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

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(a)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.