Re: [PATCH 1/2] kvm: Fix mmu_notifier release race

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Apr 26, 2017 at 05:03:44PM +0100, Suzuki K Poulose wrote:
> On 25/04/17 19:49, Radim Krčmář wrote:
> >2017-04-24 11:10+0100, Suzuki K Poulose:
> >>The KVM uses mmu_notifier (wherever available) to keep track
> >>of the changes to the mm of the guest. The guest shadow page
> >>tables are released when the VM exits via mmu_notifier->ops.release().
> >>There is a rare chance that the mmu_notifier->release could be
> >>called more than once via two different paths, which could end
> >>up in use-after-free of kvm instance (such as [0]).
> >>
> >>e.g:
> >>
> >>thread A                                        thread B
> >>-------                                         --------------
> >>
> >> get_signal->                                   kvm_destroy_vm()->
> >> do_exit->                                        mmu_notifier_unregister->
> >> exit_mm->                                        kvm_arch_flush_shadow_all()->
> >> exit_mmap->                                      spin_lock(&kvm->mmu_lock)
> >> mmu_notifier_release->                           ....
> >>  kvm_arch_flush_shadow_all()->                   .....
> >>  ... spin_lock(&kvm->mmu_lock)                   .....
> >>                                                  spin_unlock(&kvm->mmu_lock)
> >>                                                kvm_arch_free_kvm()
> >>   *** use after free of kvm ***
> >
> >I don't understand this race ...
> >a piece of code in mmu_notifier_unregister() says:
> >
> >  	/*
> >  	 * Wait for any running method to finish, of course including
> >  	 * ->release if it was run by mmu_notifier_release instead of us.
> >  	 */
> >  	synchronize_srcu(&srcu);
> >
> >and code before that removes the notifier from the list, so it cannot be
> >called after we pass this point.  mmu_notifier_release() does roughly
> >the same and explains it as:
> >
> >  	/*
> >  	 * synchronize_srcu here prevents mmu_notifier_release from returning to
> >  	 * exit_mmap (which would proceed with freeing all pages in the mm)
> >  	 * until the ->release method returns, if it was invoked by
> >  	 * mmu_notifier_unregister.
> >  	 *
> >  	 * The mmu_notifier_mm can't go away from under us because one mm_count
> >  	 * is held by exit_mmap.
> >  	 */
> >  	synchronize_srcu(&srcu);
> >
> >The call of mmu_notifier->release is protected by srcu in both cases and
> >while it seems possible that mmu_notifier->release would be called
> >twice, I don't see a combination that could result in use-after-free
> >from mmu_notifier_release after mmu_notifier_unregister() has returned.
> 
> Thanks for bringing it up. Even I am wondering why this is triggered ! (But it
> does get triggered for sure !!)
> 
> The only difference I can spot with _unregister & _release paths are the way
> we use src_read_lock across the deletion of the entry from the list.
> 
> In mmu_notifier_unregister() we do :
> 
>                 id = srcu_read_lock(&srcu);
>                 /*
>                  * exit_mmap will block in mmu_notifier_release to guarantee
>                  * that ->release is called before freeing the pages.
>                  */
>                 if (mn->ops->release)
>                         mn->ops->release(mn, mm);
>                 srcu_read_unlock(&srcu, id);
> 
> ## Releases the srcu lock here and then goes on to grab the spin_lock.
> 
>                 spin_lock(&mm->mmu_notifier_mm->lock);
>                 /*
>                  * Can not use list_del_rcu() since __mmu_notifier_release
>                  * can delete it before we hold the lock.
>                  */
>                 hlist_del_init_rcu(&mn->hlist);
>                 spin_unlock(&mm->mmu_notifier_mm->lock);
> 
> While in mmu_notifier_release() we hold it until the node(s) are deleted from the
> list :
>         /*
>          * SRCU here will block mmu_notifier_unregister until
>          * ->release returns.
>          */
>         id = srcu_read_lock(&srcu);
>         hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist)
>                 /*
>                  * If ->release runs before mmu_notifier_unregister it must be
>                  * handled, as it's the only way for the driver to flush all
>                  * existing sptes and stop the driver from establishing any more
>                  * sptes before all the pages in the mm are freed.
>                  */
>                 if (mn->ops->release)
>                         mn->ops->release(mn, mm);
> 
>         spin_lock(&mm->mmu_notifier_mm->lock);
>         while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
>                 mn = hlist_entry(mm->mmu_notifier_mm->list.first,
>                                  struct mmu_notifier,
>                                  hlist);
>                 /*
>                  * We arrived before mmu_notifier_unregister so
>                  * mmu_notifier_unregister will do nothing other than to wait
>                  * for ->release to finish and for mmu_notifier_unregister to
>                  * return.
>                  */
>                 hlist_del_init_rcu(&mn->hlist);
>         }
>         spin_unlock(&mm->mmu_notifier_mm->lock);
>         srcu_read_unlock(&srcu, id);
> 
> ## The lock is release only after the deletion of the node.
> 
> Both are followed by a synchronize_srcu(). Now, I am wondering if the unregister path
> could potentially miss SRCU read lock held in _release() path and go onto finish the
> synchronize_srcu before the item is deleted ? May be we should do the read_unlock
> after the deletion of the node in _unregister (like we do in the _release()) ?
> 
> >
> >Doesn't [2/2] solve the exact same issue (that the release method cannot
> >be called twice in parallel)?
> 
> Not really. This could be a race between a release() and one of the other notifier
> callbacks. e.g, In [0], we were hitting a use-after-free in kvm_unmap_hva() where,
> the unregister could have succeeded and released the KVM.
> 
> 
> [0] http://lkml.kernel.org/r/febea966-3767-21ff-3c40-1a76d1399138@suse.de
> 
> In effect this all could be due to the same reason, the synchronize in unregister
> missing another reader.

If this is at all reproducible, I suggest use of ftrace or event tracing
to work out exactly what is happening.

							Thanx, Paul