genetlink: disallow subscribing to unknown mcast groups
genetlink: disallow subscribing to unknown mcast groups

Jeff Layton reported that he could trigger the multicast unbind warning
in generic netlink using trinity. I originally thought it was a race
condition between unregistering the generic netlink family and closing
the socket, but there's a far simpler explanation: genetlink currently
allows subscribing to groups that don't (yet) exist, and the warning is
triggered when unsubscribing again while the group still doesn't exist.

Originally, I had a warning in the subscribe case and accepted it out of
userspace API concerns, but the warning was of course wrong and removed
later.

However, I now think that allowing userspace to subscribe to groups that
don't exist is wrong and could possibly become a security problem:
Consider a (new) genetlink family implementing a permission check in
the mcast_bind() function similar to the like the audit code does today;
it would be possible to bypass the permission check by guessing the ID
and subscribing to the group it exists. This is only possible in case a
family like that would be dynamically loaded, but it doesn't seem like a
huge stretch, for example wireless may be loaded when you plug in a USB
device.

To avoid this reject such subscription attempts.

If this ends up causing userspace issues we may need to add a workaround
in af_netlink to deny such requests but not return an error.

Reported-by: Jeff Layton <542dfe392278ff3cf017aafde9d989e8068d5c3e@primarydata.com>
Signed-off-by: Johannes Berg <bff32994ff0f8d048f262a8388145a71b6071bfe@intel.com>
Signed-off-by: David S. Miller <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs