linux-kernel - Re: [PATCH] ftrace: Remove possible deadlock between register_kprobe() and ftrace_run_update

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Message-ID: <alpine.DEB.2.21.1906280106360.32342@nanos.tec.linutronix.de>
Date:   Fri, 28 Jun 2019 01:09:08 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Petr Mladek <pmladek@...e.com>,
        Miroslav Benes <mbenes@...e.cz>, Jessica Yu <jeyu@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        Johannes Erdfelt <johannes@...felt.com>,
        Ingo Molnar <mingo@...nel.org>, mhiramat@...nel.org,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH] ftrace: Remove possible deadlock between register_kprobe()
 and ftrace_run_update_code()

On Thu, 27 Jun 2019, Steven Rostedt wrote:
> On Thu, 27 Jun 2019 17:47:29 -0500
> > Releasing the lock in a separate function seems a bit surprising and
> > fragile, would it be possible to do something like this instead?
> > 
> > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> > index b38c388d1087..89ea1af6fd13 100644
> > --- a/arch/x86/kernel/ftrace.c
> > +++ b/arch/x86/kernel/ftrace.c
> > @@ -37,15 +37,21 @@
> >  int ftrace_arch_code_modify_prepare(void)
> >  {
> >  	mutex_lock(&text_mutex);
> > +
> >  	set_kernel_text_rw();
> >  	set_all_modules_text_rw();
> > +
> > +	mutex_unlock(&text_mutex);
> >  	return 0;
> >  }
> >  
> >  int ftrace_arch_code_modify_post_process(void)
> >  {
> > +	mutex_lock(&text_mutex);
> > +
> >  	set_all_modules_text_ro();
> >  	set_kernel_text_ro();
> > +
> >  	mutex_unlock(&text_mutex);
> >  	return 0;
> >  }
> 
> I agree with Josh on this. As the original bug was the race between
> ftrace and live patching / modules changing the text from ro to rw and
> vice versa. Just protecting the update to the text permissions is more
> robust, and should be more self documenting when we need to handle
> other architectures for this.

How is that supposed to work?

    ftrace  	     	
	prepare()
	 setrw()
			setro()
	patch <- FAIL

Thanks,

	tglx