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

From: Jason Gunthorpe
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST
Date: Mon, 22 Nov 2021 10:01:48 -0400

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?

"no new access" makes sense to me, we have access through
read/write/mmap/splice/etc and access to pages through the private in
kernel interface (kvm, iommu)


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