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    SubjectRe: [PATCH v3 0/5] kvm "virtio pmem" device
    On Sat, Jan 12, 2019 at 5:38 PM Pankaj Gupta <pagupta@redhat.com> wrote:
    >
    >
    >
    > >
    > > 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.

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