rails Passenger - permission issue

Hi, I think I have a permissions issue:

I have only one user: "root" and when the rails web application tries to resize image using attachment_fu

I get this error:

RuntimeError (cannot generate tempfile `/…/tmp/attachment_fu/image32617-9.jpg'):…

It is ruby on rails 2.3.5, Passenger, apache 2, and Ubuntu 10.04 installed following the Linode tutorial: Ruby on Rails with Apache on Ubuntu 10.04 (Lucid)

How do I fix this permissions issue without doing chmod 777?

Thanks,

Sonney