FreeBSD -- sendmsg(2) privilege escalation

Problem Description:
When handling a 32-bit sendmsg(2) call, the compat32 subsystem copies the
control message to be transmitted (if any) into kernel memory, and adjusts
alignment of control message headers. The code which performs this work
contained a time-of-check to time-of-use (TOCTOU) vulnerability which allows a
malicious userspace program to modify control message headers after they were
validated by the kernel.
Impact:
The TOCTOU bug can be exploited by an unprivileged malicious userspace program
to trigger privilege escalation.