Re: [PATCH v3] arm64: mte: Move MTE TCF0 check in entry-common

[Mark Rutland]

Hi Vincenzo,

On Fri, Apr 09, 2021 at 02:24:19PM +0100, Vincenzo Frascino wrote:
> The check_mte_async_tcf macro sets the TIF flag non-atomically. This can
> race with another CPU doing a set_tsk_thread_flag() and all the other flags
> can be lost in the process.
> 
> Move the tcf0 check to enter_from_user_mode() and clear tcf0 in
> exit_to_user_mode() to address the problem.
> 
> Note: Moving the check in entry-common allows to use set_thread_flag()
> which is safe.
> 
> Fixes: 637ec831ea4f ("arm64: mte: Handle synchronous and asynchronous tag check faults")
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  arch/arm64/include/asm/mte.h     |  9 +++++++++
>  arch/arm64/kernel/entry-common.c |  6 ++++++
>  arch/arm64/kernel/entry.S        | 34 --------------------------------
>  arch/arm64/kernel/mte.c          | 33 +++++++++++++++++++++++++++++--
>  4 files changed, 46 insertions(+), 36 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index 9b557a457f24..c7ab681a95c3 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -49,6 +49,9 @@ int mte_ptrace_copy_tags(struct task_struct *child, long request,
>  
>  void mte_assign_mem_tag_range(void *addr, size_t size);
>  
> +void noinstr check_mte_async_tcf0(void);
> +void noinstr clear_mte_async_tcf0(void);

Can we please put the implementations in the header so that they can be
inlined? Otherwise when the HW doesn't support MTE we'll always do a pointless
branch to the out-of-line implementation.

With that, we can mark them __always_inline to avoid weirdness with an inline
noinstr function.

Otherwise, this looks good to me.

Thanks,
Mark.

> +
>  #else /* CONFIG_ARM64_MTE */
>  
>  /* unused if !CONFIG_ARM64_MTE, silence the compiler */
> @@ -83,6 +86,12 @@ static inline int mte_ptrace_copy_tags(struct task_struct *child,
>  {
>  	return -EIO;
>  }
> +static inline void check_mte_async_tcf0(void)
> +{
> +}
> +static inline void clear_mte_async_tcf0(void)
> +{
> +}
>  
>  static inline void mte_assign_mem_tag_range(void *addr, size_t size)
>  {
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 9d3588450473..837d3624a1d5 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -289,10 +289,16 @@ asmlinkage void noinstr enter_from_user_mode(void)
>  	CT_WARN_ON(ct_state() != CONTEXT_USER);
>  	user_exit_irqoff();
>  	trace_hardirqs_off_finish();
> +
> +	/* Check for asynchronous tag check faults in user space */
> +	check_mte_async_tcf0();



