Re: [PATCH] [Patch] mmu_notifier_unregister NULL Pointer deref fix.

[Robin Holt <holt@sgi.com>]

On Fri, Jan 18, 2013 at 10:42:07AM +0800, Xiao Guangrong wrote:
> On 01/17/2013 09:45 PM, Robin Holt wrote:
> > On Thu, Jan 17, 2013 at 08:19:55PM +0800, Xiao Guangrong wrote:
> >> On 01/17/2013 07:12 PM, Robin Holt wrote:
> >>> On Thu, Jan 17, 2013 at 10:45:32AM +0800, Xiao Guangrong wrote:
> >>>> On 01/17/2013 05:01 AM, Robin Holt wrote:
> >>>>>
> >>>>> There is a race condition between mmu_notifier_unregister() and
> >>>>> __mmu_notifier_release().
> >>>>>
> >>>>> Assume two tasks, one calling mmu_notifier_unregister() as a result
> >>>>> of a filp_close() ->flush() callout (task A), and the other calling
> >>>>> mmu_notifier_release() from an mmput() (task B).
> >>>>>
> >>>>>                 A                               B
> >>>>> t1                                              srcu_read_lock()
> >>>>> t2              if (!hlist_unhashed())
> >>>>> t3                                              srcu_read_unlock()
> >>>>> t4              srcu_read_lock()
> >>>>> t5                                              hlist_del_init_rcu()
> >>>>> t6                                              synchronize_srcu()
> >>>>> t7              srcu_read_unlock()
> >>>>> t8              hlist_del_rcu()  <--- NULL pointer deref.
> >>>>
> >>>> The detailed code here is:
> >>>> 	hlist_del_rcu(&mn->hlist);
> >>>>
> >>>> Can mn be NULL? I do not think so since mn is always the embedded struct
> >>>> of the caller, it be freed after calling mmu_notifier_unregister.
> >>>
> >>> If you look at __mmu_notifier_release() it is using hlist_del_init_rcu()
> >>> which will set the hlist->pprev to NULL.  When hlist_del_rcu() is called,
> >>> it attempts to update *hlist->pprev = hlist->next and that is where it
> >>> takes the NULL pointer deref.
> >>
> >> Yes, sorry for my careless. So, That can not be fixed by using
> >> hlist_del_init_rcu instead?
> > 
> > The problem is the race described above.  Thread 'A' has checked to see
> > if n->pprev != NULL.  Based upon that, it did called the mn->release()
> > method.  While it was trying to call the release method, thread 'B' ended
> > up calling hlist_del_init_rcu() which set n->pprev = NULL.  Then thread
> > 'A' got to run again and now it tries to do the hlist_del_rcu() which, as
> > part of __hlist_del(), the pprev will be set to n->pprev (which is NULL)
> > and then *pprev = n->next; hits the NULL pointer deref hits.
> 
> I mean using hlist_del_init_rcu instead of hlist_del_rcu in
> mmu_notifier_unregister(), hlist_del_init_rcu is aware of ->pprev.

How does that address the calling of the ->release() method twice?

Thanks,
Robin

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