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

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Message-ID: <20190627231952.nqkbtcculvo2ddif@treble>
Date:   Thu, 27 Jun 2019 18:19:52 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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 Fri, Jun 28, 2019 at 01:09:08AM +0200, Thomas Gleixner wrote:
> 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

/me dodges frozen shark

You are right of course.  My brain has apparently already shut off for
the day.

Maybe a comment or two would help though.

-- 
Josh