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

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Date:   Fri, 11 Feb 2022 15:33:35 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Chao Peng <chao.p.peng@...ux.intel.com>, 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>
Cc:     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 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.

(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.)