1=========================================
2Linux Secure Attention Key (SAK) handling
3=========================================
4
5:Date: 18 March 2001
6:Author: Andrew Morton
7
8An operating system's Secure Attention Key is a security tool which is
9provided as protection against trojan password capturing programs.  It
10is an undefeatable way of killing all programs which could be
11masquerading as login applications.  Users need to be taught to enter
12this key sequence before they log in to the system.
13
14From the PC keyboard, Linux has two similar but different ways of
15providing SAK.  One is the ALT-SYSRQ-K sequence.  You shouldn't use
16this sequence.  It is only available if the kernel was compiled with
17sysrq support.
18
19The proper way of generating a SAK is to define the key sequence using
20``loadkeys``.  This will work whether or not sysrq support is compiled
21into the kernel.
22
23SAK works correctly when the keyboard is in raw mode.  This means that
24once defined, SAK will kill a running X server.  If the system is in
25run level 5, the X server will restart.  This is what you want to
26happen.
27
28What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot
29the machine.  CTRL-ALT-BACKSPACE is magical to the X server.  We'll
30choose CTRL-ALT-PAUSE.
31
32In your rc.sysinit (or rc.local) file, add the command::
33
34	echo "control alt keycode 101 = SAK" | /bin/loadkeys
35
36And that's it!  Only the superuser may reprogram the SAK key.
37
38
39.. note::
40
41  1. Linux SAK is said to be not a "true SAK" as is required by
42     systems which implement C2 level security.  This author does not
43     know why.
44
45
46  2. On the PC keyboard, SAK kills all applications which have
47     /dev/console opened.
48
49     Unfortunately this includes a number of things which you don't
50     actually want killed.  This is because these applications are
51     incorrectly holding /dev/console open.  Be sure to complain to your
52     Linux distributor about this!
53
54     You can identify processes which will be killed by SAK with the
55     command::
56
57	# ls -l /proc/[0-9]*/fd/* | grep console
58	l-wx------    1 root     root           64 Mar 18 00:46 /proc/579/fd/0 -> /dev/console
59
60     Then::
61
62	# ps aux|grep 579
63	root       579  0.0  0.1  1088  436 ?        S    00:43   0:00 gpm -t ps/2
64
65     So ``gpm`` will be killed by SAK.  This is a bug in gpm.  It should
66     be closing standard input.  You can work around this by finding the
67     initscript which launches gpm and changing it thusly:
68
69     Old::
70
71	daemon gpm
72
73     New::
74
75	daemon gpm < /dev/null
76
77     Vixie cron also seems to have this problem, and needs the same treatment.
78
79     Also, one prominent Linux distribution has the following three
80     lines in its rc.sysinit and rc scripts::
81
82	exec 3<&0
83	exec 4>&1
84	exec 5>&2
85
86     These commands cause **all** daemons which are launched by the
87     initscripts to have file descriptors 3, 4 and 5 attached to
88     /dev/console.  So SAK kills them all.  A workaround is to simply
89     delete these lines, but this may cause system management
90     applications to malfunction - test everything well.
91
92