Red Hat: RHSA-2012:0721 Important: Kernel Security Update

The kernel packages contain the Linux kernel, the core of any Linux operating system.
This update fixes the following security issues:
* It was found that the Xen hypervisor implementation as shipped with Red Hat Enterprise Linux 5 did not properly restrict the syscall return addresses in the sysret return path to canonical addresses. An unprivileged user in a 64-bit para-virtualized guest, that is running on a 64-bit host that has an Intel CPU, could use this flaw to crash the host or, potentially, escalate their privileges, allowing them to execute arbitrary code at the hypervisor level. (CVE-2012-0217, Important)
* It was found that guests could trigger a bug in earlier AMD CPUs, leading to a CPU hard lockup, when running on the Xen hypervisor implementation. An unprivileged user in a 64-bit para-virtualized guest could use this flaw to crash the host. Warning: After installing this update, hosts that are using an affected AMD CPU (refer to Red Hat Bugzilla bug #824966 for a list) will fail to boot. In order to boot such hosts, the new kernel parameter, allow_unsafe, can be used ("allow_unsafe=on"). This option should only be used with hosts that are running trusted guests, as setting it to "on" reintroduces the flaw (allowing guests to crash the host). (CVE-2012-2934, Moderate)
Note: For Red Hat Enterprise Linux guests, only privileged guest users can exploit the CVE-2012-0217 and CVE-2012-2934 issues.
Red Hat would like to thank the Xen project for reporting these issues. Upstream acknowledges Rafal Wojtczuk as the original reporter of CVE-2012-0217.
Users should upgrade to these updated packages, which contain backported patches to correct these issues. The system must be rebooted for this update to take effect.