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