Re: [Xen-users] domU kernel

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

Steve Wray wrote:
> Christian Horn wrote:
> > On Fri, Oct 12, 2007 at 12:14:02AM -0400, IDAGroup - R.W.Muller wrote:
> > > Hi, I found lots of threads where people talk about domU kernel 
> > > sitting in /boot of dom0.
> > > The only kernel I can see there is the one the machine and dom0 
> > > booted from (vmlinuz-2.6.18-8.el5xen)
> > Two places are common:
> > - domU-kernel placed on dom0-filesystem directly, 'kernel' option in 
> > xen-
> >   config for the domU is used then. Only possible for paravirt-domU.
> >   pros: - kernel is directly reachable from dom0
> >   cons: - domU depends on files outside of its disc-image, so you 
> > have to           keep an eye of what domU uses what kernel-file
> >         - on upgrading the domU-kernel is a bit more complicated, keep
> >           kernel, maybe existing initrd and modules-directory in sync
> >
> > - domU-kernel placed inside the domU-diskimage. Works for both HVM and
> >   paravirt-domU. One sees mostly this nowadays. Kernel is located/booted
> >   by pygrub (or a script mounting the partition, making a copy of the
> >   kernel inside to dom0, and starting it then)
> >   pros: - easy updating, i.e. just 'yum update' from the domU updates 
> > the
> >           kernel, initrd, modules and kernel is booted on next domU-boot
>
> You forgot the con.
>
> cons: Security. You now have a domU in which a local exploit could 
> result in code being executed in dom0 at the next boot of that domU. 
> By the way, this actually happened. See CVE-2007-4993
> IMHO putting the kernel in domU and using pygrub was always asking for 
> trouble.
> In my opinion it is completely crazy to expose dom0 to potential 
> exploits from domU.
> So far as I am aware this is the *only* way to so expose dom0 to domU 
> security holes and I am deeply shocked if it is true that "One sees 
> mostly this nowadays"
There's a big advantage to pygrub: kernel update procedures for DomU 
have nothing to do with Dom0's kernel. This prevents version and feature 
conflicts with package management sytems, and allows updates and reboots 
of DomU without having to write to Dom0. This was particularly 
troublesome with the Xensource kernels for RHEL 4 and RHEL 5, which had 
such different versions for kernel-xen on the server and kernel-xenU on 
the guest that it made RHEL whine quite hard about the Dom0 getting such 
an old kernel installed on it. And if you tried to use kernel-xen on the 
DomU, it created conflicts because the RHEL 4 kernel and RHEL 5 kernels 
had the same names and file names, and it could screw up which kernel 
you wound up with.
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