linux-kernel - Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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Date:   Wed, 19 Oct 2022 21:30:43 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Fuad Tabba <tabba@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, aarcange@...hat.com, ddutile@...hat.com,
        dhildenb@...hat.com, Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>, wei.w.wang@...el.com,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

On Mon, Oct 17, 2022 at 08:05:10PM +0100, Fuad Tabba wrote:
> Hi,
> 
> > > > Using both private_fd and userspace_addr is only needed in TDX and other
> > > > confidential computing scenarios, pKVM may only use private_fd if the fd
> > > > can also be mmaped as a whole to userspace as Sean suggested.
> > >
> > > That does work in practice, for now at least, and is what I do in my
> > > current port. However, the naming and how the API is defined as
> > > implied by the name and the documentation. By calling the field
> > > private_fd, it does imply that it should not be mapped, which is also
> > > what api.rst says in PATCH v8 5/8. My worry is that in that case pKVM
> > > would be mis/ab-using this interface, and that future changes could
> > > cause unforeseen issues for pKVM.
> >
> > That is fairly enough. We can change the naming and the documents.
> >
> > >
> > > Maybe renaming this to something like "guest_fp", and specifying in
> > > the documentation that it can be restricted, e.g., instead of "the
> > > content of the private memory is invisible to userspace" something
> > > along the lines of  "the content of the guest memory may be restricted
> > > to userspace".
> >
> > Some other candidates in my mind:
> > - restricted_fd: to pair with the mm side restricted_memfd
> > - protected_fd: as Sean suggested before
> > - fd: how it's explained relies on the memslot.flag.
> 
> All these sound good, since they all capture the potential use cases.
> Restricted might be the most logical choice if that's going to also
> become the name for the mem_fd.

Thanks, I will use 'restricted' for them. e.g.:
- memfd_restricted() syscall
- restricted_fd
- restricted_offset

The memslot flags will still be KVM_MEM_PRIVATE, since I think pKVM will
create its own one?

Chao
> 
> Thanks,
> /fuad
> 
> > Thanks,
> > Chao
> > >
> > > What do you think?
> > >
> > > Cheers,
> > > /fuad
> > >
> > > >
> > > > Thanks,
> > > > Chao
> > > > >
> > > > > Cheers,
> > > > > /fuad