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

[Kevin Wolf]

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<address@hidden>  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.

Kevin