linux-kernel - Re: [PATCH] livepatch: add locking to force and signal functions

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Message-ID: <alpine.LSU.2.21.1712201416270.27477@pobox.suse.cz>
Date:   Wed, 20 Dec 2017 14:18:16 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     jpoimboe@...hat.com, jeyu@...nel.org, jikos@...nel.org,
        jbaron@...mai.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: add locking to force and signal functions

On Wed, 20 Dec 2017, Petr Mladek wrote:

> On Wed 2017-12-20 10:28:07, Miroslav Benes wrote:
> > klp_send_signals() and klp_force_transition() do not acquire klp_mutex,
> > because it seemed to be superfluous. A potential race in
> > klp_send_signals() was harmless and there was nothing in
> > klp_force_transition() which needed to be synchronized. That changed
> > with the addition of klp_forced variable during the review process.
> > 
> > There is a small window now, when klp_complete_transition() does not see
> > klp_forced set to true while all tasks have been already transitioned to
> > the target state. module_put() is called and the module can be removed.
> > 
> > Acquire klp_mutex to prevent it. Do the same in klp_send_signals() just
> > to be sure. There is no real downside to that.
> > 
> > Reported-by: Jason Baron <jbaron@...mai.com>
> > Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> > ---
> >  kernel/livepatch/transition.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index be5bfa533ee8..3f932ff607cd 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -625,6 +625,8 @@ void klp_send_signals(void)
> >  
> >  	pr_notice("signaling remaining tasks\n");
> >  
> > +	mutex_lock(&klp_mutex);
> > +
> >  	read_lock(&tasklist_lock);
> >  	for_each_process_thread(g, task) {
> >  		if (!klp_patch_pending(task))
> > @@ -653,6 +655,8 @@ void klp_send_signals(void)
> >  		}
> >  	}
> >  	read_unlock(&tasklist_lock);
> > +
> > +	mutex_unlock(&klp_mutex);
> 
> It would be cleaner if the lock guarded also the check:
> 
> 	if (patch != klp_transition_patch)
> 		return -EINVAL;
> 
> in signal_store(). Then we could remove also the comment
> above this check.
> 
> Same is true also for the force part stuff.

And I even left obsolete comments in sysfs callbacks. Sigh. v2 is 
inevitable...

Miroslav