Re: [PATCH v4 02/12] mm/memfd: Introduce MFD_INACCESSIBLE flag

[Mike Rapoport]

On Tue, Feb 08, 2022 at 09:49:35AM +0100, David Hildenbrand wrote:
> On 07.02.22 19:51, Vlastimil Babka wrote:
> > On 1/18/22 14:21, Chao Peng wrote:
> >> Introduce a new memfd_create() flag indicating the content of the
> >> created memfd is inaccessible from userspace. It does this by force
> >> setting F_SEAL_INACCESSIBLE seal when the file is created. It also set
> >> F_SEAL_SEAL to prevent future sealing, which means, it can not coexist
> >> with MFD_ALLOW_SEALING.
> >>
> >> The pages backed by such memfd will be used as guest private memory in
> >> confidential computing environments such as Intel TDX/AMD SEV. Since
> >> page migration/swapping is not yet supported for such usages so these
> >> pages are currently marked as UNMOVABLE and UNEVICTABLE which makes
> >> them behave like long-term pinned pages.
> > 
> > Shouldn't the amount of such memory allocations be restricted? E.g. similar
> > to secretmem_mmap() doing mlock_future_check().

Heh, for me it was easy, I had the VMA :)
 
> I've raised this already in the past and Kirill wanted to look into it [1].
> 
> We'll most certainly need a way to limit/control the amount of
> unswappable + unmovable ("worse than mlock" memory) a user/process can
> consume via this mechanism.

I think the accounting can be handled in notify_fallocate() and
notify_invalidate_page().

> [1] https://lkml.kernel.org/r/20211122135933.arjxpl7wyskkwvwv@xxxxxxxxxxxxxxxxx
>
> -- 
> Thanks,
> 
> David / dhildenb

-- 
Sincerely yours,
Mike.