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Re: [Qemu-devel] [RFC Patch 0/3] Accept passed in socket 'fd' open from

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [RFC Patch 0/3] Accept passed in socket 'fd' open from outside for unix socket
Date: Thu, 2 Jun 2016 09:29:04 +0100
User-agent: Mutt/1.6.0 (2016-04-01)

On Thu, Jun 02, 2016 at 09:41:56AM +0200, Michal Privoznik wrote:
> On 01.06.2016 18:16, Wei Xu wrote:
> > On 2016年06月01日 00:44, Daniel P. Berrange wrote:
> >> On Wed, Jun 01, 2016 at 12:30:44AM +0800, address@hidden wrote:
> >>> From: Wei Xu <address@hidden>
> >>>
> >>> Recently I'm working on a fd passing issue, selinux forbids qemu to
> >>> create a unix socket for a chardev when managing VMs with libvirt,
> >>> because qemu don't have sufficient permissions in this case, and
> >>> proposal from libvirt team is opening the 'fd' in libvirt and merely
> >>> passing it to qemu.
> >>
> >> This sounds like a bug in libvirt, or selinux, or a mistaken
> >> configuration
> >> of the guest. It is entirely possible for QEMU to create a unix socket
> >> - not
> >> least because that is exactly what QEMU uses for its QMP monitor backend.
> >> Looking at your example command line, I think the issue is simply that
> >> you
> >> should be putting the sockets in a different location. ie at
> >> /var/lib/libvirt/qemu/$guest-vhost-user1.sock where QEMU has
> >> permission to
> >> create sockets already.
> > ah.. adjusting permission or file location can solve this problem, i'm
> > guessing maybe this is a more security concern, the socket is used as a
> > network interface for a vm, similar as the qcow image file, thus should
> > prevent it to be arbitrarily accessed.
> > 
> > Michael, do you have any comment on this?
> I haven't seen the patches. But in libvirt we allow users to create a
> vhostuser interface and even specify where the socket should be placed:
>     <interface type='vhostuser'>
>       <mac address='52:54:00:ee:96:6c'/>
>       <source type='unix' path='/tmp/vhost1.sock' mode='server'/>
>       <model type='virtio'/>
>     </interface>
> The following cmd line is generated by libvirt then:
> -chardev socket,id=charnet1,path=/tmp/vhost1.sock,server \
> -netdev type=vhost-user,id=hostnet1,chardev=charnet1 \
> -device
> virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:ee:96:6c,bus=pci.0,\
> Now, if we accept only /var/run/openvwitch path in
> /interface/source/@path (or whatever path to OVS is), we don't need this
> and have users manually label the dir (unless already labeled). But
> since we accept just any path in there, we should make sure that qemu is
> then able to create the socket. One possible fix would be to allow qemu
> create sockets just anywhere in the system. This, however, brings huge
> security risks and it's not acceptable IMO. The other option would be
> that libvirt would create the socket, and pass its FD to qemu (since
> libvirt already is allowed to create sockets anywhere).

There are plenty of other places where we allow arbitrary paths in the
XML, but which have restrictions imposed by the security drivers. Not
least the <channel> devices which have the exact same scenario as this
network device, and require use of /var/lib/libvirt/qemu as the directory
for the sockets. We certainly do not want to allow QEMU to create sockets

I don't think we want to grant QEMU svirtt permission to create sockets
in the /var/run/openvswitch directory either really.IMHO, users of vhost
user should really be using /var/lib/libvirt/qemu, as is used for all
other UNIX sockets we create wrt other devices.

> >> Alternatively you could enhance the SELinux policy to grant svirt_t the
> >> permission to create sockets under /var/run/openvswitch too.
> Nah, the point of using libvirt is that you don't have to configure
> anything (or just bare minimum) and libvirt makes sure your domains have
> all the permissions needed - that's why we relabel domain's disks when
> starting it up.

That's semi-true - we don't actually support arbitrary locations for
disks - there are plenty of locations that will not work, even if
libvirt labels the disk file, due to restrictions accessing the parent
directories by SELinux.

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