projects / deliverable / linux.git / blame
| Commit | Line | Data |
|---|---|---|
| 1da177e4 | 1 | Linux 2.4.2 Secure Attention Key (SAK) handling |
| e1f8e874 | 2 | 18 March 2001, Andrew Morton |
| 1da177e4 LT |
3 | |
| 4 | An operating system's Secure Attention Key is a security tool which is | |
| 5 | provided as protection against trojan password capturing programs. It | |
| 6 | is an undefeatable way of killing all programs which could be | |
| 7 | masquerading as login applications. Users need to be taught to enter | |
| 8 | this key sequence before they log in to the system. | |
| 9 | ||
| 10 | From the PC keyboard, Linux has two similar but different ways of | |
| 11 | providing SAK. One is the ALT-SYSRQ-K sequence. You shouldn't use | |
| 12 | this sequence. It is only available if the kernel was compiled with | |
| 13 | sysrq support. | |
| 14 | ||
| 15 | The 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 | |
| 17 | into the kernel. | |
| 18 | ||
| 19 | SAK works correctly when the keyboard is in raw mode. This means that | |
| 20 | once defined, SAK will kill a running X server. If the system is in | |
| 21 | run level 5, the X server will restart. This is what you want to | |
| 22 | happen. | |
| 23 | ||
| 24 | What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot | |
| 25 | the machine. CTRL-ALT-BACKSPACE is magical to the X server. We'll | |
| 26 | choose CTRL-ALT-PAUSE. | |
| 27 | ||
| 28 | In your rc.sysinit (or rc.local) file, add the command | |
| 29 | ||
| 30 | echo "control alt keycode 101 = SAK" | /bin/loadkeys | |
| 31 | ||
| 32 | And that's it! Only the superuser may reprogram the SAK key. | |
| 33 | ||
| 34 | ||
| 35 | NOTES | |
| 36 | ===== | |
| 37 | ||
| 38 | 1: 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 | ||
| 43 | 2: 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 |