>  }
>  
>  asmlinkage void noinstr exit_to_user_mode(void)
>  {
> +	/* Ignore asynchronous tag check faults in the uaccess routines */
> +	clear_mte_async_tcf0();
> +
>  	trace_hardirqs_on_prepare();
>  	lockdep_hardirqs_on_prepare(CALLER_ADDR0);
>  	user_enter_irqoff();
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index a31a0a713c85..fb57df0d453f 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -34,15 +34,11 @@
>   * user and kernel mode.
>   */
>  	.macro user_exit_irqoff
> -#if defined(CONFIG_CONTEXT_TRACKING) || defined(CONFIG_TRACE_IRQFLAGS)
>  	bl	enter_from_user_mode
> -#endif
>  	.endm
>  
>  	.macro user_enter_irqoff
> -#if defined(CONFIG_CONTEXT_TRACKING) || defined(CONFIG_TRACE_IRQFLAGS)
>  	bl	exit_to_user_mode
> -#endif
>  	.endm
>  
>  	.macro	clear_gp_regs
> @@ -147,32 +143,6 @@ alternative_cb_end
>  .L__asm_ssbd_skip\@:
>  	.endm
>  
> -	/* Check for MTE asynchronous tag check faults */
> -	.macro check_mte_async_tcf, flgs, tmp
> -#ifdef CONFIG_ARM64_MTE
> -alternative_if_not ARM64_MTE
> -	b	1f
> -alternative_else_nop_endif
> -	mrs_s	\tmp, SYS_TFSRE0_EL1
> -	tbz	\tmp, #SYS_TFSR_EL1_TF0_SHIFT, 1f
> -	/* Asynchronous TCF occurred for TTBR0 access, set the TI flag */
> -	orr	\flgs, \flgs, #_TIF_MTE_ASYNC_FAULT
> -	str	\flgs, [tsk, #TSK_TI_FLAGS]
> -	msr_s	SYS_TFSRE0_EL1, xzr
> -1:
> -#endif
> -	.endm
> -
> -	/* Clear the MTE asynchronous tag check faults */
> -	.macro clear_mte_async_tcf
> -#ifdef CONFIG_ARM64_MTE
> -alternative_if ARM64_MTE
> -	dsb	ish
> -	msr_s	SYS_TFSRE0_EL1, xzr
> -alternative_else_nop_endif
> -#endif
> -	.endm
> -
>  	.macro mte_set_gcr, tmp, tmp2
>  #ifdef CONFIG_ARM64_MTE
>  	/*
> @@ -243,8 +213,6 @@ alternative_else_nop_endif
>  	ldr	x19, [tsk, #TSK_TI_FLAGS]
>  	disable_step_tsk x19, x20
>  
> -	/* Check for asynchronous tag check faults in user space */
> -	check_mte_async_tcf x19, x22
>  	apply_ssbd 1, x22, x23
>  
>  	ptrauth_keys_install_kernel tsk, x20, x22, x23
> @@ -775,8 +743,6 @@ SYM_CODE_START_LOCAL(ret_to_user)
>  	cbnz	x2, work_pending
>  finish_ret_to_user:
>  	user_enter_irqoff
> -	/* Ignore asynchronous tag check faults in the uaccess routines */
> -	clear_mte_async_tcf
>  	enable_step_tsk x19, x2
>  #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>  	bl	stackleak_erase
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index b3c70a612c7a..84a942c25870 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -166,14 +166,43 @@ static void set_gcr_el1_excl(u64 excl)
>  	 */
>  }
>  
> -void flush_mte_state(void)
> +void noinstr check_mte_async_tcf0(void)
> +{
> +	u64 tcf0;
> +
> +	if (!system_supports_mte())
> +		return;
> +
> +	/*
> +	 * dsb(ish) is not required before the register read
> +	 * because the TFSRE0_EL1 is automatically synchronized
> +	 * by the hardware on exception entry as SCTLR_EL1.ITFSB
> +	 * is set.
> +	 */
> +	tcf0 = read_sysreg_s(SYS_TFSRE0_EL1);
> +
> +	if (tcf0 & SYS_TFSR_EL1_TF0)
> +		set_thread_flag(TIF_MTE_ASYNC_FAULT);
> +
> +	write_sysreg_s(0, SYS_TFSRE0_EL1);
> +}
> +
> +void noinstr clear_mte_async_tcf0(void)
>  {
>  	if (!system_supports_mte())
>  		return;
>  
> -	/* clear any pending asynchronous tag fault */
>  	dsb(ish);
>  	write_sysreg_s(0, SYS_TFSRE0_EL1);
> +}
> +
> +void flush_mte_state(void)
> +{
> +	if (!system_supports_mte())
> +		return;
> +
> +	/* clear any pending asynchronous tag fault */
> +	clear_mte_async_tcf0();
>  	clear_thread_flag(TIF_MTE_ASYNC_FAULT);
>  	/* disable tag checking */
>  	set_sctlr_el1_tcf0(SCTLR_EL1_TCF0_NONE);
> -- 
> 2.30.2
>