As documented in pam_unix(8), the nullok_secure option means that null
passwords are allowed for this module only when connected via one of the
"secure" (that is, generally speaking, "local") ttys listed in
/etc/securetty.  The tty is not determined directly by pam_unix, but is
instead provided to the PAM stack by the calling application setting the
PAM_TTY value with pam_set_item().  This is further documented in
pam_set_item(3) to be:

       PAM_TTY
           The terminal name: prefixed by /dev/ if it is a device file; for
           graphical, X-based, applications the value for this item should be
           the $DISPLAY variable.

If gdm is not setting the PAM_TTY item, then that's a bug in gdm, not in
PAM.

The /etc/securetty list of values is regrettably not comprehensive
(because e.g. it doesn't support pattern matching), so if your system
has been changed to run gdm on an X display beyond :3, the default value
will not work for you.  However, I don't see any reason this would occur
unless the settings of gdm itself had been changed away from the
defaults, and if you've done this you will need to adjust the contents
of /etc/securetty to match.

** Changed in: pam (Ubuntu)
       Status: Triaged => Invalid

-- 
You received this bug notification because you are a member of Ubuntu
Touch seeded packages, which is subscribed to pam in Ubuntu.
https://bugs.launchpad.net/bugs/1561302

Title:
  gdm won't allow passwordless login

Status in gdm:
  Confirmed
Status in Ubuntu GNOME:
  Triaged
Status in casper package in Ubuntu:
  New
Status in gdm3 package in Ubuntu:
  Triaged
Status in gnome-shell package in Ubuntu:
  New
Status in pam package in Ubuntu:
  Invalid

Bug description:
  The Live session user does not have a password set, however gdm won't
  allow login in this case since the "Sign In" button is disabled.

  Normally this should be too much of an issue, since the login screen
  should only be hit on logout from the live session, some people are
  reporting hitting the gdm login screen on boot due to ubiquity-dm
  failing to boot.

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: gdm3 3.18.2-1ubuntu1
  ProcVersionSignature: Ubuntu 4.4.0-15.31-generic 4.4.6
  Uname: Linux 4.4.0-15-generic x86_64
  ApportVersion: 2.20-0ubuntu3
  Architecture: amd64
  CurrentDesktop: GNOME
  Date: Thu Mar 24 13:55:19 2016
  InstallationDate: Installed on 2016-03-23 (0 days ago)
  InstallationMedia: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Beta amd64 
(20160323)
  SourcePackage: gdm3
  UpgradeStatus: No upgrade log present (probably fresh install)

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