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

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Date:	Thu, 5 Mar 2015 08:18:45 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Petr Mladek <pmladek@...e.cz>, 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>, 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 Thu, Mar 05, 2015 at 09:52:41AM +0900, Masami Hiramatsu wrote:
> (2015/03/04 22:17), 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.
> 
> No, that's not true if you try_get_module() before patching. After the
> module state goes GOING (more correctly say, after try_release_module_ref()
> succeeded), all try_get_module() must fail :)
> So, please make sure to get module when applying patches.

Hi Masami,

As Jikos pointed out elsewhere, try_get_module() won't solve all the
GOING races.

The module can be in GOING before mod->exit() is called.  If we apply a
patch between GOING getting set and mod->exit(), try_module_get() will
fail and the module won't be patched.  But module code can still run
before or during mod->exit(), so the unpatched module code might
interact badly with new patched code elsewhere.

-- 
Josh
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