linux-kernel - Re: weird use-after-free bug in module

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Message-ID: <20121019173639.GD2152@core.coreip.homeip.net>
Date:	Fri, 19 Oct 2012 10:36:39 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: weird use-after-free bug in module_put

On Fri, Oct 19, 2012 at 06:09:51PM +0100, Al Viro wrote:
> On Fri, Oct 19, 2012 at 09:33:18AM -0700, Dmitry Torokhov wrote:
> 
> > We are now removing instance of character device corresponding to input
> > device when input device disappears.
> > 
> > Ah, I know... cdev is embedded in evdev, but lives longer.. I do want to
> > keep cdev embedded as it allows me to easily get to evdev in
> > evdev_open(), but I need to be able to add and then drop reference to
> > evdev from cdev's ->release() method. This means I need to override it.
> > 
> > Or I could have cdev separately allocated, but then I'd like to have a
> > void pointer in "struct cdev" so I could get from it back to
> > corresponding evdev.
> 
> Your real problem is that you have two kobjects embedded into the same
> thing.  It can work, but you need to make the secondary (one that does
> *not* free in its ->release()) pin the primary.  Sigh...  Device model
> sucks, film at 11...

Right, but "cdev" is currently "sealed": it does not allow specifying a
custom release function from which I could unpin primary (evdev). You
are the author/owner of cdev code, so that is why I was asking for
your opinion as to what is the best way to proceed:

1. Allocate cdev separately and add void * to struct cdev so that it is
   easy to get to corresponding structure on evdev_open.

2. Keep cdev embedded in evdev but export cdev's cleanup method and
   have evdev override ->release with its own version that calls
   cdev_default_release() and then unpins evdev stucture.

3. Add struct device *parent to struct cdev and have it pin and unpin it
   for us (if it is set up).

Thanks.

-- 
Dmitry
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