About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / SAK.txt




Custom Search

Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1	=========================================
2	Linux Secure Attention Key (SAK) handling
3	=========================================
4	
5	:Date: 18 March 2001
6	:Author: Andrew Morton
7	
8	An operating system's Secure Attention Key is a security tool which is
9	provided as protection against trojan password capturing programs.  It
10	is an undefeatable way of killing all programs which could be
11	masquerading as login applications.  Users need to be taught to enter
12	this key sequence before they log in to the system.
13	
14	From the PC keyboard, Linux has two similar but different ways of
15	providing SAK.  One is the ALT-SYSRQ-K sequence.  You shouldn't use
16	this sequence.  It is only available if the kernel was compiled with
17	sysrq support.
18	
19	The 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
21	into the kernel.
22	
23	SAK works correctly when the keyboard is in raw mode.  This means that
24	once defined, SAK will kill a running X server.  If the system is in
25	run level 5, the X server will restart.  This is what you want to
26	happen.
27	
28	What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot
29	the machine.  CTRL-ALT-BACKSPACE is magical to the X server.  We'll
30	choose CTRL-ALT-PAUSE.
31	
32	In your rc.sysinit (or rc.local) file, add the command::
33	
34		echo "control alt keycode 101 = SAK" | /bin/loadkeys
35	
36	And 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.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.