linux-kernel - Re: [PATCH 2/4] ftrace/x86: Do not crash when reading wrong ftrace func

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Date:	Mon, 27 Jun 2016 10:48:58 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH 2/4] ftrace/x86: Do not crash when reading wrong ftrace
 func

On Mon, 27 Jun 2016 15:54:35 +0200
Petr Mladek <pmladek@...e.com> wrote:

> Ftrace modifies the code on many locations. It is paranoid
> and avoid a kernel crash using probe_kernel_read() and
> probe_kernel_write(). The only exception is update_ftrace_func()
> where where we read the old code using memcpy().
> 
> It is true that this function is used only to modify well
> defined functions that are part of the ftrace API. But
> it might still make sense to be paranoid and be consistent
> with the writing side.

I'm not so sure I'm too hot on this patch. I left it with the memcpy()
because it was a well known location. If this is wrong, then we should
crash the kernel.

I'm not totally against it. But a comment should be added stating
something like:

	/*
	 * ip points to the ftrace infrastructure. If this fails,
	 * then something is totally messed up.
	 */

and perhaps even add a WARN_ON() here too.

-- Steve

> 
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  arch/x86/kernel/ftrace.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 42ea69d35dfd..8305c6792ad2 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -233,7 +233,8 @@ static int update_ftrace_func(unsigned long ip, void *new)
>  	unsigned char old[MCOUNT_INSN_SIZE];
>  	int ret;
>  
> -	memcpy(old, (void *)ip, MCOUNT_INSN_SIZE);
> +	if (probe_kernel_read(old, (void *)ip, MCOUNT_INSN_SIZE))
> +		return -EFAULT;
>  
>  	/*
>  	 * Make sure that we replace 5-byte instruction that