+			return;
+	}
+
+	/*
+	 * For Software Tag-Based KASAN, kasan_mem_to_shadow() uses the
+	 * arithmetic shift. Normally, this would make checking for a possible
+	 * shadow address complicated, as the shadow address computation
+	 * operation would overflow only for some memory addresses. However, due
+	 * to the chosen KASAN_SHADOW_OFFSET values and the fact the
+	 * kasan_mem_to_shadow() only operates on pointers with the tag reset,
+	 * the overflow always happens.
+	 *
+	 * For arm64, the top byte of the pointer gets reset to 0xFF. Thus, the
+	 * possible shadow addresses belong to a region that is the result of
+	 * kasan_mem_to_shadow() applied to the memory range
+	 * [0xFF000000000000, 0xFFFFFFFFFFFFFFFF]. Despite the overflow, the
+	 * resulting possible shadow region is contiguous, as the overflow
+	 * happens for both 0xFF000000000000 and 0xFFFFFFFFFFFFFFFF.
+	 */
+	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) && IS_ENABLED(CONFIG_ARM64)) {
+		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0xFFULL << 56)) ||
+		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
+			return;
+	}
 
 	orig_addr = (unsigned long)kasan_shadow_to_mem((void *)addr);
 
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 7571aebbe650..2e63f3dedd53 100644
--- a/scripts/gdb/linux/mm.py
+++ b/scripts/gdb/linux/mm.py
@@ -110,12 +110,13 @@ 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)
-- 
2.52.0

Re: [PATCH v8 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

[Andrey Ryabinin]

On 1/12/26 6:27 PM, Maciej Wieczor-Retman wrote:
  
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 62c01b4527eb..b5beb1b10bd2 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -642,11 +642,39 @@ void kasan_non_canonical_hook(unsigned long addr)
>  	const char *bug_type;
>  
>  	/*
> -	 * All addresses that came as a result of the memory-to-shadow mapping
> -	 * (even for bogus pointers) must be >= KASAN_SHADOW_OFFSET.
> +	 * For Generic KASAN, kasan_mem_to_shadow() uses the logical right shift
> +	 * and never overflows with the chosen KASAN_SHADOW_OFFSET values (on
> +	 * both x86 and arm64). Thus, the possible shadow addresses (even for
> +	 * bogus pointers) belong to a single contiguous region that is the
> +	 * result of kasan_mem_to_shadow() applied to the whole address space.
>  	 */
> -	if (addr < KASAN_SHADOW_OFFSET)
> -		return;
> +	if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
> +		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0ULL)) ||
> +		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
> +			return;
> +	}
> +
> +	/*
> +	 * For Software Tag-Based KASAN, kasan_mem_to_shadow() uses the
> +	 * arithmetic shift. Normally, this would make checking for a possible
> +	 * shadow address complicated, as the shadow address computation
> +	 * operation would overflow only for some memory addresses. However, due
> +	 * to the chosen KASAN_SHADOW_OFFSET values and the fact the
> +	 * kasan_mem_to_shadow() only operates on pointers with the tag reset,
> +	 * the overflow always happens.
> +	 *
> +	 * For arm64, the top byte of the pointer gets reset to 0xFF. Thus, the
> +	 * possible shadow addresses belong to a region that is the result of
> +	 * kasan_mem_to_shadow() applied to the memory range
> +	 * [0xFF000000000000, 0xFFFFFFFFFFFFFFFF]. Despite the overflow, the
                  ^ Missing couple 00 here

> +	 * resulting possible shadow region is contiguous, as the overflow
> +	 * happens for both 0xFF000000000000 and 0xFFFFFFFFFFFFFFFF.
                                  ^ same as above

> +	 */
> +	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) && IS_ENABLED(CONFIG_ARM64)) {
> +		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0xFFULL << 56)) ||

This will not work for inline mode because compiler uses logical shift.
Consider NULL-ptr derefernce. Compiler will calculate shadow address for 0 as:
      (((0x0 | 0xffULL) << 56) >> 4)+0xffff800000000000ULL = 0x0fef8000....0
Which is less than ((0xFF00...00LL) >> 4) +  0xffff800000000000ULL = 0xffff800...0
So we will bail out here.
Perhaps we could do addr |= 0xFFLL to fix this

> +		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
> +			return;
> +	}
>  
>  	orig_addr = (unsigned long)kasan_shadow_to_mem((void *)addr);
>  

Re: [PATCH v8 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

[Maciej Wieczor-Retman]

Thanks for looking at the patches :)

