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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Linux 2.4.2 Secure Attention Key (SAK) handling
Francois Camie1f8e872008-10-15 22:01:59 -0700218 March 2001, Andrew Morton
Linus Torvalds1da177e2005-04-16 15:20:36 -07003
4An operating system's Secure Attention Key is a security tool which is
5provided as protection against trojan password capturing programs. It
6is an undefeatable way of killing all programs which could be
7masquerading as login applications. Users need to be taught to enter
8this key sequence before they log in to the system.
9
10From the PC keyboard, Linux has two similar but different ways of
11providing SAK. One is the ALT-SYSRQ-K sequence. You shouldn't use
12this sequence. It is only available if the kernel was compiled with
13sysrq support.
14
15The 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
17into the kernel.
18
19SAK works correctly when the keyboard is in raw mode. This means that
20once defined, SAK will kill a running X server. If the system is in
21run level 5, the X server will restart. This is what you want to
22happen.
23
24What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot
25the machine. CTRL-ALT-BACKSPACE is magical to the X server. We'll
26choose CTRL-ALT-PAUSE.
27
28In your rc.sysinit (or rc.local) file, add the command
29
30 echo "control alt keycode 101 = SAK" | /bin/loadkeys
31
32And that's it! Only the superuser may reprogram the SAK key.
33
34
35NOTES
36=====
37
381: 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
432: 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