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@xxxxxxx
>
> 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