On 2026-01-15 at 23:42:02 +0100, Andrey Ryabinin wrote:
>
>
>On 1/12/26 6:27 PM, Maciej Wieczor-Retman wrote:
>  
>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
>> index 62c01b4527eb..b5beb1b10bd2 100644
>> --- a/mm/kasan/report.c
>> +++ b/mm/kasan/report.c
>> @@ -642,11 +642,39 @@ void kasan_non_canonical_hook(unsigned long addr)
>>  	const char *bug_type;
>>  
>>  	/*
>> -	 * All addresses that came as a result of the memory-to-shadow mapping
>> -	 * (even for bogus pointers) must be >= KASAN_SHADOW_OFFSET.
>> +	 * For Generic KASAN, kasan_mem_to_shadow() uses the logical right shift
>> +	 * and never overflows with the chosen KASAN_SHADOW_OFFSET values (on
>> +	 * both x86 and arm64). Thus, the possible shadow addresses (even for
>> +	 * bogus pointers) belong to a single contiguous region that is the
>> +	 * result of kasan_mem_to_shadow() applied to the whole address space.
>>  	 */
>> -	if (addr < KASAN_SHADOW_OFFSET)
>> -		return;
>> +	if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
>> +		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0ULL)) ||
>> +		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
>> +			return;
>> +	}
>> +
>> +	/*
>> +	 * For Software Tag-Based KASAN, kasan_mem_to_shadow() uses the
>> +	 * arithmetic shift. Normally, this would make checking for a possible
>> +	 * shadow address complicated, as the shadow address computation
>> +	 * operation would overflow only for some memory addresses. However, due
>> +	 * to the chosen KASAN_SHADOW_OFFSET values and the fact the
>> +	 * kasan_mem_to_shadow() only operates on pointers with the tag reset,
>> +	 * the overflow always happens.
>> +	 *
>> +	 * For arm64, the top byte of the pointer gets reset to 0xFF. Thus, the
>> +	 * possible shadow addresses belong to a region that is the result of
>> +	 * kasan_mem_to_shadow() applied to the memory range
>> +	 * [0xFF000000000000, 0xFFFFFFFFFFFFFFFF]. Despite the overflow, the
>                  ^ Missing couple 00 here
>
>> +	 * resulting possible shadow region is contiguous, as the overflow
>> +	 * happens for both 0xFF000000000000 and 0xFFFFFFFFFFFFFFFF.
>                                  ^ same as above

Hah, right, thank you!

>
>> +	 */
>> +	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) && IS_ENABLED(CONFIG_ARM64)) {
>> +		if (addr < (unsigned long)kasan_mem_to_shadow((void *)(0xFFULL << 56)) ||
>
>This will not work for inline mode because compiler uses logical shift.
>Consider NULL-ptr derefernce. Compiler will calculate shadow address for 0 as:
>      (((0x0 | 0xffULL) << 56) >> 4)+0xffff800000000000ULL = 0x0fef8000....0
>Which is less than ((0xFF00...00LL) >> 4) +  0xffff800000000000ULL = 0xffff800...0
>So we will bail out here.
>Perhaps we could do addr |= 0xFFLL to fix this

I suppose it should work; tried it in a python script by shoving various
addresses into this check. Pushing addresses through a logical shift
memory_to_shadow normally would return early as you noticed, and after 'addr |=
0xFFLL' it seems to work as expected. And I didn't really catch any incorrect
address slipping by this scheme either. Thanks, I'll correct it.

>
>> +		    addr > (unsigned long)kasan_mem_to_shadow((void *)(~0ULL)))
>> +			return;
>> +	}
>>  
>>  	orig_addr = (unsigned long)kasan_shadow_to_mem((void *)addr);
>>  

-- 
Kind regards
Maciej Wieczór-Retman

[PATCH v8 02/14] 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)
---
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 0d044a58cbfe..84fdf497a97c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -13581,7 +13581,7 @@ 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 6490930deef8..ccd41a39e3a1 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 0f65e88cc3f6..1c7acdb5f297 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 6f959d8ca4b4..8ba91f38a794 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1949,7 +1949,7 @@ 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;
@@ -1962,7 +1962,7 @@ 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;
@@ -1981,7 +1981,7 @@ 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)
-- 
2.52.0

Re: [PATCH v8 02/14] kasan: arm64: x86: Make special tags arch specific

[Andrey Konovalov]

On Mon, Jan 12, 2026 at 6:27 PM Maciej Wieczor-Retman
<m.wieczorretman@pm.me> 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)
> ---
> 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 0d044a58cbfe..84fdf497a97c 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -13581,7 +13581,7 @@ 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 6490930deef8..ccd41a39e3a1 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 */

One thing that stood out to me here was that for x86, KASAN_TAG_KERNEL
is defined as a 4-bit value (0xF). Which makes sense, as
KASAN_TAG_WIDTH == 4.

But for arm64, KASAN_TAG_KERNEL and others are defined as 8-bit values
(0xFF, etc.), even though for HW_TAGS, KASAN_TAG_WIDTH is also == 4
and only the lower 4 bits of these values define the tags.

