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

[David Hildenbrand]

On 16.02.21 17:25, James Bottomley wrote:
> On Mon, 2021-02-15 at 20:20 +0100, Michal Hocko wrote:
> [...]
> > > >    What kind of flags are we talking about and why would that be a
> > > > problem with memfd_create interface? Could you be more specific
> > > > please?
> > >
> > > You mean what were the ioctl flags in the patch series linked
> > > above? They were SECRETMEM_EXCLUSIVE and SECRETMEM_UNCACHED in
> > > patch 3/5.
> >
> > OK I see. How many potential modes are we talking about? A few or
> > potentially many?
>   
> Well I initially thought there were two (uncached or not) until you
> came up with the migratable or non-migratable, which affects the
> security properties.  But now there's also potential for hardware
> backing, like mktme,  described by flags as well.  I suppose you could
> also use RDT to restrict which cache the data goes into: say L1 but not
> L2 on to lessen the impact of fully uncached (although the big thrust
> of uncached was to blunt hyperthread side channels).  So there is
> potential for quite a large expansion even though I'd be willing to bet
> that a lot of the modes people have thought about turn out not to be
> very effective in the field.

Thanks for the insight. I remember that even the "uncached" parts was 
effectively nacked by x86 maintainers (I might be wrong). For the other 
parts, the question is what we actually want to let user space configure.

Being able to specify "Very secure" "maximum secure" "average secure" 
all doesn't really make sense to me. 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, ...).

--
Thanks,

David / dhildenb