Re: [RFC 0/4] Adding Virtual Memory Fuses to Xen

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, Dec 13, 2022 at 08:55:28PM +0000, Julien Grall wrote:
> On 13/12/2022 19:48, Smith, Jackson wrote:
> > Hi Xen Developers,
> 
> Hi Jackson,
> 
> Thanks for sharing the prototype with the community. Some questions/remarks
> below.

[snip]

> > With this technique, we protect the integrity and confidentiality of
> > guest memory. However, a compromised hypervisor can still read/write
> > register state during traps, or refuse to schedule a guest, denying
> > service. We also recognize that because this technique precludes
> > modifying Xen's page tables after startup, it may not be compatible
> > with all of Xen's potential use cases. On the other hand, there are
> > some uses cases (in particular statically defined embedded systems)
> > where our technique could be adopted with minimal friction.
> 
> From what you wrote, this sounds very much like the project Citrix and
> Amazon worked on called "Secret-free hypervisor" with a twist. In your case,
> you want to prevent the hypervisor to map/unmap the guest memory.
> 
> You can find some details in [1]. The code is x86 only, but I don't see any
> major blocker to port it on arm64.

Is there any way the secret-free hypervisor code could be upstreamed?
My understanding is that it would enable guests to use SMT without
risking the host, which would be amazing.

> >     Virtualized MMIO on arm needs to decode certain load/store
> >     instructions
> 
> On Arm, this can be avoided of the guest OS is not using such instruction.
> In fact they were only added to cater "broken" guest OS.
> 
> Also, this will probably be a lot more difficult on x86 as, AFAIK, there is
> no instruction syndrome. So you will need to decode the instruction in order
> to emulate the access.

Is requiring the guest to emulate such instructions itself an option?
μXen, SEV-SNP, and TDX all do this.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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