linux-kernel - Re: [PATCH 1/2] drivers: core: Don't try to use a dead glue

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Date:   Tue, 10 Jul 2018 09:50:29 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Joel Stanley <joel@....id.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] drivers: core: Don't try to use a dead glue_dir

On Sat, 2018-07-07 at 18:51 +0200, Greg Kroah-Hartman wrote:
> 
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index b610816eb887..e9eff2099896 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -1517,11 +1517,13 @@ static struct kobject *get_device_parent(struct device *dev,
> >  
> >  		/* find our class-directory at the parent and reference it */
> >  		spin_lock(&dev->class->p->glue_dirs.list_lock);
> > -		list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
> > +		list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry) {
> >  			if (k->parent == parent_kobj) {
> > -				kobj = kobject_get(k);
> > -				break;
> > +				kobj = kobject_get_unless_zero(k);
> > +				if (kobj)
> > +					break;
> 
> A parent directory _should_ not ever be able to be removed before the
> object being removed was, as we should have had a reference to it,
> right?  So I don't see how this can get hit "in real life".
> 
> Yes, enabling kobject debugging does keep objects around for a long time
> in order to try to help figure out where people are messing up their
> usage of them.  What subsystem is doing this in a way that causes
> problems here?  Shouldn't we fix that up instead?

The broken subsystem is the driver core itself :-) See the descriptions
here and in patch 2/2.

Note: This is a more generic problem with ksets vs relying on the magic
sysfs cleanup happening in kobject_release().

Any kobject that is a member of a kset and doesn't get explicitely
removed from sysfs with kobject_del() prior to dropping the last
reference with kobject_put() (and thus relies instead on the automatic
cleanup done by kobject_release()) will be exposed to the race:

The last kobject_put() will drop the refcount to 0 while the object is
still in the kset. Only some amount of time later (which can be very
short or very long if you enable kobject debugging), will
kobject_release() take it out of sysfs and out of the kset.

Thus the object will be visible, with a 0 refcount, to anything that
"walks" the kset during that period.

This is exactly what happens with the gluedirs in the device core, but
it could happen elsewhere for all I know.

Cheers,
Ben.