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

[Robin Holt <holt@sgi.com>]

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.

Thanks,
Robin

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