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

From: Wei Xu
Subject: Re: [Qemu-devel] [RFC Patch 0/3] Accept passed in socket 'fd' open from outside for unix socket
Date: Fri, 3 Jun 2016 02:07:00 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 2016年06月02日 19:38, Michal Privoznik wrote:
On 02.06.2016 10:29, Daniel P. Berrange wrote:
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
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
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'/>

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 \

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
AFAIK, Vhost user is an interface for third party implementations, and ovs/dpdk is one of the most popular choices, if it limits the socket location of a fixed and unprivileged directory to qemu, actually this should be the default and only one option, this maybe also a security consideration, so we'll have no other choice but ask sys admin to manipulate the permission, looks accepting a safe passed in 'fd' from libvirt is more rigorous and convenient, i'm not sure if this is a typical or a corner scenario.

How do you think about this with a general purpose? does qemu need such a feature?

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.

Okay. I can live with that; but in that case we should document it
somewhere, that we guarantee only paths under /var/lib/libvirt/ to be
accessible and for the rest we do our best but maybe require sys admin
intervention (e.g. to label the whole tree for a non-standard location).


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