chroot directory, and sshd

[Ron Leach <ronleach@tesco.net>]

List, good evening,

AIUI, sshd requires that a chroot directory, and all directories above 
it, including "/", must be owned by root, and not be writable except 
by root.  '755' permissions.

While trying to set up an sftp-only service, and using this stanza in 
/etc/ssh/sshd_config :

Match Group sftp_users
  X11Forwarding no
  AllowTcpForwarding no
  ChrootDirectory /home/sftp
  ForceCommand internal-sftp

sftp login attempts fail after password entry, and
/var/log/auth.log shows :

bad ownership or modes for chroot directory component "/"

I've rechecked ownership and access settings for
/home
/home/sftp
and both these are owned by root, and writable only by root as owner 
(755).

I note that the auth failure does seem to suggest there is something 
wrong with permissions for "/" itself.  I haven't been able to find 
out how to check the permissions on "/", and I'd appreciate a 
suggestion how to do that if - as it seems - that might be what sshd 
is complaining about.

If that isn't the problem, I'm not sure what else could be.  The 
directories below /home/sftp, such as
/home/sftp/mary
are owned by and writable by user mary.  mary is also in the group 
'sftp-users'.

OS is Wheezy, and ssh updates are applied.

I'd be very grateful for any insights,

regards, Ron