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

[Suzuki K Poulose]

On 25/04/17 19:49, Radim KrÄmÃÅ 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.

Suzuki

> Thanks.