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

[Robin Holt <holt@sgi.com>]

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.

> 
> > 
> > Tested with this patch applied.  My test case which was failing
> > approximately every 300th iteration passed 25,000 tests.
> 
> Could you please share your test case?

I could but it would be very useless.  It depends upon having a SGI
UV system with GRUs and and xpmem kernel module loaded.  If you would
really like all the bits, I could provide them, but you will not be able
to reproduce the failure.

Thanks,
Robin

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>