FreeBSD/Linux Kernel Cross Reference
sys/Documentation/SAK.txt

     Linux 2.4.2 Secure Attention Key (SAK) handling
     18 March 2001, Andrew Morton
     
     An operating system's Secure Attention Key is a security tool which is
     provided as protection against trojan password capturing programs.  It
     is an undefeatable way of killing all programs which could be
     masquerading as login applications.  Users need to be taught to enter
     this key sequence before they log in to the system.
     
    From the PC keyboard, Linux has two similar but different ways of
    providing SAK.  One is the ALT-SYSRQ-K sequence.  You shouldn't use
    this sequence.  It is only available if the kernel was compiled with
    sysrq support.
    
    The proper way of generating a SAK is to define the key sequence using
    `loadkeys'.  This will work whether or not sysrq support is compiled
    into the kernel.
    
    SAK works correctly when the keyboard is in raw mode.  This means that
    once defined, SAK will kill a running X server.  If the system is in
    run level 5, the X server will restart.  This is what you want to
    happen.
    
    What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot
    the machine.  CTRL-ALT-BACKSPACE is magical to the X server.  We'll
    choose CTRL-ALT-PAUSE.
    
    In your rc.sysinit (or rc.local) file, add the command
    
            echo "control alt keycode 101 = SAK" | /bin/loadkeys
    
    And that's it!  Only the superuser may reprogram the SAK key.
    
    
    NOTES
    =====
    
    1: Linux SAK is said to be not a "true SAK" as is required by
       systems which implement C2 level security.  This author does not
       know why.
    
    
    2: On the PC keyboard, SAK kills all applications which have
       /dev/console opened.
    
       Unfortunately this includes a number of things which you don't
       actually want killed.  This is because these applications are
       incorrectly holding /dev/console open.  Be sure to complain to your
       Linux distributor about this!
    
       You can identify processes which will be killed by SAK with the
       command
    
            # ls -l /proc/[0-9]*/fd/* | grep console
            l-wx------    1 root     root           64 Mar 18 00:46 /proc/579/fd/0 -> /dev/console
    
       Then:
    
            # ps aux|grep 579
            root       579  0.0  0.1  1088  436 ?        S    00:43   0:00 gpm -t ps/2
    
       So `gpm' will be killed by SAK.  This is a bug in gpm.  It should
       be closing standard input.  You can work around this by finding the
       initscript which launches gpm and changing it thusly:
    
       Old:
    
            daemon gpm
    
       New:
    
            daemon gpm < /dev/null
    
       Vixie cron also seems to have this problem, and needs the same treatment.
    
       Also, one prominent Linux distribution has the following three
       lines in its rc.sysinit and rc scripts:
    
            exec 3<&0
            exec 4>&1
            exec 5>&2
    
       These commands cause *all* daemons which are launched by the
       initscripts to have file descriptors 3, 4 and 5 attached to
       /dev/console.  So SAK kills them all.  A workaround is to simply
       delete these lines, but this may cause system management
       applications to malfunction - test everything well.
    

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