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SAK, ctrl-alt-del, and Linux keyboard mapping

A common problem with Linux systems is when Windows users press CTRL-ALT-DEL at the login prompt and reboot the machine.

To fix this some people change the ^ca line in /etc/inittab to just disable the reboot function. However this is not desirable because sometimes you want to reboot a machine with a simple keypress.

Another problem that has not been widely considered is the use of fake login prompts by attackers. This can be implemented in either text mode or graphics mode. All the fake login prompt has to do is display something that looks like a real login prompt, accept a user-name and password, verify the password (a localhost ssh connection is a good way of doing this) and then abort. In the case of a text-mode login the user will think that they entered the wrong password, in the case of a GUI login via an XDM program the user will think that the login program just crashed. Then the attacker has access to their account.

The solution to the fake-login problem is the use of the Secure Attention Keyboard (SAK) feature. When invoked this feature makes the kernel kill all processes that are on the virtual console in question. If you make CTRL-ALT-DEL the SAK combination then pressing those keys will cause the kernel to kill any processes that are attached to the current virtual console and preventing the ability of hostile programs to forge a login prompt (which is the same as it’s purpose in Windows).

The next thing to do is to make another combination used for system boot. A reasonable combination seems to be CTRL-ALT-BREAK as those keys are widely separated and the combination is not used for anything else.

If you put the following in a file named (or whatever you want to call it) then the command loadkeys will apply the change. Note that when creating a keyboard map you should do it on a machine for which you don’t mind being forced to perform a hardware reboot. It’s easy to make a mistake and give yourself a keyboard mapping that is not usable. Another possibility is to do such testing on a machine that allows ssh logins, you can then login via ssh and run loadkeys -d to correct any errors you might make.

control alt keycode 119 = Boot
control alt keycode 83 = SAK
control alt keycode 111 = SAK
control altgr keycode 119 = Boot
control altgr keycode 83 = SAK
control altgr keycode 111 = SAK

Note that the above covers both ALT and ALT-Gr keys as well as the numeric keypad and regular versions of the delete key.

dumpkeys -l gives you a list of all possible keyboard combinations. showkey will display the number matching any key you press and will exit after 10 seconds of inactivity.

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