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

[Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>]

On 2025-02-20 at 00:30:09 +0100, Andrey Konovalov wrote:
>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.

Not really necessary, I just thought all the general kasan patches from the
risc-v series could be added here at once. But you're right, I'll let Samuel
send these two (2nd and 3rd) patches since they relate to risc-v and not x86.

>
>>
>> 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
>>

-- 
Kind regards
Maciej Wieczór-Retman