Re: [PATCH 4/5] arm64: mte: Add asymmetric mode support

[Andrey Konovalov]

On Mon, Sep 13, 2021 at 10:14 AM Vincenzo Frascino
<vincenzo.frascino@xxxxxxx> wrote:
>
> MTE provides an asymmetric mode for detecting tag exceptions. In
> particular, when such a mode is present, the CPU triggers a fault
> on a tag mismatch during a load operation and asynchronously updates
> a register when a tag mismatch is detected during a store operation.
>
> Add support for MTE asymmetric mode.
>
> Note: If the CPU does not support MTE asymmetric mode the kernel falls
> back on synchronous mode which is the default for kasan=on.
>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  arch/arm64/include/asm/memory.h    |  1 +
>  arch/arm64/include/asm/mte-kasan.h |  5 +++++
>  arch/arm64/kernel/mte.c            | 26 ++++++++++++++++++++++++++
>  3 files changed, 32 insertions(+)
>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index f1745a843414..1b9a1e242612 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -243,6 +243,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
>  #ifdef CONFIG_KASAN_HW_TAGS
>  #define arch_enable_tagging_sync()             mte_enable_kernel_sync()
>  #define arch_enable_tagging_async()            mte_enable_kernel_async()
> +#define arch_enable_tagging_asymm()            mte_enable_kernel_asymm()
>  #define arch_force_async_tag_fault()           mte_check_tfsr_exit()
>  #define arch_get_random_tag()                  mte_get_random_tag()
>  #define arch_get_mem_tag(addr)                 mte_get_mem_tag(addr)
> diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
> index 22420e1f8c03..478b9bcf69ad 100644
> --- a/arch/arm64/include/asm/mte-kasan.h
> +++ b/arch/arm64/include/asm/mte-kasan.h
> @@ -130,6 +130,7 @@ 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);
>
>  #else /* CONFIG_ARM64_MTE */
>
> @@ -161,6 +162,10 @@ static inline void mte_enable_kernel_async(void)
>  {
>  }
>
> +static inline void mte_enable_kernel_asymm(void)
> +{
> +}
> +
>  #endif /* CONFIG_ARM64_MTE */
>
>  #endif /* __ASSEMBLY__ */
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 9d314a3bad3b..ef5484ecb2da 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -137,6 +137,32 @@ void mte_enable_kernel_async(void)
>         if (!system_uses_mte_async_mode())
>                 static_branch_enable(&mte_async_mode);
>  }
> +
> +void mte_enable_kernel_asymm(void)
> +{
> +       if (cpus_have_cap(ARM64_MTE_ASYMM)) {
> +               __mte_enable_kernel("asymmetric", SCTLR_ELx_TCF_ASYMM);
> +
> +               /*
> +                * MTE asymm mode behaves as async mode for store
> +                * operations. The mode is set system wide by the
> +                * first PE that executes this function.
> +                *
> +                * Note: If in future KASAN acquires a runtime switching
> +                * mode in between sync and async, this strategy needs
> +                * to be reviewed.
> +                */
> +               if (!system_uses_mte_async_mode())
> +                       static_branch_enable(&mte_async_mode);

This part is confusing: mte_async_mode gets enabled for the asymm
mode, which contradicts the comment next to the mte_async_mode
definition.

> +       } else {
> +               /*
> +                * If the CPU does not support MTE asymmetric mode the
> +                * kernel falls back on synchronous mode which is the
> +                * default for kasan=on.
> +                */
> +               mte_enable_kernel_sync();
> +       }
> +}
>  #endif
>
>  #ifdef CONFIG_KASAN_HW_TAGS
> --
> 2.33.0
>