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

From: Aaron Conole
Subject: Re: [Qemu-devel] [RFC Patch 0/3] Accept passed in socket 'fd' open from outside for unix socket
Date: Tue, 14 Jun 2016 10:21:22 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1.50 (gnu/linux)

Wei Xu <address@hidden> writes:

> On 2016年06月09日 05:48, Aaron Conole wrote:
>> Flavio Leitner <address@hidden> writes:
>>> Adding Aaron who is fixing exactly that on the OVS side.
>>> Aaron, please see the last question in the bottom of this email.
>>> On Wed, Jun 08, 2016 at 06:07:29AM -0400, Amnon Ilan wrote:
>>>> ----- Original Message -----
>>>>> From: "Michal Privoznik" <address@hidden>
>>>>> To: "Daniel P. Berrange" <address@hidden>
>>>>> Cc: address@hidden, "amit shah" <address@hidden>,
>>>>> address@hidden, "Wei Xu" <address@hidden>,
>>>>> address@hidden
>>>>> Sent: Thursday, June 2, 2016 2:38:53 PM
>>>>> Subject: Re: [Qemu-devel] [RFC Patch 0/3] Accept passed in socket
>>>>> 'fd' open from outside for unix socket
>>>>> 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
>>>>>>>>> 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
>>>>>> anywhere.
>>>>>> 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).
>>>> Does OVS has some limit for it's sockets to be only in 
>>>> /var/run/openvswitch ?
>> As of a recent commit, it can only be in /var/run/openvswitch or a
>> subdirectory therein (found in the openvswitch database).
> Aaron, thanks for your reply.
> Just a question about the usage of openvswitch, in this user case when
> launching a vhostuser/dpdk via libvirt, qemu works as server mode for
> socket under /var/run/openvswitch, but per my previous test, ovs/dpdk
> always works as server mode, which means ovs will creates the socket
> and listening for connection, thus qemu works as client mode, does
> ovs/dpdk support working in client mode? which means it's qemu's duty
> to create the socket? and ovs will connect to it on demanding?

OvS does not currently support client mode.  It will be some work to add
support, as dpdk upstream does not currently support vhostuser in client
mode, and that would need to be added before ovs would have support.

>>>> Flavio, do you know?
>>>> If not, we are good as it is today with /var/lib/libvirt/qemu, right?
>> Probably not.  There are other issues as well.  From a DAC perspective
>> (so forgetting selinux at the moment), qemu and ovs run as different
>> users.  This will mean that when ovs creates the unix domain socket
>> file, qemu will not be allowed to actually open it properly.  I have a
>> commit I'm trying to get upstream for that particular issue (see
>> bz:1281911 and upstream list discussion:
>>    http://openvswitch.org/pipermail/dev/2016-May/071453.html
>>    and
>>    http://openvswitch.org/pipermail/dev/2016-June/071978.html)
>> Ansis is (I think) attacking this from the selinux/MAC side.  It would
>> be great to hear from users, libvirt folks, or anyone else in that
>> thread to help push it to a conclusion one way or another (even if the
>> approach that I've proposed is crap and wrong - then say it there :).
>> Hope this helps,
>> -Aaron

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