seccomp: Use FIFO semantics to order notifications

Previously, the seccomp notifier used LIFO semantics, where each
notification would be added on top of the stack, and notifications
were popped off the top of the stack. This could result one process
that generates a large number of notifications preventing other
notifications from being handled. This patch moves from LIFO (stack)
semantics to FIFO (queue semantics).

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220428015447.13661-1-sargun@sargun.me

Author: sargun

kernel/seccomp.c

@@ -1100,7 +1100,7 @@ static int seccomp_do_user_notification(int this_syscall,
 	n.data = sd;
 	n.id = seccomp_next_notify_id(match);
 	init_completion(&n.ready);
-	list_add(&n.list, &match->notif->notifications);
+	list_add_tail(&n.list, &match->notif->notifications);
 	INIT_LIST_HEAD(&n.addfd);
 
 	up(&match->notif->request);