Re: [Qemu-devel] [PATCH] migration: add capability to bypass the shared memory

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Mon, Jul 02, 2018 at 09:52:08PM +0800, Peng Tao wrote:
> On Mon, Jul 2, 2018 at 9:10 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > On Sat, Mar 31, 2018 at 04:45:00PM +0800, Lai Jiangshan wrote:
> > Risks:
> > 1. If one cloned VM is exploited then all other VMs are more likely to
> >    be exploitable (e.g. kernel address space layout randomization).
> w.r.t. KASLR, any memory duplication technology would expose it. I
> remember there are CVEs (e.g., CVE-2015-2877) specific to this kind
> attack against KSM and it was stated that "Basically if you care about
> this attack vector, disable deduplication.". Share-until-written
> approaches for memory conservation among mutually untrusting tenants
> are inherently detectable for information disclosure, and can be
> classified as potentially misunderstood behaviors rather than
> vulnerabilities. [1]
> 
> I think the same applies to vm templating as well. Actually VM
> templating is more useful (than KSM) in this regard since we can
> create a template for each trusted tenant where as with KSM all VMs on
> a host are treated equally.
> 
> [1] https://access.redhat.com/security/cve/cve-2015-2877

That solves the problem between untrusted users but a breach in one
clone may reveal secrets of all other clones belonging to the same
tenant.  As a user, I would be uncomfortable knowing that if one of my
machines is breached then secrets used by all of my machines might be
exposed.

> > 2. If you give VMs cloned from the same template to untrusted users,
> >    they may be able to determine the secrets other users' VMs.
> In kata and runv, vm templating is used carefully so that we do not
> use or save any secret keys before creating the template VM. IOW, the
> feature is not supposed to be used generally to create any template
> VMs at any stage.

At what point are templates captured to avoid these problems?  Is there
code that shows how to do this?

> >  Security is a
> > major factor for using Kata, so it's important not to leak secrets
> > between cloned VMs.
> >
> Yes, indeed! And it is all about trade-offs, VM templating or KSM. If
> we want security above anything, we should just disable all the
> sharing. But there is actually no ceiling (think about physical
> isolation!). So it's more about trade-offs. With Kata, VM templating
> and KSM give users options to achieve better performance and lower
> memory footprint with little sacrifice. The security advantage of
> running VM-based containers is still there.

Adding options to enable/disable features leads to confusion among
users, makes performance comparisons harder, and increases support
overhead.

Technical solutions to the security problems are possible.  I'm
interested in progress in this area because it means users don't need to
make a choice, they can benefit from the feature without sacrificing
security.

Stefan

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