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

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Message-ID: <20221003110129.bbee7kawhw5ed745@box.shutemov.name>
Date:   Mon, 3 Oct 2022 14:01:29 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Fuad Tabba <tabba@...gle.com>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        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, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.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>,
        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>, luto@...nel.org,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.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
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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