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Re: [libvirt] [Qemu-devel] [RFC 0/5] block: File descriptor passing using -open-hook-fd



On Wed, May 2, 2012 at 9:20 AM, Kevin Wolf <kwolf redhat com> wrote:
> Am 01.05.2012 22:25, schrieb Anthony Liguori:
>> Thanks for sending this out Stefan.
>>
>> On 05/01/2012 10:31 AM, Stefan Hajnoczi wrote:
>>> Libvirt can take advantage of SELinux to restrict the QEMU process and prevent
>>> it from opening files that it should not have access to.  This improves
>>> security because it prevents the attacker from escaping the QEMU process if
>>> they manage to gain control.
>>>
>>> NFS has been a pain point for SELinux because it does not support labels (which
>>> I believe are stored in extended attributes).  In other words, it's not
>>> possible to use SELinux goodness on QEMU when image files are located on NFS.
>>> Today we have to allow QEMU access to any file on the NFS export rather than
>>> restricting specifically to the image files that the guest requires.
>>>
>>> File descriptor passing is a solution to this problem and might also come in
>>> handy elsewhere.  Libvirt or another external process chooses files which QEMU
>>> is allowed to access and provides just those file descriptors - QEMU cannot
>>> open the files itself.
>>>
>>> This series adds the -open-hook-fd command-line option.  Whenever QEMU needs to
>>> open an image file it sends a request over the given UNIX domain socket.  The
>>> response includes the file descriptor or an errno on failure.  Please see the
>>> patches for details on the protocol.
>>>
>>> The -open-hook-fd approach allows QEMU to support file descriptor passing
>>> without changing -drive.  It also supports snapshot_blkdev and other commands
>>> that re-open image files.
>>>
>>> Anthony Liguori<aliguori us ibm com>  wrote most of these patches.  I added a
>>> demo -open-hook-fd server and added some small fixes.  Since Anthony is
>>> traveling right now I'm sending the RFC for discussion.
>>
>> What I like about this approach is that it's useful outside the block layer and
>> is conceptionally simple from a QEMU PoV.  We simply delegate open() to libvirt
>> and let libvirt enforce whatever rules it wants.
>>
>> This is not meant to be an alternative to blockdev, but even with blockdev, I
>> think we still want to use a mechanism like this even with blockdev.
>
> What does it provide on top?

It solves the problem of snapshot_blkdev and other operations that
re-open files.  Using -blockdev and hotplug for image files as file
descriptors only solves the static configuration problem, not the
runtime problem we get with snapshot_blkdev.  That's why this approach
is more powerful than -blockdev fd=N.

Stefan


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