kernel: Missing namespace check in net/netlink/af_netlink.c allows for network monitors to observe systemwide activity

The netlinkdelivertapskb function in net/netlink/afnetlink.c in the Linux kernel, through 4.14.4, does not restrict observations of Netlink messages to a single net namespace, when CONFIGNLMON is enabled. This allows local users to obtain sensitive information by leveraging the CAPNETADMIN...