Re: [Qemu-devel] [libvirt] [PATCH v4] Add support for fd: protocol

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

On 08/22/2011 03:25 PM, Anthony Liguori wrote:
> On 08/22/2011 01:22 PM, Daniel P. Berrange wrote:
>> On Mon, Aug 22, 2011 at 12:25:25PM -0500, Anthony Liguori wrote:
>>> On 08/22/2011 11:50 AM, Daniel P. Berrange wrote:
>>>> On Mon, Aug 22, 2011 at 11:29:12AM -0500, Anthony Liguori wrote:
>>>>> I don't think it makes sense to have qemu-fe do dynamic labelling.
>>>>> You certainly could avoid the fd passing by having qemu-fe do the
>>>>> open though and just let qemu-fe run without the restricted security
>>>>> context.
>>>>
>>>> qemu-fe would also not be entirely simple,
>>>
>>> Indeed.
>>>
>>>> because it will need to act
>>>> as a proxy for the monitor, in order to make hotplug work. ie the mgmt
>>>> app would be sending 'drive_add file:/foo/bar' to qemu-fe, which would
>>>> then have to open the file and send 'drive_add fd:NN' onto the real
>>>> QEMU,
>>>> and then pass the results on back.
>>>>
>>>> In addition qemu-fe would still have to be under some kind of
>>>> restricted
>>>> security context for it to be acceptable. This is going to want to
>>>> be as
>>>> locked down as possible.
>>>
>>> I think there's got to be some give and take here.
>>>
>>> It should at least be as locked down as libvirtd. From a security
>>> point of view, we should be able to agree that we want libvirtd to
>>> be as locked down as possible.
>>>
>>> But there shouldn't be a hard requirement to lock down qemu-fe more
>>> than libvirtd. Instead, the requirement should be for qemu-fe to be
>>> as/more vigilant in not trusting qemu-system-x86_64 as libvirtd is.
>>>
>>> The fundamental problem here, is that there is some logic in
>>> libvirtd that rightly belongs in QEMU. In order to preserve the
>>> security model, that means that we're going to have to take a
>>> subsection of QEMU and trust it more.
>>
>> Well we have a process that makes security decisions, and a process
>> which applies those security decisions and a process which is confined
>> by those decisions. Currently libvirtd makes& applies the decisions,
>> and qemu is confined. A qemu-fe model would mean that libvirt is making
>> the decisions, but is then relying on qemu-fe to apply them. IMHO that
>> split is undesirable, but that's besides the point, since this is not
>> a decision that needs to be made now.
>>
>> 'qemu-fe' needs to have a way to communicate with the confined process
>> ('qemu-system-XXX') to supply it the resources (file FDs) it needs to
>> access. The requirements of such a comms channel for qemu-fe are going
>> to be the same as those needed by libvirtd talking to QEMU today, or
>> indeed by any process that is applying security decisions to QEMU.
>
> But the fundamental difference is that libvirtd uses what's ostensible a
> public, supported interface. That means when we add things like this,
> we're stuck supporting it for general use cases.
>
> It's much more palatable to do these things using a private interface
> such that we can change these things down the road without worrying
> about compatibility with third-party tools.
>
> Regards,
>
> Anthony Liguori
>

Is this a nack for the fd: protocol?  Or do we want to implement the fd: 
protocol as a stepping stone on the way to a privilege-separated qemu 
model?  I know the fd: protocol is not ideal, but it does provide NFS 
image isolation, perhaps much sooner than privilege-separated qemu can.

-- 
Regards,
Corey