Re: [PATCH 2/4] arm64: mte: Add asynchronous mode support

[Andrey Konovalov]

On Wed, Jan 6, 2021 at 12:56 PM Vincenzo Frascino
<vincenzo.frascino@xxxxxxx> wrote:
>
> MTE provides an asynchronous mode for detecting tag exceptions. In
> particular instead of triggering a fault the arm64 core updates a
> register which is checked by the kernel at the first entry after the tag
> exception has occurred.
>
> Add support for MTE asynchronous mode.
>
> The exception handling mechanism will be added with a future patch.
>
> Note: KASAN HW activates async mode via kasan.mode kernel parameter.
> The default mode is set to synchronous.
>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  arch/arm64/kernel/mte.c | 31 +++++++++++++++++++++++++++++--
>  1 file changed, 29 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 24a273d47df1..5d992e16b420 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -153,8 +153,35 @@ void mte_init_tags(u64 max_tag)
>
>  void mte_enable_kernel(enum kasan_arg_mode mode)
>  {
> -       /* Enable MTE Sync Mode for EL1. */
> -       sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> +       const char *m;
> +
> +       /* Preset parameter values based on the mode. */
> +       switch (mode) {
> +       case KASAN_ARG_MODE_OFF:
> +               return;
> +       case KASAN_ARG_MODE_LIGHT:
> +               /* Enable MTE Async Mode for EL1. */
> +               sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_ASYNC);
> +               m = "asynchronous";
> +               break;
> +       case KASAN_ARG_MODE_DEFAULT:
> +       case KASAN_ARG_MODE_PROD:
> +       case KASAN_ARG_MODE_FULL:
> +               /* Enable MTE Sync Mode for EL1. */
> +               sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> +               m = "synchronous";
> +               break;
> +       default:
> +               /*
> +                * kasan mode should be always set hence we should
> +                * not reach this condition.
> +                */
> +               WARN_ON_ONCE(1);
> +               return;
> +       }
> +
> +       pr_info_once("MTE: enabled in %s mode at EL1\n", m);
> +
>         isb();
>  }
>
> --
> 2.29.2
>

Hi Vincenzo,

It would be cleaner to pass a bool to mte_enable_kernel() and have it
indicate sync/async mode. This way you don't have to pull all these
KASAN constants into the arm64 code.

Thanks!