Re: [PATCH v4 01/12] mm/shmem: Introduce F_SEAL_INACCESSIBLE

[Chao Peng]

On Fri, Feb 11, 2022 at 03:33:35PM -0800, Andy Lutomirski wrote:
> On 1/18/22 05:21, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > 
> > Introduce a new seal F_SEAL_INACCESSIBLE indicating the content of
> > the file is inaccessible from userspace through ordinary MMU access
> > (e.g., read/write/mmap). However, the file content can be accessed
> > via a different mechanism (e.g. KVM MMU) indirectly.
> > 
> > It provides semantics required for KVM guest private memory support
> > that a file descriptor with this seal set is going to be used as the
> > source of guest memory in confidential computing environments such
> > as Intel TDX/AMD SEV but may not be accessible from host userspace.
> > 
> > At this time only shmem implements this seal.
> > 
> 
> I don't dislike this *that* much, but I do dislike this. F_SEAL_INACCESSIBLE
> essentially transmutes a memfd into a different type of object.  While this
> can apparently be done successfully and without races (as in this code),
> it's at least awkward.  I think that either creating a special inaccessible
> memfd should be a single operation that create the correct type of object or
> there should be a clear justification for why it's a two-step process.

Now one justification maybe from Stever's comment to patch-00: for ARM
usage it can be used with creating a normal memfd, (partially)populate
it with initial guest memory content (e.g. firmware), and then
F_SEAL_INACCESSIBLE it just before the first time lunch of the guest in
KVM (definitely the current code needs to be changed to support that).

Thanks,
Chao
> 
> (Imagine if the way to create an eventfd would be to call timerfd_create()
> and then do a special fcntl to turn it into an eventfd but only if it's not
> currently armed.  This would be weird.)