[Bug 1803993] Re: Password appears on the VT1 screen

[Balint Reczey]

** Description changed:

+ [Impact]
+ 
+  * The keyboard on the graphical login screen started on VT1 may stop
+ working and or keypresses including passwords are leaked to the terminal
+ console running 'behind' the graphical login screen or environment.
+ 
+ [Test Case]
+ 
+  * Reboot after installing the fixed systemd package.
+  * Install sysdig
+  * Start sysdig on a remote connection or on a terminal console:
+   $ sudo sysdig evt.type=ioctl | grep  request=4B4
+  * While sysdig is running log in and out 3 times in GDM and press a few keys in the graphical session to see if keyboard still works
+  * Log in and out on an other terminal console, too, running a few commands while being logged in to ensure that keyboard is working.
+  * Observe that on terminal consoles the monitored keyboard setter ioctl is called with argument=3, but where the graphical screen is active only argument=4 is used, unlike with the buggy version observed in https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1803993/comments/14
+ 
+ [Regression Potential]
+ 
+  * The fix checks the current keyboard mode of the VT and allows only
+ safe mode switches. The potential regression could be not allowing a
+ valid mode switch keeping a keyboard in a non-operational mode. Testing
+ covers that by typing the keyboard.
+ 
+ 
  (continued from bug 1767918)
  
  This was found when an administrative error made /home directory
  inaccessible.  Any users that tried to login after that, were not able
  to (which is expected) but their password appears on the VT1 screen.
  Under normal circumstances, VT1 is not visible. But once the system was
  sent into this compromised mode, one can press ctrl+alt+F1 and then
  ctrl+alt+F2 and get a momentary glance at VT1. One can keep toggling
  between these key combinations in order to make out the password(s) on
  VT1.
  
  As a further test, I wanted to see if a non-super user could cause this
  condition, and it is in fact possible. As a regular user, I made their
  own home directory not writable and then removed ~/.config and logged
  out. Then logged in as that user again, and although that user can't
  login the system does go into that mode where passwords appear on VT1
  and are viewable with the key combinations mentioned herein. Further,
  any other users that login will see no problem, but when they logon
  their passwords also appear on VT1 and are viewable.
  
  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: gdm3 3.28.3-0ubuntu18.04.3
  Uname: Linux 4.19.2-041902-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.5
  Architecture: amd64
  CurrentDesktop: ubuntu:GNOME
  Date: Mon Nov 19 08:32:59 2018
  InstallationDate: Installed on 2018-08-25 (85 days ago)
  InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: gdm3
  UpgradeStatus: No upgrade log present (probably fresh install)

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https://bugs.launchpad.net/bugs/1803993

Title:
  Password appears on the VT1 screen

Status in gdm3 package in Ubuntu:
  Invalid
Status in plymouth package in Ubuntu:
  Invalid
Status in systemd package in Ubuntu:
  In Progress

Bug description:
  [Impact]

   * The keyboard on the graphical login screen started on VT1 may stop
  working and or keypresses including passwords are leaked to the
  terminal console running 'behind' the graphical login screen or
  environment.

  [Test Case]

   * Reboot after installing the fixed systemd package.
   * Install sysdig
   * Start sysdig on a remote connection or on a terminal console:
    $ sudo sysdig evt.type=ioctl | grep  request=4B4
   * While sysdig is running log in and out 3 times in GDM and press a few keys in the graphical session to see if keyboard still works
   * Log in and out on an other terminal console, too, running a few commands while being logged in to ensure that keyboard is working.
   * Observe that on terminal consoles the monitored keyboard setter ioctl is called with argument=3, but where the graphical screen is active only argument=4 is used, unlike with the buggy version observed in https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1803993/comments/14

  [Regression Potential]

   * The fix checks the current keyboard mode of the VT and allows only
  safe mode switches. The potential regression could be not allowing a
  valid mode switch keeping a keyboard in a non-operational mode.
  Testing covers that by typing the keyboard.

  
  (continued from bug 1767918)

  This was found when an administrative error made /home directory
  inaccessible.  Any users that tried to login after that, were not able
  to (which is expected) but their password appears on the VT1 screen.
  Under normal circumstances, VT1 is not visible. But once the system
  was sent into this compromised mode, one can press ctrl+alt+F1 and
  then ctrl+alt+F2 and get a momentary glance at VT1. One can keep
  toggling between these key combinations in order to make out the
  password(s) on VT1.

  As a further test, I wanted to see if a non-super user could cause
  this condition, and it is in fact possible. As a regular user, I made
  their own home directory not writable and then removed ~/.config and
  logged out. Then logged in as that user again, and although that user
  can't login the system does go into that mode where passwords appear
  on VT1 and are viewable with the key combinations mentioned herein.
  Further, any other users that login will see no problem, but when they
  logon their passwords also appear on VT1 and are viewable.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: gdm3 3.28.3-0ubuntu18.04.3
  Uname: Linux 4.19.2-041902-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.5
  Architecture: amd64
  CurrentDesktop: ubuntu:GNOME
  Date: Mon Nov 19 08:32:59 2018
  InstallationDate: Installed on 2018-08-25 (85 days ago)
  InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: gdm3
  UpgradeStatus: No upgrade log present (probably fresh install)

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