patch-2.4.3 linux/Documentation/SAK.txt

• Lines: 89
• Date: Thu Mar 22 09:20:45 2001
• Orig file: v2.4.2/linux/Documentation/SAK.txt
• Orig date: Wed Dec 31 16:00:00 1969

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+Linux 2.4.2 Secure Attention Key (SAK) handling
+18 March 2001, Andrew Morton <andrewm@uow.edu.au>
+
+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 appliccaitons 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.
+