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

[Zhi Yong Wu <zwu.kernel@gmail.com>]

On Thu, May 17, 2012 at 9:42 PM, Stefan Hajnoczi
<stefanha@linux.vnet.ibm.com> wrote:
> 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.
I thought that the patchset can only let QEMU passively get passed fd
parameter from upper application.
>
> Stefan
>



-- 
Regards,

Zhi Yong Wu