On Tue, Nov 26, 2019 at 04:13:13PM -0800, Maciej Żenczykowski wrote:

From: Maciej Żenczykowski <redacted>

and associated inet_is_local_unbindable_port() helper function:
use it to make explicitly binding to an unbindable port return
-EPERM 'Operation not permitted'.

Autobind doesn't honour this new sysctl since:
  (a) you can simply set both if that's the behaviour you desire
  (b) there could be a use for preventing explicit while allowing auto
  (c) it's faster in the relatively critical path of doing port selection
      during connect() to only check one bitmap instead of both

...

If we *know* that certain ports are simply unusable, then it's better
nothing even gets the opportunity to try to use them.  This way we at
least get a quick failure, instead of some sort of timeout (or possibly
even corruption of the data stream of the non-kernel based use case).

This is doable with SELinux today, no?