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Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to
 create "secret" memory areas
From:   James Bottomley <jejb@linux.ibm.com>
Reply-To: jejb@linux.ibm.com
To:     Michal Hocko <mhocko@suse.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Mike Rapoport <rppt@kernel.org>,
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Date:   Tue, 16 Feb 2021 08:25:39 -0800
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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.

James