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

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Date:   Tue, 16 Feb 2021 17:51:34 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     James Bottomley <jejb@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org, Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to
 create "secret" memory areas

On Tue 16-02-21 08:25:39, 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 do not remember details about mktme but from what I still recall it
had keys associated with direct maps. Is the key management something
that fits into flags management?

> 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.

Are those very HW specific features really viable through a generic
syscall? Don't get me wrong but I find it much more likely somebody will
want a hugetlb (pretty HW independent) without a direct map than a very
close to the HW caching mode soon.

But thanks for the clarification anyway.
-- 
Michal Hocko
SUSE Labs