mount: Don't allow copying MNT_UNBINDABLE|MNT_LOCKED mounts

BugLink: https://launchpad.net/bugs/1789161

Jonathan Calmels from NVIDIA reported that he's able to bypass the
mount visibility security check in place in the Linux kernel by using
a combination of the unbindable property along with the private mount
propagation option to allow a unprivileged user to see a path which
was purposefully hidden by the root user.

Reproducer:
  # Hide a path to all users using a tmpfs
  root@castiana:~# mount -t tmpfs tmpfs /sys/devices/
  root@castiana:~#

  # As an unprivileged user, unshare user namespace and mount namespace
  stgraber@castiana:~$ unshare -U -m -r

  # Confirm the path is still not accessible
  root@castiana:~# ls /sys/devices/

  # Make /sys recursively unbindable and private
  root@castiana:~# mount --make-runbindable /sys
  root@castiana:~# mount --make-private /sys

  # Recursively bind-mount the rest of /sys over to /mnnt
  root@castiana:~# mount --rbind /sys/ /mnt

  # Access our hidden /sys/device as an unprivileged user
  root@castiana:~# ls /mnt/devices/
  breakpoint cpu cstate_core cstate_pkg i915 intel_pt isa kprobe
  LNXSYSTM:00 msr pci0000:00 platform pnp0 power software system
  tracepoint uncore_arb uncore_cbox_0 uncore_cbox_1 uprobe virtual

Solve this by teaching copy_tree to fail if a mount turns out to be
both unbindable and locked.

Cc: stable@vger.kernel.org
Fixes: 5ff9d8a6 ("vfs: Lock in place mounts from more privileged users")
Reported-by: Jonathan Calmels <jcalmels@nvidia.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
(cherry picked from commit df7342b2)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Colin King <colin.king@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

fs/namespace.c

@@ -1781,8 +1781,14 @@ struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
 		for (s = r; s; s = next_mnt(s, r)) {
 			if (!(flag & CL_COPY_UNBINDABLE) &&
 			    IS_MNT_UNBINDABLE(s)) {
-				s = skip_mnt_tree(s);
-				continue;
+				if (s->mnt.mnt_flags & MNT_LOCKED) {
+					/* Both unbindable and locked. */
+					q = ERR_PTR(-EPERM);
+					goto out;
+				} else {
+					s = skip_mnt_tree(s);
+					continue;
+				}
 			}
 			if (!(flag & CL_COPY_MNT_NS_FILE) &&
 			    is_mnt_ns_file(s->mnt.mnt_root)) {