kernel: multiple vulnerabilities

From the Xen advisory:

The Xen netback implementation contains a couple of flaws which can allow a guest to cause a DoS in the backend domain, potentially affecting other domains in the system.

CVE-2013-0216 is a failure to sanity check the ring producer/consumer pointers which can allow a guest to cause netback to loop for an extended period preventing other work from occurring.

CVE-2013-0217 is a memory leak on an error path which is guest triggerable.