Re: [PATCH v7 05/14] mm/memfd: Introduce MFD_INACCESSIBLE flag

On Fri, Aug 05, 2022 at 03:28:50PM +0200, David Hildenbrand wrote:

On 06.07.22 10:20, Chao Peng wrote:

quoted

Introduce a new memfd_create() flag indicating the content of the
created memfd 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 flag 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.

The flag can not coexist with MFD_ALLOW_SEALING, future sealing is
also impossible for a memfd created with this flag.

It's kind of weird to have it that way. Why should the user have to
care? It's the notifier requirement to have that, no?

Why can't we handle that when register a notifier? If anything is
already mapped, fail registering the notifier if the notifier has these
demands. If registering succeeds, block it internally.

Or what am I missing? We might not need the memfile set flag semantics
eventually and would not have to expose such a flag to user space.

This makes sense if doable. The major concern was: is there a reliable
way to detect this (already mapped) at the time of memslot registering.

Chao

-- 
Thanks,

David / dhildenb