[libvirt] Notes from the KVM Forum relevant to libvirt

[Daniel P. Berrange]

On Thu, Aug 25, 2011 at 08:58:27AM -0500, Serge E. Hallyn wrote:
> Quoting Stefan Hajnoczi (stefanha at gmail.com):
> > On Thu, Aug 25, 2011 at 11:03 AM, Daniel P. Berrange
> > <berrange at redhat.com> wrote:
> > > On Thu, Aug 25, 2011 at 10:10:27AM +0100, Stefan Hajnoczi wrote:
> > >> On Wed, Aug 24, 2011 at 3:46 PM, Daniel P. Berrange <berrange at redhat.com> wrote:
> > >> > On Wed, Aug 24, 2011 at 03:20:57PM +0100, Stefan Hajnoczi wrote:
> > >> >> On Tue, Aug 23, 2011 at 4:31 PM, Daniel P. Berrange <berrange at redhat.com> wrote:
> > >> >> > On Tue, Aug 23, 2011 at 04:24:46PM +0100, Stefan Hajnoczi wrote:
> > >> >> >> On Tue, Aug 23, 2011 at 12:15 PM, Daniel P. Berrange
> > >> >> >> <berrange at redhat.com> wrote:
> > >> >> >> > I was at the KVM Forum / LinuxCon last week and there were many
> > >> >> >> > interesting things discussed which are relevant to ongoing libvirt
> > >> >> >> > development. Here was the list that caught my attention. If I have
> > >> >> >> > missed any, fill in the gaps....
> > >> >> >> >
> > >> >> >> >  - Sandbox/container KVM.  The Solaris port of KVM puts QEMU inside
> > >> >> >> >   a zone so that an exploit of QEMU can't escape into the full OS.
> > >> >> >> >   Containers are Linux's parallel of Zones, and while not nearly as
> > >> >> >> >   secure yet, it would still be worth using more containers support
> > >> >> >> >   to confine QEMU.
> > >> >> >>
> > >> >> >> Can you elaborate on why Linux containers are "not nearly as secure"
> > >> >> >> [as Solaris Zones]?
> > >> >> >
> > >> >> > Mostly because the Linux namespace functionality is far from complete,
> > >> >> > notably lacking proper UID/GID/capability separation, and UID/GID
> > >> >> > virtualization wrt filesystems. The longer answer is here:
> > >> >> >
> > >> >> >   https://wiki.ubuntu.com/UserNamespace
> > >> >> >
> > >> >> > So at this time you can't build a secure container on Linux, relying
> > >> >> > just on DAC alone. You have to add in a MAC layer ontop of the container
> > >> >> > to get full security benefits, which obviously defeats the point of
> > >> >> > using the container as a backup for failure in the MAC layer.
> > >> >>
> > >> >> Thanks, that is interesting.  I still don't understand why that is a
> > >> >> problem.  Linux containers (lxc) uses a different pid namespace (no
> > >> >> ptrace worries), file system root (restricted to a subdirectory tree),
> > >> >> forbids most device nodes, etc.  Why does the user namespace matter
> > >> >> for security in this case?
> > >> >
> > >> > A number of reasons really...
> > >> >
> > >> > If user ID '0' on the host starts a container, and a process inside
> > >> > the container does 'setuid(500)', then any user outside the container
> > >> > with UID 500 will be able to kill that process. Only user ID '0' should
> > >> > have been allowed todo that.
> > >> >
> > >> > It will also let non-root user IDs on the host OS, start containers
> > >> > and have root uid=0 inside the container.
> > >> >
> > >> > Finally, any files created inside the container with, say, uid 500
> > >> > will be accessible by any other process with UID 500, in either the
> > >> > host or any other container
> > >>
> > >> These points mean that the host can peek inside containers and has
> > >> access to their processes/files.  But from the point of a libvirt
> > >> running inside a container there is no security problem.
> > >>
> > >> This is kind of like saying that root on the host can modify KVM guest
> > >> disk images.  That is true but I don't see it as a security problem
> > >> because the root on the host is the trusted part of the system.
> > >>
> > >> >> I think it matters when giving multiple containers access to the same
> > >> >> file system.  Is that what you'd like to do for libvirt?
> > >> >
> > >> > Each container would have to share a (readonly) view onto the host
> > >> > filesystem so it can see the QEMU emulator install / libraries. There
> > >> > would also have to be some writable areas per QEMU container.  QEMU
> > >> > inside the container would be set to run as some non-root UID (from
> > >> > the container's POV). So both problem 1 & 3 above would impact the
> > >> > security of this confinement.
> > >>
> > >> But is there a way to escape confinement?  If not, then this is secure.
> > >
> > > The filesystem UID/GID ownership is the most likely way you can escape
> > > the confinement. You would have to be very careful to ensure that each
> > > container's view of the filesystem did not include any directories
> > > with files that are assigned to another container, since the UID
> > > separation would not prevent access to another container's resources.
> > >
> > > This is rather tedious but could be just about doable, but it gets
> > > harder when you throw in things like sysfs and PCI device assignment.
> > > eg a guest with PCI device assigned gets given ownership of the files
> > > in /sys/bus/pci/devices/0000:00:XX:XX/ and since there is no UID
> > > namespacing, this will be accessible to any other container with the
> > > same UID. To hack around this when starting up a container you would
> > > probably have to bind mount a empty tmpfs over the top of all the
> > > PCI device paths you wanted to block in sysfs.
> 
> Which of course is easily undoable by root in the container :)

Yep, you'd have to make sure QEMU was none root for it to be at all
practical.

> > Ah, I hadn't thought of /sys/bus/pci or /sys/bus/usb!
> > 
> > Thanks for the explanation and it does seem like the design would get messy.
> 
> And plenty more, i.e.  http://blog.bofh.it/debian/id_413

Cool a nice demo :-)

> See http://sourceforge.net/mailarchive/message.php?msg_id=27878921 for
> someone actively using Smack to help mitigate this (which could also be
> done with SELinux).

Yes, I've got the same done with SELinux, but haven't posted it for
review yet, since it needs more testing and some policy additions

https://gitorious.org/~berrange/libvirt/staging/commits/lxc-svirt

Of course in the context of this discussion, QEMU already runs under
SELinux, and my desire for containers was to act as a safety net for
when SELinux fails for some reason (or is disabled by an admin) so
back to square one wrt security :-)

Daniel
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