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

[Suzuki K Poulose]

On 26/04/17 17:03, 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()) ?

I haven't been able to reproduce the mmu_notifier race condition, which leads to KVM
free, reported at [1]. I will leave it running (with tracepoints/ftrace) over the
weekend.

>
>>
>> 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.

But I can reproduce this problem [0], and we need the [2/2] for arm/arm64.

[0] http://lkml.kernel.org/r/febea966-3767-21ff-3c40-1a76d1399138@suse.de
[1] http://lkml.kernel.org/r/CAAeHK+x8udHKq9xa1zkTO6ax5E8Dk32HYWfaT05FMchL2cr48g@mail.gmail.com


Thanks
Suzuki