linux-kernel - Re: kernel bugs 2.6.31-rc6

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Date:	Mon, 24 Aug 2009 09:29:48 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Zdenek Kabelac <zdenek.kabelac@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Thielecke <christoph.thielecke@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: kernel bugs 2.6.31-rc6

On Mon, 2009-08-24 at 11:06 +0200, Zdenek Kabelac wrote:
> 2009/8/16 Eric Paris <eparis@...hat.com>:
> > On Sat, 2009-08-15 at 10:52 -0700, Linus Torvalds wrote:
> >>
> >> On Sat, 15 Aug 2009, Linus Torvalds wrote:
> >> >
> >> > For example, fsnotify_remove_priv_from_event() will remove the private
> >> > data event from the list, but what if there are _multiple_ entries with
> >> > the same 'group' entry? If so, it will remove just the first one.
> >
> > I can happen, but ONLY for the staticly declared q_overflow_event in
> > notification.c.  If the refcnt on that ever hits 0 to trigger this bug
> > we are in some serious dodo.
> >
> >> Hmm. Looking closer, that shouldn't much matter. Each time we added an
> >> entry in private_data_list, we would have done a
> >> 'fsnotify_get_event(event)' due to adding it to the 'golder->event_list'.
> >>
> >> That said, there does seem to be some dubious code there. For example,
> >> in 'inotify_ignored_and_remove_idr()', we do this:
> >>
> >>         fsnotify_add_notify_event(group, ignored_event, fsn_event_priv);
> >>
> >>         /* did the private data get added? */
> >>         if (list_empty(&fsn_event_priv->event_list))
> >>                 inotify_free_event_priv(fsn_event_priv);
> >
> >> and we do it without holding any locks at all. So as far as I can tell,
> >> what could happen is that 'fsnotify_add_notify_event()' actually adds the
> >> private event (fsn_event_priv), but then before we check that the
> >> event_list is empty, another user (on another CPU, or preempted on the
> >> same CPU - Christoph has both PREEMPT and SMP on) comes along, picks up
> >> the private event and frees it (and re-uses it).
> >
> > Actually you look correct in your assessment that there is a race here.
> > I guess I could imagine a way to make it panic like this, but I would
> > have expected a different problem in that after I freed this memory
> > (which wasn't mine any more) the other task which owned this memory
> > would have to still be able to run list_for_each_entry, but find that
> > it's group was no longer there.  Not sure how could screw up the group,
> > but not the list entries.
> >
> > I'll fix this race tonight or in the morning.
> >
> > I'm downloading and installing KDE, as I guess kde uses inotify pretty
> > hard since both Mikko and Christoph were using kde.
> 
> 
> Hi
> 
> I'm not sure how it is related - but I've got folllowing ooops while
> running yum upgrade
> I'm using gnome.
> 
> I've added the backtrace to the Bugzilla mentioned in this thread.
> 
> http://bugzilla.kernel.org/show_bug.cgi?id=13729#c7

This is a whole new problem completely unrelated!  This is a pretty
clear 'use after free' since we are seeing 6b6b6b6b all of the place.
I'm looking at this one today.

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