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

Pankaj Gupta
2019-01-13T02:17:46Z

> >
> >
> >
> > >
> > > 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.

yes. You are right I just used the pin word but it does not actually pin pages
permanently. I had gone through the discussion on existing problems with
get_user_pages and DMA e.g [1] to understand Jan's POV. It does mention GUP
pin pages so I also used the word 'pin'. But guest does not permanently pin
these pages and these pages can be reclaimed by host.

>
> > 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.

Sorry! for the confusion.

[1] https://lwn.net/Articles/753027/

Thanks,
Pankaj