FreeBSD -- Privilege escalation when returning from kernel

Problem description:

FreeBSD/amd64 runs on CPUs from different vendors. Due to varying behaviour of CPUs in 64 bit mode a sanity check of the kernel may be insufficient when returning from a system call.

Successful exploitation of the problem can lead to local kernel privilege escalation, kernel data corruption and/or crash. To exploit this vulnerability, an attacker must be able to run code with user privileges on the target system.

[source]