Re: weird use-after-free bug in module_put

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Oct 19, 2012 at 10:36:39AM -0700, Dmitry Torokhov wrote:
> 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).

The last one would be my preference, TBH, but I'm not sure how to do it
cleanly.  The thing is, kobject_del() would have to be done at some point.
_Before_ we have dropped the last references.  And once we have, we are
back to the original race, AFAICS.

What's pinning that cdev reference, BTW?