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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 12 Sep 2018 16:54:06 +0200
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Subject: Re: [PATCH v6 04/18] khwasan, arm64: adjust shadow size for CONFIG_KASAN_HW
To:     Andrey Konovalov <andreyknvl@google.com>
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On Wed, Aug 29, 2018 at 1:35 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> KWHASAN uses 1 shadow byte for 16 bytes of kernel memory, so it requires
> 1/16th of the kernel virtual address space for the shadow memory.
>
> This commit sets KASAN_SHADOW_SCALE_SHIFT to 4 when KHWASAN is enabled.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  arch/arm64/Makefile             |  2 +-
>  arch/arm64/include/asm/memory.h | 13 +++++++++----
>  2 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 106039d25e2f..17047b8ab984 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -94,7 +94,7 @@ endif
>  # KASAN_SHADOW_OFFSET = VA_START + (1 << (VA_BITS - KASAN_SHADOW_SCALE_SHIFT))
>  #                               - (1 << (64 - KASAN_SHADOW_SCALE_SHIFT))
>  # in 32-bit arithmetic
> -KASAN_SHADOW_SCALE_SHIFT := 3
> +KASAN_SHADOW_SCALE_SHIFT := $(if $(CONFIG_KASAN_HW), 4, 3)
>  KASAN_SHADOW_OFFSET := $(shell printf "0x%08x00000000\n" $$(( \
>         (0xffffffff & (-1 << ($(CONFIG_ARM64_VA_BITS) - 32))) \
>         + (1 << ($(CONFIG_ARM64_VA_BITS) - 32 - $(KASAN_SHADOW_SCALE_SHIFT))) \
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index b96442960aea..f5e262ee76c1 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -74,12 +74,17 @@
>  #define KERNEL_END        _end
>
>  /*
> - * KASAN requires 1/8th of the kernel virtual address space for the shadow
> - * region. KASAN can bloat the stack significantly, so double the (minimum)
> - * stack size when KASAN is in use.
> + * KASAN and KHWASAN require 1/8th and 1/16th of the kernel virtual address


I am somewhat confused by the terminology.
"KASAN" is not actually "CONFIG_KASAN" below, it is actually
"CONFIG_KASAN_GENERIC". While "KHWASAN" translates to "KASAN_HW" few
lines later.
I think we need some consistent terminology for comments and config
names until it's too late.


> + * space for the shadow region respectively. They can bloat the stack
> + * significantly, so double the (minimum) stack size when they are in use.
>   */
> -#ifdef CONFIG_KASAN
> +#ifdef CONFIG_KASAN_GENERIC
>  #define KASAN_SHADOW_SCALE_SHIFT 3
> +#endif
> +#ifdef CONFIG_KASAN_HW
> +#define KASAN_SHADOW_SCALE_SHIFT 4
> +#endif
> +#ifdef CONFIG_KASAN
>  #define KASAN_SHADOW_SIZE      (UL(1) << (VA_BITS - KASAN_SHADOW_SCALE_SHIFT))
>  #define KASAN_THREAD_SHIFT     1
>  #else
> --
> 2.19.0.rc0.228.g281dcd1b4d0-goog
>