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

[Andrey Konovalov]

On Thu, Feb 20, 2025 at 12:31 AM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
>
> 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
> >

Also please update the descriptions of all related options in lib/Kconfig.kasan.