linux-kernel - Re: [PATCH v4 01/12] mm/shmem: Introduce F_SEAL

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Date:   Thu, 17 Feb 2022 21:06:31 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        qemu-devel@...gnu.org, Linux API <linux-api@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        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>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [PATCH v4 01/12] mm/shmem: Introduce F_SEAL_INACCESSIBLE

On Fri, Feb 11, 2022 at 03:33:35PM -0800, Andy Lutomirski wrote:
> On 1/18/22 05:21, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > 
> > Introduce a new seal F_SEAL_INACCESSIBLE indicating the content of
> > the file is inaccessible from userspace through ordinary MMU access
> > (e.g., read/write/mmap). However, the file content can be accessed
> > via a different mechanism (e.g. KVM MMU) indirectly.
> > 
> > It provides semantics required for KVM guest private memory support
> > that a file descriptor with this seal set is going to be used as the
> > source of guest memory in confidential computing environments such
> > as Intel TDX/AMD SEV but may not be accessible from host userspace.
> > 
> > At this time only shmem implements this seal.
> > 
> 
> I don't dislike this *that* much, but I do dislike this. F_SEAL_INACCESSIBLE
> essentially transmutes a memfd into a different type of object.  While this
> can apparently be done successfully and without races (as in this code),
> it's at least awkward.  I think that either creating a special inaccessible
> memfd should be a single operation that create the correct type of object or
> there should be a clear justification for why it's a two-step process.

Now one justification maybe from Stever's comment to patch-00: for ARM
usage it can be used with creating a normal memfd, (partially)populate
it with initial guest memory content (e.g. firmware), and then
F_SEAL_INACCESSIBLE it just before the first time lunch of the guest in
KVM (definitely the current code needs to be changed to support that).

Thanks,
Chao
> 
> (Imagine if the way to create an eventfd would be to call timerfd_create()
> and then do a special fcntl to turn it into an eventfd but only if it's not
> currently armed.  This would be weird.)