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

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

On 08/22/2011 02:39 PM, Blue Swirl wrote:
> On Mon, Aug 22, 2011 at 5:42 PM, Corey Bryant<coreyb@linux.vnet.ibm.com>  wrote:
>> >
>> >
>> >  On 08/22/2011 01:25 PM, 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.
>>> >>
>> >
>> >  I do like the idea of a privileged qemu-fe performing the open and passing
>> >  the fd to a restricted qemu.
> Me too.
>
>> >    However, I get the impression that this won't
>> >  get delivered nearly as quickly as fd: passing could be.  How soon do we
>> >  need image isolation for NFS?
>> >
>> >  Btw, this sounds similar to what Blue Swirl recommended here on v1 of this
>> >  patch:http://lists.gnu.org/archive/html/qemu-devel/2011-05/msg02187.html
> I was thinking about simply doing fork() + setuid() at some point and
> using the FD passing structures directly. But would it bring
> advantages to have two separate executables, are they different from
> access control point of view vs. single but forked one?
>

We could put together an SELinux policy that would transition qemu-fe to 
a more restricted domain (ie. no open privilege on NFS files) when it 
executes qemu-system-x86_64.

-- 
Regards,
Corey

>> >  Regards,
>> >  Corey
>> >
>>>> >>>  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_addfile:/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.
>>> >>
>>>> >>>  So I'd see that you'd likely end up with the
>>>> >>>  qemu-fe security policy being identical to the qemu security policy,
>>> >>
>>> >>  Then there's no point in doing qemu-fe. qemu-fe should be thought of as
>>> >>  QEMU supplied libvirtd plugin.
>>> >>
>>>> >>>  with the exception that it would be allowed to open files on NFS without
>>>> >>>  needing them to be labelled. So I don't really see that all this gives us
>>>> >>>  any tangible benefits over just allowing the mgmt app to pass in the FDs
>>>> >>>  directly.
>>>> >>>
>>>>> >>>>  But libvirt would still need to parse image files.
>>>> >>>
>>>> >>>  Not neccessarily. As mentioned below, it is entirely possible to
>>>> >>>  enable the mgmt app to pass in details of the backing files, at
>>>> >>>  which point no image parsing is required by libvirt. Hence my
>>>> >>>  assertion that the question of who does image parsing is irrelevant
>>>> >>>  to this discussion.
>>> >>
>>> >>  That's certainly true.
>>> >>
>>> >>  Regards,
>>> >>
>>> >>  Anthony Liguori
>> >
>> >
>> >