SELinux or how to loose an hour

If your on RedHat (but it seems it can occur on any Linux) and you get a "Permission denied" coming out of nowhere. You did chmod -R 777 / and still get the issue, then you probably met SELinux. It's security above the usual security.

To desactivate it temporarily just to check it's really your current issue, do

sudo sudo sh -c 'echo 0 > /selinux/enforce'