This happens to work out: for HW_TAGS, __tag_set resets the top byte
but then uses the given value as is, so the higher 4 bits gets set to
0xF and the lower set to the tag. And for saving/restoring the tag in
page->flags, everything also works, as we only store the meaningful
lower 4 bits in flags, and restore the higher 0xF when doing ^ 0xFF.

But this is not related to this series: I think the way x86 defines
KASAN_TAG_KERNEL to be 0xF makes sense; we might just need to clean up
the arm64 implementation at some point.

> +
> +#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 0f65e88cc3f6..1c7acdb5f297 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 6f959d8ca4b4..8ba91f38a794 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1949,7 +1949,7 @@ 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;
> @@ -1962,7 +1962,7 @@ 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;
> @@ -1981,7 +1981,7 @@ 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)
> --
> 2.52.0
>
>

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

Re: [PATCH v8 02/14] kasan: arm64: x86: Make special tags arch specific

[Maciej Wieczor-Retman]

On 2026-01-13 at 02:21:07 +0100, Andrey Konovalov wrote:
>On Mon, Jan 12, 2026 at 6:27 PM Maciej Wieczor-Retman
><m.wieczorretman@pm.me> wrote:
>>
>> From: Samuel Holland <samuel.holland@sifive.com>
>> 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 */
>
>One thing that stood out to me here was that for x86, KASAN_TAG_KERNEL
>is defined as a 4-bit value (0xF). Which makes sense, as
>KASAN_TAG_WIDTH == 4.
>
>But for arm64, KASAN_TAG_KERNEL and others are defined as 8-bit values
>(0xFF, etc.), even though for HW_TAGS, KASAN_TAG_WIDTH is also == 4
>and only the lower 4 bits of these values define the tags.
>
>This happens to work out: for HW_TAGS, __tag_set resets the top byte
>but then uses the given value as is, so the higher 4 bits gets set to
>0xF and the lower set to the tag. And for saving/restoring the tag in
>page->flags, everything also works, as we only store the meaningful
>lower 4 bits in flags, and restore the higher 0xF when doing ^ 0xFF.
>
>But this is not related to this series: I think the way x86 defines
>KASAN_TAG_KERNEL to be 0xF makes sense; we might just need to clean up
>the arm64 implementation at some point.
>

I suppose while there is only one such mode that stands out from the other two
there is little hint as to what should be generalized. As you said so far this
scheme we have works - altough it is somewhat convoluted.

One thing I was thinking of was cleaning up all the #ifdefs for different modes
into a more ordered structure. I think there are ~24 ifdefs in mm/kasan.h and
include/linux/kasan.h and many of them could potentially be merged.

...
>
>Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

Thanks :)

-- 
Kind regards
Maciej Wieczór-Retman

Re: [PATCH v8 02/14] kasan: arm64: x86: Make special tags arch specific

[Andrey Ryabinin]

On 1/12/26 6:27 PM, 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 Ryabinin <ryabinin.a.a@gmail.com>

[PATCH v8 12/14] arm64: Unify software tag-based KASAN inline recovery path

[Maciej Wieczor-Retman]

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

To avoid having a copy of a long comment explaining the intricacies of
the inline KASAN recovery system and issues for every architecture that
uses the software tag-based mode, a unified kasan_die_unless_recover()
function was added.

Use kasan_die_unless_recover() in the kasan brk handler to cleanup the
long comment, that's kept in the non-arch KASAN code.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Alexander Potapenko <glider@google.com>
---
Changelog v7:
- Add Alexander's Acked-by tag.

Changelog v6:
- Add Catalin's Acked-by tag.

Changelog v5:
- Split arm64 portion of patch 13/18 into this one. (Peter Zijlstra)

 arch/arm64/kernel/traps.c | 17 +----------------
 1 file changed, 1 insertion(+), 16 deletions(-)

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 914282016069..e076753576e0 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -1071,22 +1071,7 @@ int kasan_brk_handler(struct pt_regs *regs, unsigned long esr)
 
 	kasan_report(addr, size, write, pc);
 
-	/*
-	 * The instrumentation allows to control whether we can proceed after
-	 * a crash was detected. This is done by passing the -recover flag to
-	 * the compiler. Disabling recovery allows to generate more compact
-	 * code.
-	 *
-	 * Unfortunately disabling recovery doesn't work for the kernel right
-	 * now. KASAN reporting is disabled in some contexts (for example when
-	 * the allocator accesses slab object metadata; this is controlled by
-	 * current->kasan_depth). All these accesses are detected by the tool,
-	 * even though the reports for them are not printed.
-	 *
-	 * This is something that might be fixed at some point in the future.
-	 */
-	if (!recover)
-		die("Oops - KASAN", regs, esr);
+	kasan_die_unless_recover(recover, "Oops - KASAN", regs, esr, die);
 
 	/* If thread survives, skip over the brk instruction and continue: */
 	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
-- 
2.52.0