Re: [PATCH v5 19/19] x86: Make software tag-based kasan available

[Andrey Konovalov <andreyknvl@gmail.com>]

On Mon, Aug 25, 2025 at 10:32 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@intel.com> wrote:
>
> 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.
>
> Set scale macro based on KASAN mode: in software tag-based mode 16 bytes
> of memory map to one shadow byte and 8 in generic mode.
>
> Disable CONFIG_KASAN_INLINE and CONFIG_KASAN_STACK when
> CONFIG_KASAN_SW_TAGS is enabled on x86 until the appropriate compiler
> support is available.
>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> ---
> Changelog v4:
> - Add x86 specific kasan_mem_to_shadow().
> - Revert x86 to the older unsigned KASAN_SHADOW_OFFSET. Do the same to
>   KASAN_SHADOW_START/END.
> - Modify scripts/gdb/linux/kasan.py to keep x86 using unsigned offset.
> - Disable inline and stack support when software tags are enabled on
>   x86.
>
> Changelog v3:
> - Remove runtime_const from previous patch and merge the rest here.
> - Move scale shift definition back to header file.
> - Add new kasan offset for software tag based mode.
> - Fix patch message typo 32 -> 16, and 16 -> 8.
> - Update lib/Kconfig.kasan with x86 now having software tag-based
>   support.
>
> Changelog v2:
> - Remove KASAN dense code.
>
>  Documentation/arch/x86/x86_64/mm.rst | 6 ++++--
>  arch/x86/Kconfig                     | 4 +++-
>  arch/x86/boot/compressed/misc.h      | 1 +
>  arch/x86/include/asm/kasan.h         | 1 +
>  arch/x86/kernel/setup.c              | 2 ++
>  lib/Kconfig.kasan                    | 3 ++-
>  scripts/gdb/linux/kasan.py           | 4 ++--
>  7 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/arch/x86/x86_64/mm.rst b/Documentation/arch/x86/x86_64/mm.rst
> index a6cf05d51bd8..ccbdbb4cda36 100644
> --- a/Documentation/arch/x86/x86_64/mm.rst
> +++ b/Documentation/arch/x86/x86_64/mm.rst
> @@ -60,7 +60,8 @@ Complete virtual memory map with 4-level page tables
>     ffffe90000000000 |  -23    TB | ffffe9ffffffffff |    1 TB | ... unused hole
>     ffffea0000000000 |  -22    TB | ffffeaffffffffff |    1 TB | virtual memory map (vmemmap_base)
>     ffffeb0000000000 |  -21    TB | ffffebffffffffff |    1 TB | ... unused hole
> -   ffffec0000000000 |  -20    TB | fffffbffffffffff |   16 TB | KASAN shadow memory
> +   ffffec0000000000 |  -20    TB | fffffbffffffffff |   16 TB | KASAN shadow memory (generic mode)
> +   fffff40000000000 |   -8    TB | fffffbffffffffff |    8 TB | KASAN shadow memory (software tag-based mode)
>    __________________|____________|__________________|_________|____________________________________________________________
>                                                                |
>                                                                | Identical layout to the 56-bit one from here on:
> @@ -130,7 +131,8 @@ Complete virtual memory map with 5-level page tables
>     ffd2000000000000 |  -11.5  PB | ffd3ffffffffffff |  0.5 PB | ... unused hole
>     ffd4000000000000 |  -11    PB | ffd5ffffffffffff |  0.5 PB | virtual memory map (vmemmap_base)
>     ffd6000000000000 |  -10.5  PB | ffdeffffffffffff | 2.25 PB | ... unused hole
> -   ffdf000000000000 |   -8.25 PB | fffffbffffffffff |   ~8 PB | KASAN shadow memory
> +   ffdf000000000000 |   -8.25 PB | fffffbffffffffff |   ~8 PB | KASAN shadow memory (generic mode)
> +   ffeffc0000000000 |   -6    PB | fffffbffffffffff |    4 PB | KASAN shadow memory (software tag-based mode)
>    __________________|____________|__________________|_________|____________________________________________________________
>                                                                |
>                                                                | Identical layout to the 47-bit one from here on:
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b8df57ac0f28..f44fec1190b6 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -69,6 +69,7 @@ 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

Do you think it would make sense to drop the parts of the series that
add int3 handling, since the inline instrumentation does not work yet
anyway?

