Comment 59 for bug 269215

can you log in as root?
then type the following into a terminal
# chmod 777 /tmp

if you can't log in as root (because for instance no root password is set),
i suggest booting into single user/recovery mode
you'll then get a root prompt where you'll be able to change the permissions

if you don't have single user grub entry, edit the existing grub entry by typing 'e'
and add 'single' tot the line with the kernel and boot with 'b'