[libvirt] [RFC 0/5] block: File descriptor passing using -open-hook-fd
Anthony Liguori
aliguori at us.ibm.com
Tue May 1 20:25:42 UTC 2012
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 at 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.
Regards,
Anthony Liguori
>
> Anthony Liguori (3):
> block: add open() wrapper that can be hooked by libvirt
> block: add new command line parameter that and protocol description
> block: plumb up open-hook-fd option
>
> Stefan Hajnoczi (2):
> osdep: add qemu_recvmsg() wrapper
> Example -open-hook-fd server
>
> block.c | 107 ++++++++++++++++++++++++++++++++++++++
> block.h | 2 +
> block/raw-posix.c | 18 +++----
> block/raw-win32.c | 2 +-
> block/vdi.c | 2 +-
> block/vmdk.c | 6 +--
> block/vpc.c | 2 +-
> block/vvfat.c | 4 +-
> block_int.h | 12 +++++
> osdep.c | 46 +++++++++++++++++
> qemu-common.h | 2 +
> qemu-options.hx | 42 +++++++++++++++
> test-fd-passing.c | 147 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> vl.c | 3 ++
> 14 files changed, 378 insertions(+), 17 deletions(-)
> create mode 100644 test-fd-passing.c
>
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