>         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
> @@ -199,6 +200,7 @@ config X86
>         select HAVE_ARCH_JUMP_LABEL_RELATIVE
>         select HAVE_ARCH_KASAN                  if X86_64
>         select HAVE_ARCH_KASAN_VMALLOC          if X86_64
> +       select HAVE_ARCH_KASAN_SW_TAGS          if ADDRESS_MASKING
>         select HAVE_ARCH_KFENCE
>         select HAVE_ARCH_KMSAN                  if X86_64
>         select HAVE_ARCH_KGDB
> @@ -403,7 +405,7 @@ config AUDIT_ARCH
>
>  config KASAN_SHADOW_OFFSET
>         hex
> -       depends on KASAN

Line accidentally removed?

> +       default 0xeffffc0000000000 if KASAN_SW_TAGS
>         default 0xdffffc0000000000
>
>  config HAVE_INTEL_TXT
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index db1048621ea2..ded92b439ada 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -13,6 +13,7 @@
>  #undef CONFIG_PARAVIRT_SPINLOCKS
>  #undef CONFIG_KASAN
>  #undef CONFIG_KASAN_GENERIC
> +#undef CONFIG_KASAN_SW_TAGS
>
>  #define __NO_FORTIFY
>
> diff --git a/arch/x86/include/asm/kasan.h b/arch/x86/include/asm/kasan.h
> index f3e34a9754d2..385f4e9daab3 100644
> --- a/arch/x86/include/asm/kasan.h
> +++ b/arch/x86/include/asm/kasan.h
> @@ -7,6 +7,7 @@
>  #include <linux/types.h>
>  #define KASAN_SHADOW_OFFSET _AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
>  #ifdef CONFIG_KASAN_SW_TAGS
> +#define KASAN_SHADOW_SCALE_SHIFT 4
>
>  /*
>   * LLVM ABI for reporting tag mismatches in inline KASAN mode.
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 1b2edd07a3e1..5b819f84f6db 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1207,6 +1207,8 @@ void __init setup_arch(char **cmdline_p)
>
>         kasan_init();
>
> +       kasan_init_sw_tags();
> +
>         /*
>          * Sync back kernel address range.
>          *
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index f82889a830fa..9ddbc6aeb5d5 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -100,7 +100,8 @@ 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.
> diff --git a/scripts/gdb/linux/kasan.py b/scripts/gdb/linux/kasan.py
> index fca39968d308..4b86202b155f 100644
> --- a/scripts/gdb/linux/kasan.py
> +++ b/scripts/gdb/linux/kasan.py
> @@ -7,7 +7,7 @@
>  #
>
>  import gdb
> -from linux import constants, mm
> +from linux import constants, utils, mm
>  from ctypes import c_int64 as s64
>
>  def help():
> @@ -40,7 +40,7 @@ class KasanMemToShadow(gdb.Command):
>          else:
>              help()
>      def kasan_mem_to_shadow(self, addr):
> -        if constants.CONFIG_KASAN_SW_TAGS:
> +        if constants.CONFIG_KASAN_SW_TAGS and not utils.is_target_arch('x86'):

This change seems to belong to the patch that changes how the shadow
memory address is calculated.


>              addr = s64(addr)
>          return (addr >> self.p_ops.KASAN_SHADOW_SCALE_SHIFT) + self.p_ops.KASAN_SHADOW_OFFSET
>
> --
> 2.50.1
>