Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device

[Dan Williams]

On Sat, Jan 12, 2019 at 5:38 PM Pankaj Gupta <address@hidden> wrote:
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>
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> >
> > On Thu 10-01-19 12:26:17, Dave Chinner wrote:
> > > On Wed, Jan 09, 2019 at 08:17:31PM +0530, Pankaj Gupta wrote:
> > > >  This patch series has implementation for "virtio pmem".
> > > >  "virtio pmem" is fake persistent memory(nvdimm) in guest
> > > >  which allows to bypass the guest page cache. This also
> > > >  implements a VIRTIO based asynchronous flush mechanism.
> > >
> > > Hmmmm. Sharing the host page cache direct into the guest VM. Sounds
> > > like a good idea, but.....
> > >
> > > This means the guest VM can now run timing attacks to observe host
> > > side page cache residency, and depending on the implementation I'm
> > > guessing that the guest will be able to control host side page
> > > cache eviction, too (e.g. via discard or hole punch operations).
> > >
> > > Which means this functionality looks to me like a new vector for
> > > information leakage into and out of the guest VM via guest
> > > controlled host page cache manipulation.
> > >
> > > https://arxiv.org/pdf/1901.01161
> > >
> > > I might be wrong, but if I'm not we're going to have to be very
> > > careful about how guest VMs can access and manipulate host side
> > > resources like the page cache.....
> >
> > Right. Thinking about this I would be more concerned about the fact that
> > guest can effectively pin amount of host's page cache upto size of the
> > device/file passed to guest as PMEM, can't it Pankaj? Or is there some QEMU
> > magic that avoids this?
>
> Yes, guest will pin these host page cache pages using 'get_user_pages' by
> elevating the page reference count. But these pages can be reclaimed by host
> at any time when there is memory pressure.

Wait, how can the guest pin the host pages? I would expect this to
happen only when using vfio and device assignment. Otherwise, no the
host can't reclaim a pinned page, that's the whole point of a pin to
prevent the mm from reclaiming ownership.

> KVM does not permanently pin pages. vfio does that but we are not using
> it here.

Right, so I'm confused by your pin assertion above.