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Re: [PATCH v1 00/12] virtio-mem: Expose device memory via multiple memsl

From: Michael S. Tsirkin
Subject: Re: [PATCH v1 00/12] virtio-mem: Expose device memory via multiple memslots
Date: Tue, 2 Nov 2021 07:35:31 -0400

On Tue, Nov 02, 2021 at 09:33:55AM +0100, David Hildenbrand wrote:
> On 01.11.21 23:15, Michael S. Tsirkin wrote:
> > On Wed, Oct 27, 2021 at 02:45:19PM +0200, David Hildenbrand wrote:
> >> This is the follow-up of [1], dropping auto-detection and vhost-user
> >> changes from the initial RFC.
> >>
> >> Based-on: 20211011175346.15499-1-david@redhat.com
> >>
> >> A virtio-mem device is represented by a single large RAM memory region
> >> backed by a single large mmap.
> >>
> >> Right now, we map that complete memory region into guest physical addres
> >> space, resulting in a very large memory mapping, KVM memory slot, ...
> >> although only a small amount of memory might actually be exposed to the VM.
> >>
> >> For example, when starting a VM with a 1 TiB virtio-mem device that only
> >> exposes little device memory (e.g., 1 GiB) towards the VM initialliy,
> >> in order to hotplug more memory later, we waste a lot of memory on metadata
> >> for KVM memory slots (> 2 GiB!) and accompanied bitmaps. Although some
> >> optimizations in KVM are being worked on to reduce this metadata overhead
> >> on x86-64 in some cases, it remains a problem with nested VMs and there are
> >> other reasons why we would want to reduce the total memory slot to a
> >> reasonable minimum.
> >>
> >> We want to:
> >> a) Reduce the metadata overhead, including bitmap sizes inside KVM but also
> >>    inside QEMU KVM code where possible.
> >> b) Not always expose all device-memory to the VM, to reduce the attack
> >>    surface of malicious VMs without using userfaultfd.
> > 
> > I'm confused by the mention of these security considerations,
> > and I expect users will be just as confused.
> Malicious VMs wanting to consume more memory than desired is only
> relevant when running untrusted VMs in some environments, and it can be
> caught differently, for example, by carefully monitoring and limiting
> the maximum memory consumption of a VM. We have the same issue already
> when using virtio-balloon to logically unplug memory. For me, it's a
> secondary concern ( optimizing a is much more important ).
> Some users showed interest in having QEMU disallow access to unplugged
> memory, because coming up with a maximum memory consumption for a VM is
> hard. This is one step into that direction without having to run with
> uffd enabled all of the time.

Sorry about missing the memo - is there a lot of overhead associated
with uffd then?

> ("security is somewhat the wrong word. we won't be able to steal any
> information from the hypervisor.)

Right. Let's just spell it out.
Further, removing memory still requires guest cooperation.

> > So let's say user wants to not be exposed. What value for
> > the option should be used? What if a lower option is used?
> > Is there still some security advantage?
> My recommendation will be to use 1 memslot per gigabyte as default if
> possible in the configuration. If we have a virtio-mem devices with a
> maximum size of 128 GiB, the suggestion will be to use memslots=128.
> Some setups will require less (e.g., vhost-user until adjusted, old
> KVM), some setups can allow for more. I assume that most users will
> later set "memslots=0", to enable auto-detection mode.
> Assume we have a virtio-mem device with a maximum size of 1 TiB and we
> hotplugged 1 GiB to the VM. With "memslots=1", the malicious VM could
> actually access the whole 1 TiB. With "memslots=1024", the malicious VM
> could only access additional ~ 1 GiB. With "memslots=512", ~ 2 GiB.
> That's the reduced attack surface.
> Of course, it's different after we hotunplugged memory, before we have
> VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE support in QEMU, because all memory
> inside the usable region has to be accessible and we cannot "unplug" the
> memslots.
> Note: With upcoming VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE changes in QEMU,
> one will be able to disallow any access for malicious VMs by setting the
> memblock size just as big as the device block size.
> So with a 128 GiB virtio-mem device with memslots=128,block-size=1G, or
> with memslots=1024,block-size=128M we could make it impossible for a
> malicious VM to consume more memory than intended. But we lose
> flexibility due to the block size and the limited number of available
> memslots.
> But again, for "full protection against malicious VMs" I consider
> userfaultfd protection more flexible. This approach here gives some
> advantage, especially when having large virtio-mem devices that start
> out small.
> -- 
> Thanks,
> David / dhildenb

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