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

Jason Gunthorpe
2021-11-22T15:10:01Z

On Mon, Nov 22, 2021 at 03:57:17PM +0100, David Hildenbrand wrote:
> On 22.11.21 15:01, Jason Gunthorpe wrote:
> > On Mon, Nov 22, 2021 at 02:35:49PM +0100, David Hildenbrand wrote:
> >> On 22.11.21 14:31, Jason Gunthorpe wrote:
> >>> On Mon, Nov 22, 2021 at 10:26:12AM +0100, David Hildenbrand wrote:
> >>>
> >>>> I do wonder if we want to support sharing such memfds between processes
> >>>> in all cases ... we most certainly don't want to be able to share
> >>>> encrypted memory between VMs (I heard that the kernel has to forbid
> >>>> that). It would make sense in the use case you describe, though.
> >>>
> >>> If there is a F_SEAL_XX that blocks every kind of new access, who
> >>> cares if userspace passes the FD around or not?
> >> I was imagining that you actually would want to do some kind of "change
> >> ownership". But yeah, the intended semantics and all use cases we have
> >> in mind are not fully clear to me yet. If it's really "no new access"
> >> (side note: is "access" the right word?) then sure, we can pass the fd
> >> around.
> >
> > What is "ownership" in a world with kvm and iommu are reading pages
> > out of the same fd?
>
> In the world of encrypted memory / TDX, KVM somewhat "owns" that memory
> IMHO (for example, only it can migrate or swap out these pages; it's
> might be debatable if the TDX module or KVM actually "own" these pages ).

Sounds like it is a swap provider more than an owner?

Jason