Re: [libvirt] Notes from the KVM Forum relevant to libvirt

Quoting Stefan Hajnoczi (stefanha(a)gmail.com): > On Thu, Aug 25, 2011 at 11:03 AM, Daniel P. Berrange > <berrange(a)redhat.com&gt; 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(a)redhat.com&gt; 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(a)redhat.com&gt; 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(a)redhat.com&gt; 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 :)

> 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

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).