linux-kernel - Re: [PATCH RFC 00/24] userfaultfd: write protection support

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Message-ID: <c36071dd-da8a-22fa-8f9a-262c942fcdf4@redhat.com>
Date:   Tue, 22 Jan 2019 09:59:34 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Jerome Glisse <jglisse@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH RFC 00/24] userfaultfd: write protection support

On 22.01.19 04:18, Peter Xu wrote:
> On Mon, Jan 21, 2019 at 03:33:21PM +0100, David Hildenbrand wrote:
> 
> [...]
> 
>> Does this series fix the "false positives" case I experienced on early
>> prototypes of uffd-wp? (getting notified about a write access although
>> it was not a write access?)
> 
> Hi, David,
> 
> Yes it should solve it.

Terrific, as my use case for uffd-wp really rely on not having false
positives these are good news :)

... however it will take a while until I actually have time to look back
into it (too much stuff on my table).

Just for reference (we talked about this offline once):

My plan is to use this for virtio-mem in QEMU. Memory that a virtio-mem
device provides to a guest can either be plugged or unplugged. When
unplugging, memory will be MADVISE_DONTNEED'ed and uffd-wp'ed. The guest
can still read memory (e.g. for dumping) but writing to it is considered
bad (as the guest could this way consume more memory as intended). So I
can detect malicious guests without too much overhead this way.

False positives would mean that I would detect guests as malicious
although they are not. So it really would be harmful.

Thanks!

> 
> The early prototype in Andrea's tree hasn't yet applied the new
> PTE/swap bits for uffd-wp hence it was not able to avoid those fause
> positives.  This series has applied all those ideas (which actually
> come from Andrea as well) so the protection information will be
> persisent per PTE rather than per VMA and it will be kept even through
> swapping and page migrations.
> 
> Thanks,
> 


-- 

Thanks,

David / dhildenb