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Diffstat (limited to 'kernel/Documentation/SAK.txt')
-rw-r--r-- | kernel/Documentation/SAK.txt | 88 |
1 files changed, 88 insertions, 0 deletions
diff --git a/kernel/Documentation/SAK.txt b/kernel/Documentation/SAK.txt new file mode 100644 index 000000000..74be14679 --- /dev/null +++ b/kernel/Documentation/SAK.txt @@ -0,0 +1,88 @@ +Linux 2.4.2 Secure Attention Key (SAK) handling +18 March 2001, Andrew Morton + +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 applications 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. + |