CVE Request: systemd v209+: local denial-of-service attack

[Andrew Ayer <agwa () andrewayer name>]

systemd[1] fails an assertion in manager_invoke_notify_message[2] when
a zero-length message is received over its notification socket.
After failing the assertion, PID 1 hangs in the pause system call.
It is no longer possible to start and stop daemons or cleanly reboot
the system. Inetd-style services managed by systemd no longer accept
connections.

Since the notification socket, /run/systemd/notify, is world-writable,
this allows a local user to perform a denial-of-service attack against
systemd.

Proof-of-concept:

        NOTIFY_SOCKET=/run/systemd/notify systemd-notify ""

This vulnerability is present in all versions of systemd since at
least v209[3].

This has been reported to systemd.[4]

[1] https://github.com/systemd/systemd/
[2] https://github.com/systemd/systemd/blob/b8fafaf4a1cffd02389d61ed92ca7acb1b8c739c/src/core/manager.c#L1666
[3] 
https://github.com/systemd/systemd/commit/5ba6985b6c8ef85a8bcfeb1b65239c863436e75b#diff-ab78220e12703ee63fa1e6a2caa16bebR1325
[4] https://github.com/systemd/systemd/issues/4234