Documentation/SAK.txt

   1 Linux 2.4.2 Secure Attention Key (SAK) handling
   2 18 March 2001, Andrew Morton
   3
   4 An operating system's Secure Attention Key is a security tool which is
   5 provided as protection against trojan password capturing programs.  It
   6 is an undefeatable way of killing all programs which could be
   7 masquerading as login applications.  Users need to be taught to enter
   8 this key sequence before they log in to the system.
   9
  10 From the PC keyboard, Linux has two similar but different ways of
  11 providing SAK.  One is the ALT-SYSRQ-K sequence.  You shouldn't use
  12 this sequence.  It is only available if the kernel was compiled with
  13 sysrq support.
  14
  15 The proper way of generating a SAK is to define the key sequence using
  16 `loadkeys'.  This will work whether or not sysrq support is compiled
  17 into the kernel.
  18
  19 SAK works correctly when the keyboard is in raw mode.  This means that
  20 once defined, SAK will kill a running X server.  If the system is in
  21 run level 5, the X server will restart.  This is what you want to
  22 happen.
  23
  24 What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot
  25 the machine.  CTRL-ALT-BACKSPACE is magical to the X server.  We'll
  26 choose CTRL-ALT-PAUSE.
  27
  28 In your rc.sysinit (or rc.local) file, add the command
  29
  30         echo "control alt keycode 101 = SAK" | /bin/loadkeys
  31
  32 And that's it!  Only the superuser may reprogram the SAK key.
  33
  34
  35 NOTES
  36 =====
  37
  38 1: Linux SAK is said to be not a "true SAK" as is required by
  39    systems which implement C2 level security.  This author does not
  40    know why.
  41
  42
  43 2: On the PC keyboard, SAK kills all applications which have
  44    /dev/console opened.
  45
  46    Unfortunately this includes a number of things which you don't
  47    actually want killed.  This is because these applications are
  48    incorrectly holding /dev/console open.  Be sure to complain to your
  49    Linux distributor about this!
  50
  51    You can identify processes which will be killed by SAK with the
  52    command
  53
  54         # ls -l /proc/[0-9]*/fd/* | grep console
  55         l-wx------    1 root     root           64 Mar 18 00:46 /proc/579/fd/0 -> /dev/console
  56
  57    Then:
  58
  59         # ps aux|grep 579
  60         root       579  0.0  0.1  1088  436 ?        S    00:43   0:00 gpm -t ps/2
  61
  62    So `gpm' will be killed by SAK.  This is a bug in gpm.  It should
  63    be closing standard input.  You can work around this by finding the
  64    initscript which launches gpm and changing it thusly:
  65
  66    Old:
  67
  68         daemon gpm
  69
  70    New:
  71
  72         daemon gpm < /dev/null
  73
  74    Vixie cron also seems to have this problem, and needs the same treatment.
  75
  76    Also, one prominent Linux distribution has the following three
  77    lines in its rc.sysinit and rc scripts:
  78
  79         exec 3<&0
  80         exec 4>&1
  81         exec 5>&2
  82
  83    These commands cause *all* daemons which are launched by the
  84    initscripts to have file descriptors 3, 4 and 5 attached to
  85    /dev/console.  So SAK kills them all.  A workaround is to simply
  86    delete these lines, but this may cause system management
  87    applications to malfunction - test everything well.
  88