Re: [Qemu-devel] [PATCH] Add support for fd: protocol

[Kevin Wolf <kwolf@redhat.com>]

Am 20.05.2011 21:53, schrieb Blue Swirl:
> On Fri, May 20, 2011 at 10:42 PM, Anthony Liguori <anthony@codemonkey.ws> wrote:
>> On 05/20/2011 02:25 PM, Blue Swirl wrote:
>>>
>>> On Fri, May 20, 2011 at 9:48 PM, Corey Bryant<bryntcor@us.ibm.com>  wrote:
>>>>
>>>> sVirt provides SELinux MAC isolation for Qemu guest processes and their
>>>> corresponding resources (image files). sVirt provides this support
>>>> by labeling guests and resources with security labels that are stored
>>>> in file system extended attributes. Some file systems, such as NFS, do
>>>> not support the extended attribute security namespace, which is needed
>>>> for image file isolation when using the sVirt SELinux security driver
>>>> in libvirt.
>>>>
>>>> The proposed solution entails a combination of Qemu, libvirt, and
>>>> SELinux patches that work together to isolate multiple guests' images
>>>> when they're stored in the same NFS mount. This results in an
>>>> environment where sVirt isolation and NFS image file isolation can both
>>>> be provided.
>>>
>>> Very nice. QEMU should use this to support privilege separation. We
>>> already have chroot and runas switches, a new switch should convert
>>> all file references to fd references internally for that process. If
>>> this can be made transparent, this should even be the default way of
>>> operation.
>>
>> You mean, QEMU starts up, opens all disk images, reinvokes itself in a
>> confined context, and then passes fds to the child?
> 
> And exit after that, or do the same without forking.
> 
> This wouldn't work now for the native CDROM devices which need to
> reopen the device. For that, an explicit reopen method could be added.
> The method could even chat with the privileged process to get that to
> do the reopening, but I'd leave that to libvirt and fail without it
> for plain QEMU.

There are more cases where we reopen the image file. One example is the
'commit' monitor command which temporarily reopens the backing file r/w.
Or Christoph's patch that allows guests to toggle the write-cache
enabled bit. Same for live snapshots. So we'll need a solution for them
before doing anything like this.

And breaking qemu without libvirt isn't really an option for me.

Kevin