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

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Message-ID: <20121019170951.GM2616@ZenIV.linux.org.uk>
Date:	Fri, 19 Oct 2012 18:09:51 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
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 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...
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