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Re: [Qemu-block] [PATCH v2 2/5] virtio-blk: add "discard-wzeroes" boolea
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-block] [PATCH v2 2/5] virtio-blk: add "discard-wzeroes" boolean property |
Date: |
Mon, 4 Feb 2019 08:37:28 -0500 |
On Mon, Feb 04, 2019 at 11:16:14AM +0100, Stefano Garzarella wrote:
> On Mon, Feb 04, 2019 at 11:33:07AM +0800, Stefan Hajnoczi wrote:
> > On Fri, Feb 01, 2019 at 06:18:52PM +0100, Stefano Garzarella wrote:
> > > On Fri, Feb 1, 2019 at 4:17 PM Michael S. Tsirkin <address@hidden> wrote:
> > > > On Thu, Jan 31, 2019 at 04:19:11PM +0100, Stefano Garzarella wrote:
> > > > > In order to avoid migration issues, we enable DISCARD and
> > > > > WRITE ZEROES features only for machine type >= 4.0
> > > > >
> > > > > Suggested-by: Dr. David Alan Gilbert <address@hidden>
> > > > > Signed-off-by: Stefano Garzarella <address@hidden>
> > > > > ---
> > > > > hw/block/virtio-blk.c | 2 ++
> > > > > hw/core/machine.c | 1 +
> > > > > include/hw/virtio/virtio-blk.h | 1 +
> > > > > 3 files changed, 4 insertions(+)
> > > > >
> > > > > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> > > > > index 8a6754d9a2..542ec52536 100644
> > > > > --- a/hw/block/virtio-blk.c
> > > > > +++ b/hw/block/virtio-blk.c
> > > > > @@ -1026,6 +1026,8 @@ static Property virtio_blk_properties[] = {
> > > > > DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size,
> > > > > 128),
> > > > > DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread,
> > > > > TYPE_IOTHREAD,
> > > > > IOThread *),
> > > > > + DEFINE_PROP_BIT("discard-wzeroes", VirtIOBlock,
> > > > > conf.discard_wzeroes, 0,
> > > > > + true),
> > > > > DEFINE_PROP_END_OF_LIST(),
> > > > > };
> > > > >
> > > >
> > > > Thinking about it, are there security implications for discard?
> > > > Should we make it default false?
> > >
> > > Hi Michael,
> > >
> > > I'm not completely sure but if the guest can write on a specific sector,
> > > discard or write_zeroes operations should not have a security implication.
> > >
> > > Do I miss something?
> >
> > Recently page cache attacks have been discussed in the Linux community:
> > https://arxiv.org/pdf/1901.01161.pdf
> >
> > I guess the scenario Michael is thinking about involves either -drive
> > cache.direct=off (including cache=writeback or cache=writethrough) or
> > maybe a timing side-channel in the storage appliance.
> >
> > The discard operation may allow a guest to evict the cache, an important
> > primitive for page cache attacks.
> >
> > Most of the time disk images are not shared between guests at all.
> > Therefore the discard operation cannot be used to learn information
> > about other guests.
> >
> > Let's focus on shared disk images: shared disk images are either
> > read-only (then discard isn't allowed anyway) or they are shared
> > writable (but this already implies a trust relationship between the
> > guests).
> >
> > My opinion is that discard is safe because virtualization use cases are
> > quite different from the attacks possible with shared library files
> > between userspace processes. I'm curious if anyone has figured out a
> > realistic scenario where it does matter though...
>
> Many thanks for the explanation!
>
> I'll wait to send the v3 in order to understand if Michael agrees to
> leave discard feature enabled to default.
>
> Thanks,
> Stefano
OK. Maybe mention the above in the commit log.
--
MST
Re: [Qemu-block] [PATCH v2 2/5] virtio-blk: add "discard-wzeroes" boolean property, Michael S. Tsirkin, 2019/02/05