Unfortunately it has to be a privileged operation, otherwise any
application could set the attribute and then have access to user
namespaces. The problem with unprivileged user namespaces is that it
makes privileged interfaces available to the user in ways that they
weren't designed for, leading to vulnerabilities. Yes it tries to
mitigate and control this in some ways, but the reality is the kernel is
always adding new interfaces that are privileged, so its a game of
whack-a-mole.

To quote Linus about adding user namespaces "it was a mistake. We're
stuck with it". This is just an after the fact mitigation, and as such
there is going to be a somewhat painful transition period.

There is another reason to not use a single attribute as well. This is a
stepping stone to bringing much tighter/finer confinement to the
desktop. Having unique labels on the applications will allow us to start
deploying finer controls over who can talk to who. This is really
important when one of those entities have elevated privileges, which is
the case for applications making use of unprivileged user namespaces.

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https://bugs.launchpad.net/bugs/2046844

Title:
  AppArmor user namespace creation restrictions cause many applications
  to crash with SIGTRAP

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