On Wed, May 2, 2012 at 9:20 AM, Kevin Wolf <kw...@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
>>> 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
>>> 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
>>> 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
>>> 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.
>>> response includes the file descriptor or an errno on failure. Please see
>>> 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
>>> that re-open image files.
>>> Anthony Liguori<aligu...@us.ibm.com> wrote most of these patches. I added
>>> 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
>> is conceptionally simple from a QEMU PoV. We simply delegate open() to
>> 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.