Re: [RFC][PATCH] kprobes/x86: While list ftrace locations in kprobe blacklist areas

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Steve,

On Thu, 10 Oct 2019 17:52:16 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt (VMware) <rostedt@goodmis.org>
> 
> I noticed some of my old tests failing on kprobes, and realized that
> this was due to black listing irq_entry functions on x86 from being
> used by kprobes. IIRC, this was due to the cr2 being corrupted and
> such, and I believe other things were to cause. But black listing all
> irq_entry code is a big hammer to this.

OK, I think if we can use ftrace for hooking, probing on "that address"
is good, but the function body still can not be probed.

> 
>  (See commit 0eae81dc9f026 "x86/kprobes: Prohibit probing on IRQ
>  handlers directly" for more details)
> 
> Anyway, if kprobes is using ftrace as a hook, there shouldn't be any
> problems here. If we white list ftrace locations in the range of
> kprobe_add_area_blacklist(), it should be safe.

Agreed.

> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index d9770a5393c8..9d28a279282c 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -2124,6 +2124,11 @@ int kprobe_add_area_blacklist(unsigned long start, unsigned long end)
>  	int ret = 0;
>  
>  	for (entry = start; entry < end; entry += ret) {
> +#ifdef CONFIG_KPROBES_ON_FTRACE
> +		/* We are safe if using ftrace */
> +		if (ftrace_location(entry))
> +			continue;
> +#endif

Have you tested the patch? it doesn't measure the entry function's size.
(kprobe_add_ksym_blacklist(entry) returns the function size)

Could you do this in kprobe_add_ksym_blacklist()?
Instead of continue, increment ent->start_addr by MCOUNT size(?) will be OK.
(Note that since each blacklist symbol is managed independently, you can make
 a "gap" between them as a safe area)

Thank you,

>  		ret = kprobe_add_ksym_blacklist(entry);
>  		if (ret < 0)
>  			return ret;


-- 
Masami Hiramatsu <mhiramat@kernel.org>