Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas

[David Hildenbrand]

On 22.02.21 10:38, David Hildenbrand wrote:
> On 17.02.21 17:19, James Bottomley wrote:
> > On Tue, 2021-02-16 at 18:16 +0100, David Hildenbrand wrote:
> > [...]
> > > > >     The discussion regarding migratability only really popped up
> > > > > because this is a user-visible thing and not being able to
> > > > > migrate can be a real problem (fragmentation, ZONE_MOVABLE, ...).
> > > >
> > > > I think the biggest use will potentially come from hardware
> > > > acceleration.  If it becomes simple to add say encryption to a
> > > > secret page with no cost, then no flag needed.  However, if we only
> > > > have a limited number of keys so once we run out no more encrypted
> > > > memory then it becomes a costly resource and users might want a
> > > > choice of being backed by encryption or not.
> > >
> > > Right. But wouldn't HW support with configurable keys etc. need more
> > > syscall parameters (meaning, even memefd_secret() as it is would not
> > > be sufficient?). I suspect the simplistic flag approach might not
> > > be sufficient. I might be wrong because I have no clue about MKTME
> > > and friends.
> >
> > The theory I was operating under is key management is automatic and
> > hidden, but key scarcity can't be, so if you flag requesting hardware
> > backing then you either get success (the kernel found a key) or failure
> > (the kernel is out of keys).  If we actually want to specify the key
> > then we need an extra argument and we *must* have a new system call.
> >
> > > Anyhow, I still think extending memfd_create() might just be good
> > > enough - at least for now.
> >
> > I really think this is the wrong approach for a user space ABI.  If we
> > think we'll ever need to move to a separate syscall, we should begin
> > with one.  The pain of trying to shift userspace from memfd_create to a
> > new syscall would be enormous.  It's not impossible (see clone3) but
> > it's a pain we should avoid if we know it's coming.
>
> Sorry for the late reply, there is just too much going on :)
>
> *If* we ever realize we need to pass more parameters we can easily have
> a new syscall for that purpose. *Then*, we know how that syscall will
> look like. Right now, it's just pure speculation.
>
> Until then, going with memfd_create() works just fine IMHO.
>
> The worst think that could happen is that we might not be able to create
> all fancy sectremem flavors in the future via memfd_create() but only
> via different, highly specialized syscall. I don't see a real problem
> with that.

Adding to that, I'll give up arguing now as I have more important things 
to do. It has been questioned by various people why we need a dedicate 
syscall and at least for me, without a satisfying answer.

Worst thing is that we end up with a syscall that could have been 
avoided, for example, because
1. We add existing/future memfd_create() flags to memfd_secret() as well 
when we need them (sealing, hugetlb., ..).
2. We decide in the future to still add MFD_SECRET support to 
memfd_secret().

So be it.

--
Thanks,

David / dhildenb