Re: [PATCH v13 4/7] arm64: mte: Enable TCO in functions that can read beyond buffer limits

[Catalin Marinas]

On Thu, Feb 11, 2021 at 03:33:50PM +0000, Vincenzo Frascino wrote:
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 706b7ab75f31..65ecb86dd886 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -26,6 +26,10 @@ u64 gcr_kernel_excl __ro_after_init;
>  
>  static bool report_fault_once = true;
>  
> +/* Whether the MTE asynchronous mode is enabled. */
> +DEFINE_STATIC_KEY_FALSE(mte_async_mode);
> +EXPORT_SYMBOL_GPL(mte_async_mode);
> +
>  static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
>  {
>  	pte_t old_pte = READ_ONCE(*ptep);
> @@ -119,12 +123,24 @@ static inline void __mte_enable_kernel(const char *mode, unsigned long tcf)
>  void mte_enable_kernel_sync(void)
>  {
>  	__mte_enable_kernel("synchronous", SCTLR_ELx_TCF_SYNC);
> +
> +	/*
> +	 * This function is called on each active smp core at boot
> +	 * time, hence we do not need to take cpu_hotplug_lock again.
> +	 */
> +	static_branch_disable_cpuslocked(&mte_async_mode);
>  }
>  EXPORT_SYMBOL_GPL(mte_enable_kernel_sync);
>  
>  void mte_enable_kernel_async(void)
>  {
>  	__mte_enable_kernel("asynchronous", SCTLR_ELx_TCF_ASYNC);
> +
> +	/*
> +	 * This function is called on each active smp core at boot
> +	 * time, hence we do not need to take cpu_hotplug_lock again.
> +	 */
> +	static_branch_enable_cpuslocked(&mte_async_mode);
>  }

Sorry, I missed the cpuslocked aspect before. Is there any reason you
need to use this API here? I suggested to add it to the
mte_enable_kernel_sync() because kasan may at some point do this
dynamically at run-time, so the boot-time argument doesn't hold. But
it's also incorrect as this function will be called for hot-plugged
CPUs as well after boot.

The only reason for static_branch_*_cpuslocked() is if it's called from
a region that already invoked cpus_read_lock() which I don't think is
the case here.

-- 
Catalin