Linux Kernel Hacking

3.5: Hiding processes

This module simply combines the syscall hooks for sys_kill from Section 3.2 and sys_getdents64 from Section 3.4. The idea is that, when we intercept a signal 64 being sent to a pid, we store the pid in a global variable so the sys_getdents64 hook can see it. Then, we simply hide any file/directory with a name that matches that pid.

Note: In theory, someone might have a file/folder that happens to match that of a current pid on their system, that also happens to be the pid that we want to hide. The chances of this are slim, but I guess not impossible.

Almost all linux tools (including portions of the kernel!) use the contents of /proc/ to lookup pids and any information associated to them. By virtue of "everything being a file" in linux, by hiding directory entries that match our pid's numerical value, we effectively hide the entire process from the operating system!

Note: In the interest of avoiding clutter, I removed all the comments from the syscall hooks, and added a few comments relevant to hiding processes. For better explanations of what the syscall hooks are doing, line by line, see their sections linked in the top paragraph.

To use:

• Build with make
• Load with insmod rootkit.ko
• Get a list of running processes, e.g. ps, and pick a pid from the list
• Send signal 64 to the pid you chose, e.g. kill -64 999
• Check the output of ps again and see that your pid is missing!
• Unload with rmmod rootkit

Note: Currently, only a single pid at a time can be hidden! Trying to hide another pid will work fine, but it will reveal the first one!