linux-kernel - Re: [PATCH 2/2] livepatch: fix patched module loading race

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Date:	Wed, 4 Mar 2015 15:18:25 +0100
From:	Petr Mladek <pmladek@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Seth Jennings <sjenning@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Miroslav Benes <mbenes@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	mingo@...nel.org, mathieu.desnoyers@...icios.com, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, andi@...stfloor.org,
	rostedt@...dmis.org, tglx@...utronix.de
Subject: Re: [PATCH 2/2] livepatch: fix patched module loading race

On Wed 2015-03-04 14:17:52, Petr Mladek wrote:
> On Tue 2015-03-03 17:02:22, Josh Poimboeuf wrote:
> > It's possible for klp_register_patch() to see a module before the COMING
> > notifier is called, or after the GOING notifier is called.
> > 
> > That can cause all kinds of ugly races.  As Pter Mladek reported:
> > 
> >   "The problem is that we do not keep the klp_mutex lock all the time when
> >   the module is being added or removed.
> > 
> >   First, the module is visible even before ftrace is ready. If we enable a patch
> >   in this time frame, adding ftrace ops will fail and the patch will get rejected
> >   just because bad timing.
> 
> Ah, this is not true after all. I did not properly check when
> MODULE_STATE_COMING was set. I though that it was before ftrace was
> initialized but it was not true.
> 
> 
> >   Second, if we are "lucky" and enable the patch for the coming module when the
> >   ftrace is ready but before the module notifier has been called. The notifier
> >   will try to enable the patch as well. It will detect that it is already patched,
> >   return error, and the module will get rejected just because bad
> >   timing. The more serious problem is that it will not call the notifier for
> >   going module, so that the mess will stay there and we wont be able to load
> >   the module later.
> 
> Ah, the race is there but the effect is not that serious in the
> end. It seems that errors from module notifiers are ignored. In fact,
> we do not propagate the error from klp_module_notify_coming(). It means
> that WARN() from klp_enable_object() will be printed but the module
> will be loaded and patched.
> 
> I am sorry, I was confused by kGraft where kgr_module_init() was
> called directly from module_load(). The errors were propagated. It
> means that kGraft rejects module when the patch cannot be applied.
> 
> Note that the current solution is perfectly fine for the simple
> consistency model.
> 
> 
> >   Third, similar problems are there for going module. If a patch is enabled after
> >   the notifier finishes but before the module is removed from the list of modules,
> >   the new patch will be applied to the module. The module might disappear at
> >   anytime when the patch enabling is in progress, so there might be an access out
> >   of memory. Or the whole patch might be applied and some mess will be left,
> >   so it will not be possible to load/patch the module again."
> 
> This is true.
> 
> 
> > Fix these races by letting the first one who sees the module do the
> > needed work.
> > 
> > Reported-by: Petr Mladek <pmladek@...e.cz>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >  kernel/livepatch/core.c | 53 +++++++++++++++++++++++++++++++++++++++++++++----
> > @@ -965,10 +990,30 @@ static int klp_module_notify(struct notifier_block *nb, unsigned long action,
> >  				continue;
> >  
> >  			if (action == MODULE_STATE_COMING) {
> > +
> > +				/*
> > +				 * Check for a small window where the register
> > +				 * path already initialized the object.
> > +				 */
> s/path/patch/
> 
> 
> 
> > +				if (obj->mod)
> > +					continue;
> 
> This might break stacking. The recently registered patch might become
> the last on the stack and thus unused.

Going through the stack when registering new patch would be quite
ugly. I am going to provide yet another solution.

Best Regards,
Petr
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