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

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  Re: [PATCH 1/2] kvm: Fix mmu_notifier release race Radim Krčmář <rkrcmar@redhat.com> - 2017-04-25 20:50 +0200
    Re: [PATCH 1/2] kvm: Fix mmu_notifier release race Suzuki K Poulose <Suzuki.Poulose@arm.com> - 2017-04-26 18:10 +0200
      Re: [PATCH 1/2] kvm: Fix mmu_notifier release race "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-26 18:20 +0200
      Re: [PATCH 1/2] kvm: Fix mmu_notifier release race Suzuki K Poulose <Suzuki.Poulose@arm.com> - 2017-04-28 19:30 +0200
        Re: [PATCH 1/2] kvm: Fix mmu_notifier release race Suzuki K Poulose <Suzuki.Poulose@arm.com> - 2017-05-03 15:20 +0200

#1630904 — Re: [PATCH 1/2] kvm: Fix mmu_notifier release race

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.

Doesn't [2/2] solve the exact same issue (that the release method cannot
be called twice in parallel)?

Thanks.

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#1631583

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.

Suzuki

>
> Thanks.
>

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#1631591

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

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#1633044

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

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#1634970

On 28/04/17 18:20, Suzuki K Poulose wrote:
> 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.
>

I couldn't reproduce the proposed "mmu_notifier race" reported in [0].
However I found some other use-after-free cases in the unmap_stage2_range()
code due to the introduction of cond_resched_lock(). It may be just that the
IP reported in [0] was for wrong line of code ? i.e, arch_spin_is_locked instead
of unmap_stage2_range ?
Anyways, I will send a new version of the patches in a separate series.

[0] https://marc.info/?l=linux-kernel&m=149201399018791&w=2

Suzuki

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