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

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Date:   Fri, 12 Feb 2021 00:59:29 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     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>,
        David Hildenbrand <david@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.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 Thu, Feb 11, 2021 at 01:30:42PM +0100, Michal Hocko wrote:
> On Thu 11-02-21 13:20:08, Mike Rapoport wrote:
> [...]
> > Sealing is anyway controlled via fcntl() and I don't think
> > MFD_ALLOW_SEALING makes much sense for the secretmem because it is there to
> > prevent rogue file sealing in tmpfs/hugetlbfs.
> 
> This doesn't really match my understanding. The primary usecase for the
> sealing is to safely and predictably coordinate over shared memory. I
> absolutely do not see why this would be incompatible with an additional
> requirement to unmap the memory from the kernel to prevent additional
> interference from the kernel side. Quite contrary it looks like a very
> nice extension to this model.

I didn't mean that secretmem should not support sealing. I meant that
MFD_ALLOW_SEALING flag does not make sense. Unlike tmpfs, the secretmem fd
does not need protection from somebody unexpectedly sealing it.

> > As for the huge pages, I'm not sure at all that supporting huge pages in
> > secretmem will involve hugetlbfs.
> 
> Have a look how hugetlb proliferates through our MM APIs. I strongly
> suspect this is strong signal that this won't be any different.
> 
> > And even if yes, adding SECRETMEM_HUGE
> > flag seems to me less confusing than saying "from kernel x.y you can use
> > MFD_CREATE | MFD_SECRET | MFD_HUGE" etc for all possible combinations.
> 
> I really fail to see your point. This is a standard model we have. It is
> quite natural that flags are added. Moreover adding a new syscall will
> not make it any less of a problem.

Nowadays adding a new syscall is not as costly as it used to be. And I
think it'll provide better extensibility when new features would be added
to secretmem. 

For instance, for creating a secretmem fd backed with sealing we'd have

	memfd_secretm(SECRETMEM_HUGE);

rather than

	memfd_create(MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_SECRET);


Besides, if we overload memfd_secret we add complexity to flags validation
of allowable flag combinations even with the simplest initial
implementation.
And what it will become when more features are added to secretmem?
 
> > > I by no means do not insist one way or the other but from what I have
> > > seen so far I have a feeling that the interface hasn't been thought
> > > through enough.
> > 
> > It has been, but we have different thoughts about it ;-)
> 
> Then you must be carrying a lot of implicit knowledge which I want you
> to document.

I don't have any implicit knowledge, we just have a different perspective.

-- 
Sincerely yours,
Mike.