Subject: [PATCH] arm64: only advance singlestep for user instruction traps

From: [email-masked] (AKASHI Takahiro)
Date: Thu, 5 Apr 2018 10:51:45 +0900

On Tue, Apr 03, 2018 at 11:22:51AM +0100, Mark Rutland wrote:
> Our arm64_skip_faulting_instruction() helper advances the userspace
> singlestep state machine, but this is also called by the kernel BRK
> handler, as used for WARN*().
> 
> Thus, if we happen to hit a WARN*() while the user singlestep state
> machine is in the active-no-pending state, we'll advance to the
> active-pending state without having executed a user instruction, and
> will take a step exception earlier than expected when we return to
> userspace.
> 
> Let's fix this by only advancing the state machine when skipping a user
> instruction.

Is it possible to have TIF_SINGLESTEP set even if !user_mode()?

If WARN*() is only an issue, why not fix bug_handler() directly?

-Takahiro AKASHI

> Signed-off-by: Mark Rutland <[email-masked]>
> Cc: Andrey Konovalov <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> ---
>  arch/arm64/kernel/traps.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index ba964da31a25..75625a401a4e 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -277,7 +277,8 @@ void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
>  	 * If we were single stepping, we want to get the step exception after
>  	 * we return from the trap.
>  	 */
> -	user_fastforward_single_step(current);
> +	if (user_mode(regs))
> +		user_fastforward_single_step(current);
>  }
>  
>  static LIST_HEAD(undef_hook);
> -- 
> 2.11.0
> 
> 
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