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Re: [libvirt] RFC [3/3]: Lock manager usage scenarios



----- "Daniel P. Berrange" <berrange redhat com> wrote:

> On Mon, Sep 13, 2010 at 03:49:38PM +0200, Saggi Mizrahi wrote:
> > On Mon, 2010-09-13 at 14:29 +0100, Daniel P. Berrange wrote:
> > > On Mon, Sep 13, 2010 at 03:20:13PM +0200, Saggi Mizrahi wrote:
> > > > On Mon, 2010-09-13 at 13:35 +0100, Daniel P. Berrange wrote:
> > > > > > 
> > > > > > Overall, this looks workable to me.  As proposed, this
> assumes a 1:1 
> > > > > > relation between LockManager process and managed VMs.  But I
> guess you 
> > > > > > can still have a central manager process that manages all
> the VMs, by 
> > > > > > having the lock manager plugin spawn a simple shim process
> that does all 
> > > > > > the communication with the central lock manager.
> > > > > 
> > > > > I could have decided it such that it didn't assume presence of
> a angel
> > > > > process around each VM, but I think it is easier to be able to
> presume
> > > > > that there is one. It can be an incredibly thin stub if
> desired, so I
> > > > > don't think it'll be too onerous on implementations
> > > > 
> > > > > We are looking into the possibility of not having a process
> manage a
> > > > VM but rather having the sync_manager process register with a
> central
> > > > daemon and exec into qemu (or anything else) so assuming there
> is a
> > > > process per VM is essentially false. But the verb could be used
> for
> > > > "unregistering" the current instance with the main manager so
> the verb
> > > > does have it use.
> > > > 
> > > > Further more, even if we decide to leave the current
> 'sync_manager
> > > > process per child process' system as is for now. The general
> direction
> > > > is a central deamon per host for managing all the leases and
> guarding
> > > > all processes. So be sure to keep that in mind while assembling
> the API.
> > > 
> > > Having a single daemon per host that exec's the VMs is explicitly
> *not*
> > > something we intend to support because the QEMU process needs to
> inherit
> > > its process execution state from libvirtd. It is fundamental to
> the
> > > security architecture that processes are completely isolated the
> moment
> > > that libvirtd has spawned them. We don't want to offload all the
> security
> > > driver setup into a central lock manager daemon. Aside from this
> we also
> > > pass open file descriptors down from libvirtd to the QEMU daemon.
> > My explanation might have been confusing or ill phrased. I'll try
> again.
> > What the suggestion was:
> > instead of libvirt running sync manager that will fork off and run
> qemu.
> > libvirt would run sync_manager wrapper that will register with the
> > central daemon wait for it to acquire leases and then exec to qemu
> (in
> > process). From that moment the central daemon monitors the process
> and
> > when it quits frees it's leases.
> > This way we still keep all the context stuff from libvirt and have
> only
> > 1 process managing the leases.
> > But, as I said, this is only a suggestion and is still in very
> early
> > stages. We might not implement in the initial version and leave the
> > current forking method.
> 
> That is probably possible with the current security driver
> implementations
> but more generally I think it will still hit some trouble.
> Specifically 
> one of the items on our todo list is a new security driver that makes
> use
> of Linux container namespace functionality to isolate the VMs, so they
> 
> can't even see other resources / processes on the host. This may well
> prevent the sync manager wrapper talking to a central sync mnager
> process
> The general rule we aim for is that once libvirtd has spawned a VM
> they
> are completely isolated with exception of any disks marked with
> <shareable/>
> In other words, any communictions channels must be
> initiated/established 
> by the mgmt layer to the VM process, with nothing to be established in
> the
> reverse direction.
Correct me if I'm wrong, but the security limitations (selinux context) would only take effect after the "exec", no? so the process could still communicate with the daemon, open an FD and then exec.  After exec, the VM would be locked down but the daemon could still wait on the FD to see whether VM has died.

> 
> Daniel
> -- 
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