FIFO event channels control block related ordering

ISSUE DESCRIPTION

Recording of the per-vCPU control block mapping maintained by Xen and that of pointers into the control block is reversed. The consumer assumes, seeing the former initialized, that the latter are also ready for use.

IMPACT

Malicious or buggy guest kernels can mount a Denial of Service (DoS) attack affecting the entire system.

VULNERABLE SYSTEMS

All Xen versions from 4.4 onwards are vulnerable. Xen versions 4.3 and earlier are not vulnerable.