diff -Nru livecd-rootfs-2.408.16/debian/changelog livecd-rootfs-2.408.17/debian/changelog
--- livecd-rootfs-2.408.16/debian/changelog	2017-09-12 15:25:58.000000000 -0500
+++ livecd-rootfs-2.408.17/debian/changelog	2017-09-13 12:00:59.000000000 -0500
@@ -1,3 +1,13 @@
+livecd-rootfs (2.408.17) xenial; urgency=medium
+
+  * live-build/ubuntu-cpc/functions: Add a function, teardown_mountpoint,
+    to reverse the work done in setup_mountpoint.  Lack of this function
+    has forced users of setup_mountpoint to implement this separately
+    and the implementations have diverged. (LP: #1716992)
+  * live-build/ubuntu-cpc/functions: Remove umount_settle function.
+    The was only used where teardown_mountpoint was lacking.
+ -- Robert C Jennings <robert.jennings@canonical.com>  Wed, 13 Sep 2017 12:00:59 -0500
+
 livecd-rootfs (2.408.16) xenial; urgency=medium
 
   * live-build/ubuntu-cpc/functions: call apt-get update in the chroot
diff -Nru livecd-rootfs-2.408.16/live-build/ubuntu-cpc/functions livecd-rootfs-2.408.17/live-build/ubuntu-cpc/functions
--- livecd-rootfs-2.408.16/live-build/ubuntu-cpc/functions	2017-09-12 15:24:23.000000000 -0500
+++ livecd-rootfs-2.408.17/live-build/ubuntu-cpc/functions	2017-09-13 12:00:59.000000000 -0500
@@ -94,6 +94,21 @@
 
 }
 
+teardown_mountpoint() {
+    # Reverse the operations from setup_mountpoint
+    local mountpoint="$1"
+
+    umount "$mountpoint/var/cache/apt"
+    umount "$mountpoint/var/lib/apt"
+    umount "$mountpoint/tmp"
+    umount "$mountpoint/sys"
+    umount "$mountpoint/proc"
+    umount -R "$mountpoint/dev"
+    udevadm settle
+    sleep 3
+    mv resolv.conf.tmp "$mountpoint/etc/resolv.conf"
+}
+
 mount_partition() {
     partition="$1"
     mountpoint="$2"
@@ -126,16 +141,9 @@
 
 }
 
-umount_settle() {
-    # Unmount device, and let it settle
-    umount $1
-    udevadm settle
-    sleep 3
-}
-
 umount_partition() {
     local mountpoint=${1}
-    mv resolv.conf.tmp "$mountpoint/etc/resolv.conf"
+    teardown_mountpoint $mountpoint
     umount -R $mountpoint
     udevadm settle
 
diff -Nru livecd-rootfs-2.408.16/live-build/ubuntu-cpc/hooks/030-root-tarball.binary livecd-rootfs-2.408.17/live-build/ubuntu-cpc/hooks/030-root-tarball.binary
--- livecd-rootfs-2.408.16/live-build/ubuntu-cpc/hooks/030-root-tarball.binary	2017-09-12 15:25:58.000000000 -0500
+++ livecd-rootfs-2.408.17/live-build/ubuntu-cpc/hooks/030-root-tarball.binary	2017-09-13 12:00:59.000000000 -0500
@@ -25,9 +25,4 @@
 chroot binary/boot/filesystem.dir dpkg-divert --remove --local --rename /usr/sbin/grub-probe
 
 mv resolv.conf.tmp "binary/boot/filesystem.dir/etc/resolv.conf"
-umount "binary/boot/filesystem.dir/proc"
-umount "binary/boot/filesystem.dir/sys"
-umount -R "binary/boot/filesystem.dir/dev"
-umount "binary/boot/filesystem.dir/tmp"
-umount "binary/boot/filesystem.dir/var/lib/apt"
-umount "binary/boot/filesystem.dir/var/cache/apt"
+teardown_mountpoint binary/boot/filesystem.dir