Quite. I'd say this security bug was about as serious as the obscure ext4 bug that ate my filesystem: both have serious consequences, but neither is going to happen unless you make some unusual changes to your system: in my case, mounting with certain mount options and then rebooting in the middle of a umount; in this case, turning on a config option which pretty much screams SECURITY PROBLEMS HERE, and which can't be turned on in conjunction with a lot of other commonly-used options which are probably already on. I know I wasn't faced with the *option* to turn on userspace namespaces until 3.9: many people, e.g. those using XFS, probably still can't see it. In both cases the fault was really one of documentation: the ext4 mount option didn't say it was experimental and dangerous, so tweakers-for-tweaking's-sake like me might well turn it on without realizing the danger; this option, too, was missing that disclaimer in the Kconfig help text.