[PATCH v7 4/4] arm64: mte: Enable async tag check fault

[Andrey Konovalov]

On Fri, Jan 22, 2021 at 3:11 PM Vincenzo Frascino
<vincenzo.frascino at arm.com> wrote:
>
> MTE provides a mode that asynchronously updates the TFSR_EL1 register
> when a tag check exception is detected.
>
> To take advantage of this mode the kernel has to verify the status of
> the register at:
>   1. Context switching
>   2. Return to user/EL0 (Not required in entry from EL0 since the kernel
>   did not run)
>   3. Kernel entry from EL1
>   4. Kernel exit to EL1
>
> If the register is non-zero a trace is reported.
>
> Add the required features for EL1 detection and reporting.
>
> Note: ITFSB bit is set in the SCTLR_EL1 register hence it guaranties that
> the indirect writes to TFSR_EL1 are synchronized at exception entry to
> EL1. On the context switch path the synchronization is guarantied by the
> dsb() in __switch_to().
> The dsb(nsh) in mte_check_tfsr_exit() is provisional pending
> confirmation by the architects.
>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
> ---
>  arch/arm64/include/asm/mte.h     | 32 +++++++++++++++++++++++
>  arch/arm64/kernel/entry-common.c |  6 +++++
>  arch/arm64/kernel/mte.c          | 44 ++++++++++++++++++++++++++++++++
>  3 files changed, 82 insertions(+)
>
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index d02aff9f493d..237bb2f7309d 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -92,5 +92,37 @@ static inline void mte_assign_mem_tag_range(void *addr, size_t size)
>
>  #endif /* CONFIG_ARM64_MTE */
>
> +#ifdef CONFIG_KASAN_HW_TAGS
> +void mte_check_tfsr_el1(void);
> +
> +static inline void mte_check_tfsr_entry(void)
> +{
> +       mte_check_tfsr_el1();
> +}
> +
> +static inline void mte_check_tfsr_exit(void)
> +{
> +       /*
> +        * The asynchronous faults are sync'ed automatically with
> +        * TFSR_EL1 on kernel entry but for exit an explicit dsb()
> +        * is required.
> +        */
> +       dsb(nsh);
> +       isb();
> +
> +       mte_check_tfsr_el1();
> +}
> +#else
> +static inline void mte_check_tfsr_el1(void)
> +{
> +}
> +static inline void mte_check_tfsr_entry(void)
> +{
> +}
> +static inline void mte_check_tfsr_exit(void)
> +{
> +}
> +#endif /* CONFIG_KASAN_HW_TAGS */
> +
>  #endif /* __ASSEMBLY__ */
>  #endif /* __ASM_MTE_H  */
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 5346953e4382..31666511ba67 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -37,6 +37,8 @@ static void noinstr enter_from_kernel_mode(struct pt_regs *regs)
>         lockdep_hardirqs_off(CALLER_ADDR0);
>         rcu_irq_enter_check_tick();
>         trace_hardirqs_off_finish();
> +
> +       mte_check_tfsr_entry();
>  }
>
>  /*
> @@ -47,6 +49,8 @@ static void noinstr exit_to_kernel_mode(struct pt_regs *regs)
>  {
>         lockdep_assert_irqs_disabled();
>
> +       mte_check_tfsr_exit();
> +
>         if (interrupts_enabled(regs)) {
>                 if (regs->exit_rcu) {
>                         trace_hardirqs_on_prepare();
> @@ -243,6 +247,8 @@ asmlinkage void noinstr enter_from_user_mode(void)
>
>  asmlinkage void noinstr exit_to_user_mode(void)
>  {
> +       mte_check_tfsr_exit();
> +
>         trace_hardirqs_on_prepare();
>         lockdep_hardirqs_on_prepare(CALLER_ADDR0);
>         user_enter_irqoff();
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 92078e1eb627..7763ac1f2917 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -182,6 +182,37 @@ bool mte_report_once(void)
>         return READ_ONCE(report_fault_once);
>  }
>
> +#ifdef CONFIG_KASAN_HW_TAGS
> +void mte_check_tfsr_el1(void)
> +{
> +       u64 tfsr_el1;
> +
> +       if (!system_supports_mte())
> +               return;
> +
> +       tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
> +
> +       /*
> +        * The kernel should never trigger an asynchronous fault on a
> +        * TTBR0 address, so we should never see TF0 set.
> +        * For futexes we disable checks via PSTATE.TCO.
> +        */
> +       WARN_ONCE(tfsr_el1 & SYS_TFSR_EL1_TF0,
> +                 "Kernel async tag fault on TTBR0 address");
> +
> +       if (unlikely(tfsr_el1 & SYS_TFSR_EL1_TF1)) {
> +               /*
> +                * Note: isb() is not required after this direct write
> +                * because there is no indirect read subsequent to it
> +                * (per ARM DDI 0487F.c table D13-1).
> +                */
> +               write_sysreg_s(0, SYS_TFSR_EL1);
> +
> +               kasan_report_async();
> +       }
> +}
> +#endif
> +
>  static void update_sctlr_el1_tcf0(u64 tcf0)
>  {
>         /* ISB required for the kernel uaccess routines */
> @@ -247,6 +278,19 @@ void mte_thread_switch(struct task_struct *next)
>         /* avoid expensive SCTLR_EL1 accesses if no change */
>         if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
>                 update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
> +       else
> +               isb();
> +
> +       /*
> +        * Check if an async tag exception occurred at EL1.
> +        *
> +        * Note: On the context switch path we rely on the dsb() present
> +        * in __switch_to() to guarantee that the indirect writes to TFSR_EL1
> +        * are synchronized before this point.
> +        * isb() above is required for the same reason.
> +        *
> +        */
> +       mte_check_tfsr_el1();
>  }
>
>  void mte_suspend_exit(void)
> --
> 2.30.0
>

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