Re: [PATCH v2 14/14] x86: Make software tag-based kasan available

[Andrey Konovalov]

On Tue, Feb 18, 2025 at 9:20 AM Maciej Wieczor-Retman
<maciej.wieczor-retman@xxxxxxxxx> 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 32 bytes
> of memory map to one shadow byte and 16 in generic mode.

These should be 16 and 8.

>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
> ---
> Changelog v2:
> - Remove KASAN dense code.
>
>  arch/x86/Kconfig                | 6 ++++++
>  arch/x86/boot/compressed/misc.h | 1 +
>  arch/x86/include/asm/kasan.h    | 2 +-
>  arch/x86/kernel/setup.c         | 2 ++
>  4 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index f4ef64bf824a..dc48eb5b664f 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -195,6 +195,7 @@ 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
> @@ -402,6 +403,11 @@ config KASAN_SHADOW_OFFSET
>         hex
>         default 0xdffffc0000000000 if KASAN_GENERIC
>
> +config KASAN_SHADOW_SCALE_SHIFT
> +       int
> +       default 4 if KASAN_SW_TAGS
> +       default 3

What's the purpose of this config option? I think we can just change
the value of the KASAN_SHADOW_SCALE_SHIFT define when KASAN_SW_TAGS is
enabled.


> +
>  config HAVE_INTEL_TXT
>         def_bool y
>         depends on INTEL_IOMMU && ACPI
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index dd8d1a85f671..f6a87e9ad200 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 4bfd3641af84..cfc31e4a2f70 100644
> --- a/arch/x86/include/asm/kasan.h
> +++ b/arch/x86/include/asm/kasan.h
> @@ -6,7 +6,7 @@
>  #include <linux/kasan-tags.h>
>  #include <linux/types.h>
>
> -#define KASAN_SHADOW_SCALE_SHIFT 3
> +#define KASAN_SHADOW_SCALE_SHIFT CONFIG_KASAN_SHADOW_SCALE_SHIFT
>
>  /*
>   * Compiler uses shadow offset assuming that addresses start
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index cebee310e200..768990c573ea 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1124,6 +1124,8 @@ void __init setup_arch(char **cmdline_p)
>
>         kasan_init();
>
> +       kasan_init_sw_tags();
> +
>         /*
>          * Sync back kernel address range.
>          *
> --
> 2.47.1
>