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



On Tue, Aug 23, 2011 at 05:50:03PM +0200, Kevin Wolf wrote:
> Am 23.08.2011 17:26, schrieb Daniel P. Berrange:
> > On Tue, Aug 23, 2011 at 11:13:34AM -0400, Corey Bryant wrote:
> >>
> >>
> >> 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.
> > 
> > Thinking about this some more, I don't really think the idea of delegating
> > open of NFS files to a separate qemu-fe is very desirable. Libvirt makes the
> > decision on the security policy that the VM will run under, and provides
> > audit records to log what resources are being assigned to the VM. From that
> > point onwards, we must be able to guarantee that MAC will be enforced on
> > the VM, according to what we logged via the auditd system.
> > 
> > In the case where we delegate opening of the files to qemu-fe, and allow
> > its policy to open NFS files, we no longer have a guarentee that the MAC
> > policy will be enforced as we originally intended. Yes, qemu-fe will very
> > likely honour what we tell it and open the correct files, and yes qmeu-fe
> > has lower attack surface wrt the guest than the real qemu does, but we
> > still loose the guarentee of MAC enforcement from libvirt's POV.
> 
> On the other hand, from a qemu POV libvirt is only one possible
> management tool. In practice, another very popular "management tool" for
> qemu is bash. With qemu-fe all the other tools, including direct
> invocation from the command line, would get some protection, too.

That's why I said a qemu-fe like tool need not be mutually exclusive
with exposing FD passing to a management tool like libvirt. Both
qemu-fe and libvirt need to some mechanism to pass open FDs to the
real QEMU.  We could needlessly invent a new communication channel
between qemu-fe and qemu that only it can use, or we can use the
channel we already have - QMP - to provide an interface that either
qemu-fe or libvirt, can use to pass FDs into the real QEMU.

Daniel
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