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.