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From:   Fuad Tabba <tabba@google.com>
Date:   Tue, 4 Oct 2022 16:39:15 +0100
Message-ID: <CA+EHjTzS1YDS0DxMyKaUqgykKp5G637NoGPQxdORQfLfKRNqrQ@mail.gmail.com>
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
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        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
        Steven Price <steven.price@arm.com>,
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        Muchun Song <songmuchun@bytedance.com>, wei.w.wang@intel.com
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Hi,

On Mon, Oct 3, 2022 at 12:01 PM Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Mon, Oct 03, 2022 at 08:33:13AM +0100, Fuad Tabba wrote:
> > > I think it is "don't do that" category. inaccessible_register_notifier()
> > > caller has to know what file it operates on, no?
> >
> > The thing is, you could oops the kernel from userspace. For that, all
> > you have to do is a memfd_create without the MFD_INACCESSIBLE,
> > followed by a KVM_SET_USER_MEMORY_REGION using that as the private_fd.
> > I ran into this using my port of this patch series to arm64.
>
> My point is that it has to be handled on a different level. KVM has to
> reject private_fd if it is now inaccessible. It should be trivial by
> checking file->f_inode->i_sb->s_magic.

Yes, that makes sense.

Thanks,
/fuad

> --
>   Kiryl Shutsemau / Kirill A. Shutemov