Message-ID: <CA+fCnZd=EQ+5b=rBQ66LkJ3Bz2GrKHvnYk0DQLbs=o9=k0C69g@mail.gmail.com>
Date: Thu, 14 Aug 2025 07:03:35 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Yeoreum Yun <yeoreum.yun@....com>, glider@...gle.com, Marco Elver <elver@...gle.com>
Cc: ryabinin.a.a@...il.com, dvyukov@...gle.com, vincenzo.frascino@....com, 
	corbet@....net, catalin.marinas@....com, will@...nel.org, 
	akpm@...ux-foundation.org, scott@...amperecomputing.com, jhubbard@...dia.com, 
	pankaj.gupta@....com, leitao@...ian.org, kaleshsingh@...gle.com, 
	maz@...nel.org, broonie@...nel.org, oliver.upton@...ux.dev, 
	james.morse@....com, ardb@...nel.org, hardevsinh.palaniya@...iconsignals.io, 
	david@...hat.com, yang@...amperecomputing.com, kasan-dev@...glegroups.com, 
	workflows@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH v2 1/2] kasan/hw-tags: introduce kasan.store_only option

On Wed, Aug 13, 2025 at 7:53 PM Yeoreum Yun <yeoreum.yun@....com> wrote:
>
> Since Armv8.9, FEATURE_MTE_STORE_ONLY feature is introduced to restrict
> raise of tag check fault on store operation only.
> Introcude KASAN store only mode based on this feature.
>
> KASAN store only mode restricts KASAN checks operation for store only and
> omits the checks for fetch/read operation when accessing memory.
> So it might be used not only debugging enviroment but also normal
> enviroment to check memory safty.
>
> This features can be controlled with "kasan.store_only" arguments.
> When "kasan.store_only=on", KASAN checks store only mode otherwise
> KASAN checks all operations.

I'm thinking if we should name this "kasan.write_only" instead of
"kasan.store_only". This would align the terms with the
"kasan.fault=panic_on_write" parameter we already have. But then it
would be different from "FEATURE_MTE_STORE_ONLY", which is what Arm
documentation uses (right?).

Marco, Alexander, any opinion?

>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  Documentation/dev-tools/kasan.rst  |  3 ++
>  arch/arm64/include/asm/memory.h    |  1 +
>  arch/arm64/include/asm/mte-kasan.h |  6 +++
>  arch/arm64/kernel/cpufeature.c     |  6 +++
>  arch/arm64/kernel/mte.c            | 14 ++++++
>  include/linux/kasan.h              |  2 +
>  mm/kasan/hw_tags.c                 | 76 +++++++++++++++++++++++++++++-
>  mm/kasan/kasan.h                   | 10 ++++
>  8 files changed, 116 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index 0a1418ab72fd..fcb70dd821ec 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -143,6 +143,9 @@ disabling KASAN altogether or controlling its features:
>    Asymmetric mode: a bad access is detected synchronously on reads and
>    asynchronously on writes.
>
> +- ``kasan.store_only=off`` or ``kasan.store_only=on`` controls whether KASAN
> +  checks the store (write) accesses only or all accesses (default: ``off``)
> +
>  - ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
>    allocations (default: ``on``).
>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 5213248e081b..ae29cd3db78d 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -308,6 +308,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
>  #define arch_enable_tag_checks_sync()          mte_enable_kernel_sync()
>  #define arch_enable_tag_checks_async()         mte_enable_kernel_async()
>  #define arch_enable_tag_checks_asymm()         mte_enable_kernel_asymm()
> +#define arch_enable_tag_checks_store_only()    mte_enable_kernel_store_only()
>  #define arch_suppress_tag_checks_start()       mte_enable_tco()
>  #define arch_suppress_tag_checks_stop()                mte_disable_tco()
>  #define arch_force_async_tag_fault()           mte_check_tfsr_exit()
> diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
> index 2e98028c1965..3e1cc341d47a 100644
> --- a/arch/arm64/include/asm/mte-kasan.h
> +++ b/arch/arm64/include/asm/mte-kasan.h
> @@ -200,6 +200,7 @@ static inline void mte_set_mem_tag_range(void *addr, size_t size, u8 tag,
>  void mte_enable_kernel_sync(void);
>  void mte_enable_kernel_async(void);
>  void mte_enable_kernel_asymm(void);
> +int mte_enable_kernel_store_only(void);
>
>  #else /* CONFIG_ARM64_MTE */
>
> @@ -251,6 +252,11 @@ static inline void mte_enable_kernel_asymm(void)
>  {
>  }
>
> +static inline int mte_enable_kenrel_store_only(void)

Typo in the function name. Please build/boot test without MTE/KASAN enabled.

