Newer versions of the code in question has been included in Debian/Lenny. So if you want to use namespaces for a login session on a Lenny system you can do the following:
chmod 0 /tmp-inst
echo “/tmp /tmp-inst/ user root” >> /etc/security/namespace.conf
echo “session required pam_namespace.so” >> /etc/pam.d/common-session
Then every user will have their own unique /tmp and be unable to mess with other users.
If you want to use the shared-subtrees facility to have mount commands which don’t affect /tmp be propagated to other sessions then you need to have the following commands run at boot (maybe from /etc/rc.local):
mount –make-shared /
mount –bind /tmp /tmp
mount –make-private /tmp
The functionality in pam_namespace.so to use the SE Linux security context to instantiate the directory seems broken in Lenny. I’ll write a patch for this shortly.
While my paper is not particularly useful as documentation of pam_namespace.so (things changed after I wrote it), it does cover the threats that you face in terms of hostile use of /tmp and how namespaces may be used to solve them.