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

[David Hildenbrand]

On 18.05.21 18:19, Claudio Imbrenda wrote:
> On Tue, 18 May 2021 18:04:11 +0200
> Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
>
> > On Tue, 18 May 2021 17:36:24 +0200
> > Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx> wrote:
> >
> > > On Tue, 18 May 2021 17:05:37 +0200
> > > Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
> > >    
> > > > On Mon, 17 May 2021 22:07:47 +0200
> > > > Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx> 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