> +{
> +       return -EINVAL;
> +}
> +
>  #endif /* CONFIG_ARM64_MTE */
>
>  #endif /* __ASSEMBLY__ */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9ad065f15f1d..7b724fcf20a7 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2404,6 +2404,11 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
>
>         kasan_init_hw_tags_cpu();
>  }
> +
> +static void cpu_enable_mte_store_only(struct arm64_cpu_capabilities const *cap)
> +{
> +       kasan_late_init_hw_tags_cpu();
> +}
>  #endif /* CONFIG_ARM64_MTE */
>
>  static void user_feature_fixup(void)
> @@ -2922,6 +2927,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>                 .capability = ARM64_MTE_STORE_ONLY,
>                 .type = ARM64_CPUCAP_SYSTEM_FEATURE,
>                 .matches = has_cpuid_feature,
> +               .cpu_enable = cpu_enable_mte_store_only,
>                 ARM64_CPUID_FIELDS(ID_AA64PFR2_EL1, MTESTOREONLY, IMP)
>         },
>  #endif /* CONFIG_ARM64_MTE */
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index e5e773844889..8eb1f66f2ccd 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -157,6 +157,20 @@ void mte_enable_kernel_asymm(void)
>                 mte_enable_kernel_sync();
>         }
>  }
> +
> +int mte_enable_kernel_store_only(void)
> +{
> +       if (!cpus_have_cap(ARM64_MTE_STORE_ONLY))
> +               return -EINVAL;
> +
> +       sysreg_clear_set(sctlr_el1, SCTLR_EL1_TCSO_MASK,
> +                        SYS_FIELD_PREP(SCTLR_EL1, TCSO, 1));
> +       isb();
> +
> +       pr_info_once("MTE: enabled stonly mode at EL1\n");
> +
> +       return 0;
> +}
>  #endif
>
>  #ifdef CONFIG_KASAN_HW_TAGS
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 890011071f2b..28951b29c593 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -552,9 +552,11 @@ static inline void kasan_init_sw_tags(void) { }
>  #ifdef CONFIG_KASAN_HW_TAGS
>  void kasan_init_hw_tags_cpu(void);
>  void __init kasan_init_hw_tags(void);
> +void kasan_late_init_hw_tags_cpu(void);
>  #else
>  static inline void kasan_init_hw_tags_cpu(void) { }
>  static inline void kasan_init_hw_tags(void) { }
> +static inline void kasan_late_init_hw_tags_cpu(void) { }
>  #endif
>
>  #ifdef CONFIG_KASAN_VMALLOC
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index 9a6927394b54..c2f90c06076e 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -41,9 +41,16 @@ enum kasan_arg_vmalloc {
>         KASAN_ARG_VMALLOC_ON,
>  };
>
> +enum kasan_arg_store_only {
> +       KASAN_ARG_STORE_ONLY_DEFAULT,
> +       KASAN_ARG_STORE_ONLY_OFF,
> +       KASAN_ARG_STORE_ONLY_ON,
> +};
> +
>  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;
> +static enum kasan_arg_store_only kasan_arg_store_only __ro_after_init;
>
>  /*
>   * Whether KASAN is enabled at all.
> @@ -67,6 +74,9 @@ DEFINE_STATIC_KEY_FALSE(kasan_flag_vmalloc);
>  #endif
>  EXPORT_SYMBOL_GPL(kasan_flag_vmalloc);
>
> +DEFINE_STATIC_KEY_FALSE(kasan_flag_store_only);

Is there a reason to have this as a static key? I think a normal
global bool would work, just as a normal variable works for
kasan_mode.

> +EXPORT_SYMBOL_GPL(kasan_flag_store_only);
> +
>  #define PAGE_ALLOC_SAMPLE_DEFAULT      1
>  #define PAGE_ALLOC_SAMPLE_ORDER_DEFAULT        3
>
> @@ -141,6 +151,23 @@ static int __init early_kasan_flag_vmalloc(char *arg)
>  }
>  early_param("kasan.vmalloc", early_kasan_flag_vmalloc);
>
> +/* kasan.store_only=off/on */
> +static int __init early_kasan_flag_store_only(char *arg)
> +{
> +       if (!arg)
> +               return -EINVAL;
> +
> +       if (!strcmp(arg, "off"))
> +               kasan_arg_store_only = KASAN_ARG_STORE_ONLY_OFF;
> +       else if (!strcmp(arg, "on"))
> +               kasan_arg_store_only = KASAN_ARG_STORE_ONLY_ON;
> +       else
> +               return -EINVAL;
> +
> +       return 0;
> +}
> +early_param("kasan.store_only", early_kasan_flag_store_only);
> +
>  static inline const char *kasan_mode_info(void)
>  {
>         if (kasan_mode == KASAN_MODE_ASYNC)
> @@ -219,6 +246,20 @@ void kasan_init_hw_tags_cpu(void)
>         kasan_enable_hw_tags();
>  }
>
> +/*
> + * kasan_late_init_hw_tags_cpu_post() is called for each CPU after
> + * all cpus are bring-up at boot.

"CPUs"
"brought up"

And please spell-check other comments.

> + * Not marked as __init as a CPU can be hot-plugged after boot.
> + */
> +void kasan_late_init_hw_tags_cpu(void)
> +{
> +       /*
> +        * Enable stonly mode only when explicitly requested through the command line.

"store-only"

> +        * If system doesn't support, kasan checks all operation.

"If the system doesn't support this mode, KASAN will check both load
and store operations."

