[libvirt] [Qemu-devel] live snapshot wiki updated

Kevin Wolf kwolf at redhat.com
Wed Jul 20 10:01:11 UTC 2011

Am 20.07.2011 10:25, schrieb Jes Sorensen:
> On 07/19/11 18:14, Anthony Liguori wrote:
>>>> As nice as that sentiment is, it will never fly, because it would be a
>>>> regression in current behavior.  The whole reason that the virt_use_nfs
>>>> SELinux bool exists is that some people are willing to make the partial
>>>> security tradeoff.  Besides, the use of sVirt via SELinux is more than
>>>> just open() protection - while the current virt_use_nfs bool makes NFS
>>>> less secure than otherwise possible, it still gives some nice guarantees
>>>> to the rest of the qemu process such as passthrough accesses to local
>>>> pci devices.
>>> Well leaving things at status quo is not making it worse, it just leaves
>>> an evil in place.
>> NFS and SELinux is a fundamental problem with SELinux and NFS.  We can
>> piss and moan as much as we want about it but it's reality.  SELinux
>> fundamentally requires extended attributes.  By the time NFS adds
>> extended attribute support, we'll all be flying around in hover cars.
>> As terrible as NFS is, people use it all of the time.
>> It would be nice if libvirt had the ability to make better use of DAC to
>> support isolation.  The fact that MAC is the only way you can do
>> isolation between guests is pretty unfortunate.  If I could assign
>> specific UIDs to a guest and use that to enforce isolation, it would go
>> a long ways to solving this problem.
> Right, we're stuck with the two horros of NFS and selinux, so we need
> something that gets around the problem. In a sane world we would simply
> say 'no NFS, no selinux', but as you say that will never happen.
> My suggestion of a callback mechanism where libvirt registers the
> callback with QEMU for open() calls, allowing libvirt to perform the
> open and return the open file descriptor would get around this problem.

To me this sounds more like a problem than a solution. It basically
means that during an open (which may even be initiated by a monitor
command), you need monitor interaction. It basically means that open
becomes asynchronous, and requires clients to deal with that, which
sounds at least "interesting"... Also you have to add some magic to all
places opening something.

I think if libvirt wants qemu to use an fd instead of a file name, it
shouldn't pass a file name but an fd in the first place. Which means
that the two that we need are support for an fd: protocol (patches on
the list, need review), and a way for libvirt to override the backing
file of an image.


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