Re: [PATCH] arm64: kprobes: Drop open-coded exception fixup

[Masami Hiramatsu]

Hi Robin,

On Thu, 17 Oct 2019 16:31:42 +0100
Robin Murphy <robin.murphy@xxxxxxx> wrote:

> The short-circuit call to fixup_exception() from kprobe_fault_handler()
> poses a problem now that the former wants to consume the fault address
> too, since the common kprobes API offers us no way to pass it through.
> Fortunately, however, it works out to be unnecessary:

Thank you for pointing it out!

> 
> - uaccess instructions themselves are not probeable, so at most we
>   should only ever expect to take a fixable fault from the pre or post
>   handlers.

Right. If it is not fixable, we should handle it as a kernel bug.
(to avoid it we can use probe_kernel_read() in kprobe handler)

> - the pre and post handler run with preemption disabled, thus for any
>   fault they may cause, an unhandled return from kprobe_page_fault()
>   will proceed directly to __do_kernel_fault() thanks to the
>   faulthandler_disabled() check.

OK, this is reasonable.

> - __do_kernel_fault() will immediately call fixup_exception() unless
>   we're in an EL1 instruction abort, and if we've somehow taken one of
>   those on what we think is the middle of a uaccess routine, then the
>   world is already very on fire.

OK, this looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thank you!

> 
> Thus we can reasonably drop the call from kprobe_fault_handler() and
> leave uaccess fixups to the regular flow.
> 
> Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index c4452827419b..422fbd5c6c55 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -334,13 +334,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
>  		 */
>  		if (cur->fault_handler && cur->fault_handler(cur, regs, fsr))
>  			return 1;
> -
> -		/*
> -		 * In case the user-specified fault handler returned
> -		 * zero, try to fix up.
> -		 */
> -		if (fixup_exception(regs))
> -			return 1;
>  	}
>  	return 0;
>  }
> -- 
> 2.21.0.dirty
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>