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

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Fri, May 04, 2012 at 11:28:47AM +0800, Zhi Yong Wu wrote:
> On Tue, May 1, 2012 at 11:31 PM, Stefan Hajnoczi
> <stefanha@linux.vnet.ibm.com> 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
> By the way, How will it support them?

The problem with snapshot_blkdev is that closing a file and opening a
new file cannot be done by the QEMU process when an SELinux policy is in
place to prevent opening files.

The -open-hook-fd approach works even when the QEMU process is not
allowed to open files since file descriptor passing over a UNIX domain
socket is used to open files on behalf of QEMU.

Stefan