FreeBSD: VID-f8e1e2a6-9791-11eb-b87a-901b0ef719ab (CVE-2021-29627): FreeBSD -- double free in accept_filter(9) socket configuration interface

Problem Description: An unprivileged process can configure an accept filter on a listening socket. This is done using the setsockopt(2) system call. The process supplies the name of the accept filter which is to be attached to the socket, as well as a string containing filter-specific information. If the filter implements the accf_create callback, the socket option handler attempts to preserve the process-supplied argument string. A bug in the socket option handler caused this string to be freed prematurely, leaving a dangling pointer. Additional operations on the socket can turn this into a double free or a use-after-free. Impact: The bug may be exploited to trigger local privilege escalation or kernel memory disclosure.