Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

Jason Gunthorpe
2021-11-21T00:05:30Z

On Sat, Nov 20, 2021 at 01:23:16AM +0000, Sean Christopherson wrote:
> On Fri, Nov 19, 2021, Jason Gunthorpe wrote:
> > On Fri, Nov 19, 2021 at 10:21:39PM +0000, Sean Christopherson wrote:
> > > On Fri, Nov 19, 2021, Jason Gunthorpe wrote:
> > > > On Fri, Nov 19, 2021 at 07:18:00PM +0000, Sean Christopherson wrote:
> > > > > No ideas for the kernel API, but that's also less concerning since
> > > > > it's not set in stone. I'm also not sure that dedicated APIs for
> > > > > each high-ish level use case would be a bad thing, as the semantics
> > > > > are unlikely to be different to some extent. E.g. for the KVM use
> > > > > case, there can be at most one guest associated with the fd, but
> > > > > there can be any number of VFIO devices attached to the fd.
> > > >
> > > > Even the kvm thing is not a hard restriction when you take away
> > > > confidential compute.
> > > >
> > > > Why can't we have multiple KVMs linked to the same FD if the memory
> > > > isn't encrypted? Sure it isn't actually useful but it should work
> > > > fine.
> > >
> > > Hmm, true, but I want the KVM semantics to be 1:1 even if memory
> > > isn't encrypted.
> >
> > That is policy and it doesn't belong hardwired into the kernel.
>
> Agreed. I had a blurb typed up about that policy just being an "exclusive" flag
> in the kernel API that KVM would set when creating a confidential
> VM,

I still think that is policy in the kernel, what is wrong with
userspace doing it?

> > Your explanation makes me think that the F_SEAL_XX isn't defined
> > properly. It should be a userspace trap door to prevent any new
> > external accesses, including establishing new kvms, iommu's, rdmas,
> > mmaps, read/write, etc.
>
> Hmm, the way I was thinking of it is that it the F_SEAL_XX itself would prevent
> mapping/accessing it from userspace, and that any policy beyond that would be
> done via kernel APIs and thus handled by whatever in-kernel agent can access the
> memory. E.g. in the confidential VM case, without support for trusted devices,
> KVM would require that it be the sole owner of the file.

And how would kvm know if there is support for trusted devices?
Again seems like policy choices that should be left in userspace.

Especially for what could be a general in-kernel mechanism with many
users and not tightly linked to KVM as imagined here.

Jason