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

[Will Deacon]

On Thu, Apr 08, 2021 at 03:37:23PM +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 the flag can be
> lost in the process.

Actually, it's all the *other* flags that get lost!

> 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>
> Reported-by: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  arch/arm64/include/asm/mte.h     |  8 ++++++++
>  arch/arm64/kernel/entry-common.c |  6 ++++++
>  arch/arm64/kernel/entry.S        | 30 ------------------------------
>  arch/arm64/kernel/mte.c          | 25 +++++++++++++++++++++++--
>  4 files changed, 37 insertions(+), 32 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index 9b557a457f24..188f778c6f7b 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -31,6 +31,8 @@ void mte_invalidate_tags(int type, pgoff_t offset);
>  void mte_invalidate_tags_area(int type);
>  void *mte_allocate_tag_storage(void);
>  void mte_free_tag_storage(char *storage);
> +void check_mte_async_tcf0(void);
> +void clear_mte_async_tcf0(void);
>  
>  #ifdef CONFIG_ARM64_MTE
>  
> @@ -83,6 +85,12 @@ static inline int mte_ptrace_copy_tags(struct task_struct *child,
>  {
>  	return -EIO;
>  }
> +void check_mte_async_tcf0(void)
> +{
> +}
> +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();
>  }

Is enter_from_user_mode() always called when we enter the kernel from EL0?
afaict, some paths (e.g. el0_irq()) only end up calling it if
CONTEXT_TRACKING or TRACE_IRQFLAGS are enabled.

>  
>  asmlinkage void noinstr exit_to_user_mode(void)
>  {
> +	/* Ignore asynchronous tag check faults in the uaccess routines */
> +	clear_mte_async_tcf0();
> +

and this one seems to be called even less often.

Will