> +        */
> +       kasan_enable_store_only();
> +}
> +
>  /* kasan_init_hw_tags() is called once on boot CPU. */
>  void __init kasan_init_hw_tags(void)
>  {
> @@ -257,15 +298,28 @@ void __init kasan_init_hw_tags(void)
>                 break;
>         }
>
> +       switch (kasan_arg_store_only) {
> +       case KASAN_ARG_STORE_ONLY_DEFAULT:
> +               /* Default is specified by kasan_flag_store_only definition. */
> +               break;
> +       case KASAN_ARG_STORE_ONLY_OFF:
> +               static_branch_disable(&kasan_flag_store_only);
> +               break;
> +       case KASAN_ARG_STORE_ONLY_ON:
> +               static_branch_enable(&kasan_flag_store_only);
> +               break;
> +       }

Let's move this part to kasan_late_init_hw_tags_cpu. Since that's
where the final decision of whether the store-only mode is enabled is
taken, we should just set the global flag there.

> +
>         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",
> +       pr_info("KernelAddressSanitizer initialized (hw-tags, mode=%s, vmalloc=%s, stacktrace=%s store_only=%s\n",

Let's put "store_only" here next to "mode".

You're also missing a comma.

>                 kasan_mode_info(),
>                 str_on_off(kasan_vmalloc_enabled()),
> -               str_on_off(kasan_stack_collection_enabled()));
> +               str_on_off(kasan_stack_collection_enabled()),
> +               str_on_off(kasan_store_only_enabled()));
>  }
>
>  #ifdef CONFIG_KASAN_VMALLOC
> @@ -394,6 +448,22 @@ void kasan_enable_hw_tags(void)
>                 hw_enable_tag_checks_sync();
>  }
>
> +void kasan_enable_store_only(void)

Do we need this as a separate function? I think we can just move the
code to kasan_late_init_hw_tags_cpu.

> +{
> +       if (kasan_arg_store_only == KASAN_ARG_STORE_ONLY_ON) {
> +               if (hw_enable_tag_checks_store_only()) {
> +                       static_branch_disable(&kasan_flag_store_only);
> +                       kasan_arg_store_only = KASAN_ARG_STORE_ONLY_OFF;
> +                       pr_warn_once("KernelAddressSanitizer: store only mode isn't supported (hw-tags)\n");

No need for the "KernelAddressSanitizer" prefix, it's already defined
via pr_fmt().

> +               }
> +       }
> +}
> +
> +bool kasan_store_only_enabled(void)
> +{
> +       return static_branch_unlikely(&kasan_flag_store_only);
> +}
> +
>  #if IS_ENABLED(CONFIG_KASAN_KUNIT_TEST)
>
>  EXPORT_SYMBOL_IF_KUNIT(kasan_enable_hw_tags);
> @@ -404,4 +474,6 @@ VISIBLE_IF_KUNIT void kasan_force_async_fault(void)
>  }
>  EXPORT_SYMBOL_IF_KUNIT(kasan_force_async_fault);
>
> +EXPORT_SYMBOL_IF_KUNIT(kasan_store_only_enabled);
> +
>  #endif
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 129178be5e64..1d853de1c499 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -33,6 +33,7 @@ static inline bool kasan_stack_collection_enabled(void)
>  #include "../slab.h"
>
>  DECLARE_STATIC_KEY_TRUE(kasan_flag_vmalloc);
> +DECLARE_STATIC_KEY_FALSE(kasan_flag_stonly);

kasan_flag_store_only

Did you build test this at all?


>
>  enum kasan_mode {
>         KASAN_MODE_SYNC,
> @@ -428,6 +429,7 @@ static inline const void *arch_kasan_set_tag(const void *addr, u8 tag)
>  #define hw_enable_tag_checks_sync()            arch_enable_tag_checks_sync()
>  #define hw_enable_tag_checks_async()           arch_enable_tag_checks_async()
>  #define hw_enable_tag_checks_asymm()           arch_enable_tag_checks_asymm()
> +#define hw_enable_tag_checks_store_only()      arch_enable_tag_checks_store_only()
>  #define hw_suppress_tag_checks_start()         arch_suppress_tag_checks_start()
>  #define hw_suppress_tag_checks_stop()          arch_suppress_tag_checks_stop()
>  #define hw_force_async_tag_fault()             arch_force_async_tag_fault()
> @@ -437,10 +439,18 @@ static inline const void *arch_kasan_set_tag(const void *addr, u8 tag)
>                         arch_set_mem_tag_range((addr), (size), (tag), (init))
>
>  void kasan_enable_hw_tags(void);
> +void kasan_enable_store_only(void);
> +bool kasan_store_only_enabled(void);
>
>  #else /* CONFIG_KASAN_HW_TAGS */
>
>  static inline void kasan_enable_hw_tags(void) { }
> +static inline void kasan_enable_store_only(void) { }
> +
> +static inline bool kasan_store_only_enabled(void)
> +{
> +       return false;
> +}
>
>  #endif /* CONFIG_KASAN_HW_TAGS */
>
> --
> LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}
>
> --
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