1Linux 2.4.2 Secure Attention Key (SAK) handling 218 March 2001, Andrew Morton 3 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 89