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Date:   Mon, 3 Oct 2022 14:01:29 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Fuad Tabba <tabba@google.com>
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>,
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        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>,
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        Michael Roth <michael.roth@amd.com>, mhocko@suse.com,
        Muchun Song <songmuchun@bytedance.com>, wei.w.wang@intel.com
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd
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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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov