xen: possible privilege escalation

The Python bindings for the xc_vcpu_setaffinity call do not properly check their inputs. Systems which allow untrusted administrators to configure guest vcpu affinity may be exploited to trigger a buffer overrun and corrupt memory.

An attacker who is able to configure a specific vcpu affinity via a toolstack which uses the Python bindings is able to exploit this issue.

Exploiting this issue leads to memory corruption which may result in a DoS against the system by crashing the toolstack. The possibility of code execution (privilege escalation) has not been ruled out.

The xend toolstack passes a cpumap to this function without sanitization. xend allows the cpumap to be configured via the guest configuration file or the SXP/XenAPI interface. Normally these interfaces are not considered safe to expose to non-trusted parties. However systems which attempt to allow guest administrator control of VCPU affinity in a safe way via xend may expose this issue.