Problem Description:
While the kernel was copying knotes during fork, a knote with
a timer-based filter could fire and be enqueued on the kqueue's
active list before the copy was complete. The copy routine did not
account for this and could enqueue the new knote a second time,
corrupting the active list. In addition, the copy routine did not
hold the appropriate locks while reading knote state, allowing
further races.
Impact:
An unprivileged local user can trigger a use-after-free in the
kernel, potentially leading to privilege escalation.