illumos-gate: Missing release of passed file descriptors in illumos nscd allows local users to exhaust kernel memory

A missing release of resources in the illumos name service cache daemon (nscd) allows a local user to exhaust kernel memory. The nscd door server procedure, switcher() in usr/src/cmd/nscd/nscd_frontend.c, does not close file descriptors that are passed with a door call but not used by the request, and the main nscd door at /var/run/name_service_door accepts passed descriptors from any user in its zone. Because nscd also runs with an unlimited file descriptor limit, an unprivileged local user, including one in a non-global zone, can repeatedly pass a descriptor to its zone's nscd in a door_call() loop, causing the file descriptor table of nscd to grow without bound in kernel memory. This causes a denial of service of nscd and can render processes in all zones on the host unresponsive. The flaw has existed since 2006 (illumos-gate commit cb5caa98), and affects any illumos distribution prior to illumos-gate commit af810a72.