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

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Date:	Wed, 4 Mar 2015 16:51:39 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
cc:	Petr Mladek <pmladek@...e.cz>, Seth Jennings <sjenning@...hat.com>,
	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, 4 Mar 2015, Josh Poimboeuf wrote:

> > CPU0					CPU1
> > 
> > delete_module()  #SYSCALL
> > 
> >    try_stop_module()
> >      mod->state = MODULE_STATE_GOING;
> > 
> >    mutex_unlock(&module_mutex);
> > 
> > 					klp_register_patch()
> > 					klp_enable_patch()
> > 
> > 					#save place to switch universe
> > 
> > 					b()     # from module that is going
> > 					  a()   # from core (patched)
> > 
> > 
> >    mod->exit();
> > 
> > 
> > Note that the function b() can be called until we call mod->exit().
> > 
> > If we do not apply patch against b() because it is in
> > MODULE_STATE_GOING. It will call patched a() with modified semantic
> > and things might get wrong.
> > 
> > Well, the above described race is rather theoretical. It will happen
> > only when a patched module is being removed and a module with a patch
> > is added at the same time. Also it means that some other CPU will
> > manage to register, enable the patch, switch the universe, and
> > call function from the patched module during the small race window.
> > 
> > I am not sure if we need to handle such a type of race if the solution
> > adds too big complexity.
> 
> But b() can't be called after the module is in MODULE_STATE_GOING,
> right?  try_stop_module() makes sure it's not being used before changing
> its state.

If b() is called from __exit() of the particular module, then you end up 
in exactly the situation described above, don't you?

Thanks,

-- 
Jiri Kosina
SUSE Labs
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