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

David Hildenbrand
2022-08-10T09:55:31Z

On 10.08.22 11:37, Chao Peng wrote:
> On Fri, Aug 05, 2022 at 03:28:50PM +0200, David Hildenbrand wrote:
>> On 06.07.22 10:20, Chao Peng wrote:
>>> 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.

If too complicated, we could simplify to "was this ever mapped" and fail
for now. Hooking into shmem_mmap() might be sufficient for that to get
notified about the first mmap.

As an alternative, mapping_mapped() or similar *might* do what we want.

--
Thanks,

David / dhildenb