[PATCH] arm64: kasan: mte: move GCR_EL1 switch to task switch when KASAN disabled

[Andrey Konovalov]

On Fri, Aug 27, 2021 at 5:07 AM Peter Collingbourne <pcc at google.com> wrote:
>
> It is not necessary to write to GCR_EL1 on every kernel entry and
> exit when HW tag-based KASAN is disabled because the kernel will not
> execute any IRG instructions in that mode. Since accessing GCR_EL1
> can be expensive on some microarchitectures, avoid doing so by moving
> the access to task switch when HW tag-based KASAN is disabled.
>
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> ---
>  arch/arm64/kernel/entry.S | 10 +++++-----
>  arch/arm64/kernel/mte.c   | 24 +++++++++++++++++++++---
>  include/linux/kasan.h     |  9 +++++++--
>  3 files changed, 33 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 923ee2ac85fd..e12ed892cde8 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -178,9 +178,9 @@ alternative_else_nop_endif
>
>         .macro mte_set_kernel_gcr, tmp, tmp2
>  #ifdef CONFIG_KASAN_HW_TAGS
> -alternative_if_not ARM64_MTE
> +alternative_cb kasan_hw_tags_enable
>         b       1f
> -alternative_else_nop_endif
> +alternative_cb_end
>         mov     \tmp, KERNEL_GCR_EL1
>         msr_s   SYS_GCR_EL1, \tmp
>  1:
> @@ -188,10 +188,10 @@ alternative_else_nop_endif
>         .endm
>
>         .macro mte_set_user_gcr, tsk, tmp, tmp2
> -#ifdef CONFIG_ARM64_MTE
> -alternative_if_not ARM64_MTE
> +#ifdef CONFIG_KASAN_HW_TAGS
> +alternative_cb kasan_hw_tags_enable
>         b       1f
> -alternative_else_nop_endif
> +alternative_cb_end
>         ldr     \tmp, [\tsk, #THREAD_MTE_CTRL]
>
>         mte_set_gcr \tmp, \tmp2
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 9d314a3bad3b..0e8edb5d5861 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -162,7 +162,7 @@ void mte_check_tfsr_el1(void)
>  }
>  #endif
>
> -static void mte_update_sctlr_user(struct task_struct *task)
> +static void mte_update_sctlr_user_and_gcr_excl(struct task_struct *task)
>  {
>         /*
>          * This must be called with preemption disabled and can only be called
> @@ -182,6 +182,24 @@ static void mte_update_sctlr_user(struct task_struct *task)
>         else if (resolved_mte_tcf & MTE_CTRL_TCF_SYNC)
>                 sctlr |= SCTLR_EL1_TCF0_SYNC;
>         task->thread.sctlr_user = sctlr;
> +
> +       /*
> +        * SYS_GCR_EL1 will be set to current->thread.mte_ctrl value by
> +        * mte_set_user_gcr() in kernel_exit, but only if KASAN is enabled.
> +        */
> +       if (!kasan_hw_tags_enabled())
> +               write_sysreg_s(((mte_ctrl >> MTE_CTRL_GCR_USER_EXCL_SHIFT) &
> +                               SYS_GCR_EL1_EXCL_MASK) | SYS_GCR_EL1_RRND,
> +                              SYS_GCR_EL1);
> +}
> +
> +void __init kasan_hw_tags_enable(struct alt_instr *alt, __le32 *origptr,
> +                                __le32 *updptr, int nr_inst)
> +{
> +       BUG_ON(nr_inst != 1); /* Branch -> NOP */
> +
> +       if (kasan_hw_tags_enabled())
> +               *updptr = cpu_to_le32(aarch64_insn_gen_nop());
>  }
>
>  void mte_thread_init_user(void)
> @@ -199,7 +217,7 @@ void mte_thread_init_user(void)
>
>  void mte_thread_switch(struct task_struct *next)
>  {
> -       mte_update_sctlr_user(next);
> +       mte_update_sctlr_user_and_gcr_excl(next);
>
>         /*
>          * Check if an async tag exception occurred at EL1.
> @@ -244,7 +262,7 @@ long set_mte_ctrl(struct task_struct *task, unsigned long arg)
>         task->thread.mte_ctrl = mte_ctrl;
>         if (task == current) {
>                 preempt_disable();
> -               mte_update_sctlr_user(task);
> +               mte_update_sctlr_user_and_gcr_excl(task);
>                 update_sctlr_el1(task->thread.sctlr_user);
>                 preempt_enable();
>         }
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index dd874a1ee862..de5f5913374d 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -89,7 +89,7 @@ static __always_inline bool kasan_enabled(void)
>         return static_branch_likely(&kasan_flag_enabled);
>  }
>
> -static inline bool kasan_has_integrated_init(void)
> +static inline bool kasan_hw_tags_enabled(void)
>  {
>         return kasan_enabled();
>  }
> @@ -104,7 +104,7 @@ static inline bool kasan_enabled(void)
>         return IS_ENABLED(CONFIG_KASAN);
>  }
>
> -static inline bool kasan_has_integrated_init(void)
> +static inline bool kasan_hw_tags_enabled(void)
>  {
>         return false;
>  }
> @@ -125,6 +125,11 @@ static __always_inline void kasan_free_pages(struct page *page,
>
>  #endif /* CONFIG_KASAN_HW_TAGS */
>
> +static inline bool kasan_has_integrated_init(void)
> +{
> +       return kasan_hw_tags_enabled();
> +}
> +
>  #ifdef CONFIG_KASAN
>
>  struct kasan_cache {
> --
> 2.33.0.259.gc128427fd7-goog
>

Acked-by: Andrey Konovalov <andreyknvl at gmail.com>

for non-arm-specific KASAN parts.