Re: [PATCH v2 3/4] arm64: mte: Enable async tag check fault

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

On 1/13/21 6:11 PM, Catalin Marinas wrote:
> On Thu, Jan 07, 2021 at 05:29:07PM +0000, Vincenzo Frascino wrote:
>> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
>> index d02aff9f493d..a60d3718baae 100644
>> --- a/arch/arm64/include/asm/mte.h
>> +++ b/arch/arm64/include/asm/mte.h
>> @@ -39,6 +39,7 @@ void mte_free_tag_storage(char *storage);
>>  /* track which pages have valid allocation tags */
>>  #define PG_mte_tagged	PG_arch_2
>>  
>> +void mte_check_tfsr_el1(void);
>>  void mte_sync_tags(pte_t *ptep, pte_t pte);
>>  void mte_copy_page_tags(void *kto, const void *kfrom);
>>  void flush_mte_state(void);
>> @@ -56,6 +57,9 @@ void mte_assign_mem_tag_range(void *addr, size_t size);
>>  /* unused if !CONFIG_ARM64_MTE, silence the compiler */
>>  #define PG_mte_tagged	0
>>  
>> +static inline void mte_check_tfsr_el1(void)
>> +{
>> +}
> 
> I think we should enable this dummy function when !CONFIG_KASAN_HW_TAGS.
> It saves us an unnecessary function call in a few places.
> 

Ok, I will add it in v3.

>>  static inline void mte_sync_tags(pte_t *ptep, pte_t pte)
>>  {
>>  }
>> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
>> index 5346953e4382..74b020ce72d7 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_el1();
>>  }
>>  
>>  /*
>> @@ -47,6 +49,8 @@ static void noinstr exit_to_kernel_mode(struct pt_regs *regs)
>>  {
>>  	lockdep_assert_irqs_disabled();
>>  
>> +	mte_check_tfsr_el1();
>> +
>>  	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_el1();
> 
> While for kernel entry the asynchronous faults are sync'ed automatically
> with TFSR_EL1, we don't have this for exit, so we'd need an explicit
> DSB. But rather than placing it here, it's better if we add a bool sync
> argument to mte_check_tfsr_el1() which issues a dsb() before checking
> the register. I think that's the only place where such argument would be
> true (for now).
> 

Good point, I will add the dsb() in mte_check_tfsr_el1() but instead of a bool
parameter I will add something more explicit.

>> +
>>  	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 5d992e16b420..26030f0b79fe 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -185,6 +185,34 @@ void mte_enable_kernel(enum kasan_arg_mode mode)
>>  	isb();
>>  }
>>  
>> +void mte_check_tfsr_el1(void)
>> +{
>> +	u64 tfsr_el1;
>> +
>> +	if (!IS_ENABLED(CONFIG_KASAN_HW_TAGS))
>> +		return;
> 
> If we define the static inline when !CONFIG_KASAN_HW_TAGS, we could add
> the #ifdef here around the whole function.
>

Ok. I will add it in v3.

>> +	if (!system_supports_mte())
>> +		return;
>> +
>> +	tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
>> +
>> +	/*
>> +	 * The kernel should never hit the condition TF0 == 1
>> +	 * at this point because for the futex code we set
>> +	 * PSTATE.TCO.
>> +	 */
>> +	WARN_ON(tfsr_el1 & SYS_TFSR_EL1_TF0);
>> +
>> +	if (tfsr_el1 & SYS_TFSR_EL1_TF1) {
>> +		write_sysreg_s(0, SYS_TFSR_EL1);
>> +		isb();
>> +
>> +		pr_err("MTE: Asynchronous tag exception detected!");
>> +	}
>> +}
>> +NOKPROBE_SYMBOL(mte_check_tfsr_el1);
> 
> Do we need this to be NOKPROBE_SYMBOL? It's not that low level.
>
It is an inheritance from when I had this code called very early. I will remove
it in the next version.

>> +
>>  static void update_sctlr_el1_tcf0(u64 tcf0)
>>  {
>>  	/* ISB required for the kernel uaccess routines */
>> @@ -250,6 +278,15 @@ 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);
>> +
>> +	/*
>> +	 * Check if an async tag exception occurred at EL1.
>> +	 *
>> +	 * Note: On the context switch patch we rely on the dsb() present
> 
> s/patch/path/
> 
>> +	 * in __switch_to() to guaranty that the indirect writes to TFSR_EL1
> 
> s/guaranty/guarantee/ (well, still valid though I think rarely used).
> 
>> +	 * are synchronized before this point.
>> +	 */
>> +	mte_check_tfsr_el1();
>>  }
>>  
>>  void mte_suspend_exit(void)
>> -- 
>> 2.30.0
> 

-- 
Regards,
Vincenzo

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