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. * 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 will fail to boot. In order to boot such hosts, the new kernel parameter, allow_unsafe, can be used . This option should only be used with hosts that are running trusted guests, as setting it to "on" reintroduces the flaw . 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.