From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Subject: Re: [PATCH] cgroups: fix possible use after free
Date: Tue, 10 Feb 2009 12:45:27 +0000
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To: Paul Menage <menage-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: Linux Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
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On Tue, Feb 10, 2009 at 02:15:36AM -0800, Paul Menage wrote:
> On Tue, Feb 10, 2009 at 1:31 AM, Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org> wrote:
> > In cgroup_kill_sb(), root is freed before sb is detached from the list,
> > so another sget() may find this sb and call cgroup_test_super(),
> > which will access the root that has been freed.
> 
> I think that I'd assumed that by the time we get to cgroup_kill_sb()
> there's no chance of the sb being resurrected by sget().

There is none.  grab_super() will fail to get it, so sget() will go
through retry logics.  Which doesn't mean that test won't be called
on it in the meanwhile.