linux-kernel - Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy

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Date:   Tue, 18 May 2021 18:22:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     kvm@...r.kernel.org, borntraeger@...ibm.com, frankja@...ux.ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 00/11] KVM: s390: pv: implement lazy destroy

On 18.05.21 18:19, Claudio Imbrenda wrote:
> On Tue, 18 May 2021 18:04:11 +0200
> Cornelia Huck <cohuck@...hat.com> wrote:
> 
>> On Tue, 18 May 2021 17:36:24 +0200
>> Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:
>>
>>> On Tue, 18 May 2021 17:05:37 +0200
>>> Cornelia Huck <cohuck@...hat.com> wrote:
>>>    
>>>> On Mon, 17 May 2021 22:07:47 +0200
>>>> Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:
>>
>>>>> This means that the same address space can have memory
>>>>> belonging to more than one protected guest, although only one
>>>>> will be running, the others will in fact not even have any
>>>>> CPUs.
>>>>
>>>> Are those set-aside-but-not-yet-cleaned-up pages still possibly
>>>> accessible in any way? I would assume that they only belong to
>>>> the
>>>
>>> in case of reboot: yes, they are still in the address space of the
>>> guest, and can be swapped if needed
>>>    
>>>> 'zombie' guests, and any new or rebooted guest is a new entity
>>>> that needs to get new pages?
>>>
>>> the rebooted guest (normal or secure) will re-use the same pages of
>>> the old guest (before or after cleanup, which is the reason of
>>> patches 3 and 4)
>>
>> Took a look at those patches, makes sense.
>>
>>>
>>> the KVM guest is not affected in case of reboot, so the userspace
>>> address space is not touched.
>>
>> 'guest' is a bit ambiguous here -- do you mean the vm here, and the
>> actual guest above?
>>
> 
> yes this is tricky, because there is the guest OS, which terminates or
> reboots, then there is the "secure configuration" entity, handled by the
> Ultravisor, and then the KVM VM
> 
> when a secure guest reboots, the "secure configuration" is dismantled
> (in this case, in a deferred way), and the KVM VM (and its memory) is
> not directly affected
> 
> what happened before was that the secure configuration was dismantled
> synchronously, and then re-created.
> 
> now instead, a new secure configuration is created using the same KVM
> VM (and thus the same mm), before the old secure configuration has been
> completely dismantled. hence the same KVM VM can have multiple secure
> configurations associated, sharing the same address space.
> 
> of course, only the newest one is actually running, the other ones are
> "zombies", without CPUs.
> 

Can a guest trigger a DoS?

-- 
Thanks,

David / dhildenb