Security update for Xen (critical)

Three security issues were found in XEN.

Two security issues are fixed by this update:

CVE-2012-0217: Due to incorrect fault handling in the
XEN hypervisor it was possible for a XEN guest domain
administrator to execute code in the XEN host environment.

CVE-2012-0218: Also a guest user could crash the
guest XEN kernel due to a protection fault bounce.

The third fix is changing the Xen behaviour on certain
hardware:

CVE-2012-2934: The issue is a denial of service issue
on older pre-SVM AMD CPUs (AMD Erratum 121).

AMD Erratum #121 is described in "Revision Guide for
AMD Athlon 64 and AMD Opteron Processors":
<a href=“http://support.amd.com/us/Processor_TechDocs/25759.pdf”>http://support.amd.com/us/Processor_TechDocs/25759.pdf</a>
<<a href=“http://support.amd.com/us/Processor_TechDocs/25759.pdf”>http://support.amd.com/us/Processor_TechDocs/25759.pdf</a>>

The following 130nm and 90nm (DDR1-only) AMD
processors are subject to this erratum:

o

First-generation AMD-Opteron™ single and
dual core processors in either 939 or 940 packages:

• AMD Opteron™ 100-Series Processors
• AMD Opteron™ 200-Series Processors
• AMD Opteron™ 800-Series Processors
• AMD Athlon™ processors in either 754,
  939 or 940 packages
• AMD Sempron™ processor in either 754
  or 939 packages
• AMD Turion™ Mobile Technology in 754
  package

This issue does not effect Intel processors.

The impact of this flaw is that a malicious PV guest
user can halt the host system.

As this is a hardware flaw, it is not fixable except
by upgrading your hardware to a newer revision, or not
allowing untrusted 64bit guestsystems.

The patch changes the behaviour of the host system
booting, which makes it unable to create guest machines
until a specific boot option is set.

There is a new XEN boot option "allow_unsafe" for
GRUB which allows the host to start guests again.

This is added to /boot/grub/menu.lst in the line
looking like this:

kernel /boot/xen.gz … allow_unsafe

Note: … in this example represents the existing
boot options for the host.