Re: [PATCH v2 02/14] kasan: sw_tags: Check kasan_flag_enabled at runtime

[Andrey Konovalov <andreyknvl@gmail.com>]

On Tue, Feb 18, 2025 at 9:16 AM Maciej Wieczor-Retman
<maciej.wieczor-retman@intel.com> wrote:
>
> From: Samuel Holland <samuel.holland@sifive.com>
>
> On RISC-V, the ISA extension required to dereference tagged pointers is
> optional, and the interface to enable pointer masking requires firmware
> support. Therefore, we must detect at runtime if sw_tags is usable on a
> given machine. Reuse the logic from hw_tags to dynamically enable KASAN.

Is this patch required on x86 as well? If so, I think it makes sense
to point it out here. And do the same in messages for other commits
that now mention RISC-V.


>
> This commit makes no functional change to the KASAN_HW_TAGS code path.
>
> Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> ---
>  include/linux/kasan-enabled.h | 15 +++++----------
>  mm/kasan/hw_tags.c            | 10 ----------
>  mm/kasan/tags.c               | 10 ++++++++++
>  3 files changed, 15 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/kasan-enabled.h b/include/linux/kasan-enabled.h
> index 6f612d69ea0c..648bda9495b7 100644
> --- a/include/linux/kasan-enabled.h
> +++ b/include/linux/kasan-enabled.h
> @@ -4,7 +4,7 @@
>
>  #include <linux/static_key.h>
>
> -#ifdef CONFIG_KASAN_HW_TAGS
> +#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
>
>  DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
>
> @@ -13,23 +13,18 @@ static __always_inline bool kasan_enabled(void)
>         return static_branch_likely(&kasan_flag_enabled);
>  }
>
> -static inline bool kasan_hw_tags_enabled(void)
> -{
> -       return kasan_enabled();
> -}
> -
> -#else /* CONFIG_KASAN_HW_TAGS */
> +#else /* CONFIG_KASAN_SW_TAGS || CONFIG_KASAN_HW_TAGS */
>
>  static inline bool kasan_enabled(void)
>  {
>         return IS_ENABLED(CONFIG_KASAN);
>  }
>
> +#endif /* CONFIG_KASAN_SW_TAGS || CONFIG_KASAN_HW_TAGS */
> +
>  static inline bool kasan_hw_tags_enabled(void)
>  {
> -       return false;
> +       return IS_ENABLED(CONFIG_KASAN_HW_TAGS) && kasan_enabled();
>  }
>
> -#endif /* CONFIG_KASAN_HW_TAGS */
> -
>  #endif /* LINUX_KASAN_ENABLED_H */
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index 9a6927394b54..7f82af13b6a6 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -45,13 +45,6 @@ static enum kasan_arg kasan_arg __ro_after_init;
>  static enum kasan_arg_mode kasan_arg_mode __ro_after_init;
>  static enum kasan_arg_vmalloc kasan_arg_vmalloc __initdata;
>
> -/*
> - * Whether KASAN is enabled at all.
> - * The value remains false until KASAN is initialized by kasan_init_hw_tags().
> - */
> -DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
> -EXPORT_SYMBOL(kasan_flag_enabled);
> -
>  /*
>   * Whether the selected mode is synchronous, asynchronous, or asymmetric.
>   * Defaults to KASAN_MODE_SYNC.
> @@ -259,9 +252,6 @@ void __init kasan_init_hw_tags(void)
>
>         kasan_init_tags();
>
> -       /* KASAN is now initialized, enable it. */
> -       static_branch_enable(&kasan_flag_enabled);
> -
>         pr_info("KernelAddressSanitizer initialized (hw-tags, mode=%s, vmalloc=%s, stacktrace=%s)\n",
>                 kasan_mode_info(),
>                 str_on_off(kasan_vmalloc_enabled()),
> diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
> index d65d48b85f90..c111d98961ed 100644
> --- a/mm/kasan/tags.c
> +++ b/mm/kasan/tags.c
> @@ -32,6 +32,13 @@ enum kasan_arg_stacktrace {
>
>  static enum kasan_arg_stacktrace kasan_arg_stacktrace __initdata;
>
> +/*
> + * Whether KASAN is enabled at all.
> + * The value remains false until KASAN is initialized by kasan_init_tags().
> + */
> +DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
> +EXPORT_SYMBOL(kasan_flag_enabled);
> +
>  /* Whether to collect alloc/free stack traces. */
>  DEFINE_STATIC_KEY_TRUE(kasan_flag_stacktrace);
>
> @@ -92,6 +99,9 @@ void __init kasan_init_tags(void)
>                 if (WARN_ON(!stack_ring.entries))
>                         static_branch_disable(&kasan_flag_stacktrace);
>         }
> +
> +       /* KASAN is now initialized, enable it. */
> +       static_branch_enable(&kasan_flag_enabled);
>  }
>
>  static void save_stack_info(struct kmem_cache *cache, void *object,
> --
> 2.47.1
>