xen: multiple vulnerabilities

The ELF parser used by the Xen tools to read domains' kernels and construct domains has multiple integer overflows, pointer dereferences based on calculations from unchecked input values, and other problems.

A malicious PV domain administrator who can specify their own kernel can escalate their privilege to that of the domain construction tools (i.e., normally, to control of the host).

Additionally a malicious HVM domain administrator who is able to supply their own firmware ("hvmloader") can do likewise; however we think this would be very unusual and it is unlikely that such configurations exist in production systems.