linux-kernel - Re: [PATCH RESEND 2/2] Fix some kallsyms

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Date:	Mon, 19 Mar 2007 13:21:07 +0300
From:	Alexey Dobriyan <adobriyan@...ru>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Ingo Molnar <mingo@...e.hu>, akpm@...l.org,
	linux-kernel@...r.kernel.org, devel@...nvz.org, tglx@...utronix.de,
	viro@...iv.linux.org.uk, pmarques@...popie.com
Subject: Re: [PATCH RESEND 2/2] Fix some kallsyms_lookup() vs rmmod races

On Sat, Mar 17, 2007 at 08:37:18PM +1100, Rusty Russell wrote:
> On Fri, 2007-03-16 at 12:51 +0100, Ingo Molnar wrote:
> > * Alexey Dobriyan <adobriyan@...ru> wrote:
> >
> > > [cc'ing folks whose proc files are affected]
> > >
> > > kallsyms_lookup() can call module_address_lookup() which iterates over
> > > modules list without module_mutex taken. Comment at the top of
> > > module_address_lookup() says it's for oops resolution so races are
> > > irrelevant, but in some cases it's reachable from regular code:
> >
> > looking at the problem from another angle: wouldnt this be something
> > that would benefit from freeze_processes()/unfreeze_processes(), and
> > hence no locking would be required?
>
> Actually, the list manipulation is done with stop_machine for this
> reason.

mmm, my changelog is slightly narrow than it should be.

	Non-emergency code is traversing modules list.
	It finds "struct module *".
	module is removed.
	"struct module *" is now meaningless, but still dereferenced.

How would all this refrigerator stuff would help? It wouldn't,

	Non-emergency code is traversing modules list.
	It finds "struct module *".
	Everything is freezed.
	Module is removed.
	Everything is unfreezed.
	"struct module *" is now meaningless, but still dereferenced.

> Alexey, is preempt enabled in your kernel?

Yes. FWIW,

CONFIG_PREEMPT=y
CONFIG_PREEMPT_BKL=y
CONFIG_DEBUG_PREEMPT=y

I very much agree with proto-patch which _copies_ all relevant
information into caller-supplied structure, keeping module_mutex private.
Time to split it sanely.

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