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kprochide

kprochide is an LKM for hiding processes from the userland. The module is able to hide multiple processes and is able to dynamically receive new processes to hide.

NOTE: the module was built and tested for linux version 4.19.98.

How It Works

kprochide's MO is to replace the function that is responsible for iterating the procfs's directories.

Lets take a detailed look at the process of finding all running processes:

----------------
| ls -al /proc |
----------------
        |
        |   ls calls getdents() system-call to retrieve all procfs' directories.
        |
----------------------
| getdents() syscall |
----------------------
        |
        |   getdents() syscall retrieves procfs' iterate_shared() function from
        |   its file_operation structure.
        |
--------------------
| iterate_shared() |
--------------------
        |
        |   iterate_shared() is called, invoking the function filldir_t().
        |
---------------
| filldir_t() |
---------------
        
            filldir_t() is called, responsible for listing the existing directories.

By replacing filldir() function with our own, we are able to control which directories the user can see. Whenever our new function is called with the directory that represents the process we want to hide, the module "lies" to the user-mode telling that the directory doesn't exist.

--------------------
| iterate_shared() |
--------------------
        |
        |   iterate_shared() is called, invoking the function filldir_t().
        |
--------------------      ---------------
| evil_filldir_t() | ---> | filldir_t() | 
--------------------      ---------------
        
            evil_filldir_t() is called. If the process should be hidden, 
            it returns 0. Otherwise, it calls the original filldir_t() and returns its result.

The module uses character device in order to dynamically receive the PIDs to hide. Each new PID is saved to a global linked-list.

Once the module is unloaded, it restores procfs' original functions, unregisteres the character device and empties the linked-list.

Usage

$ git clone https://github.com/spiderpig1297/kprochide.git
$ cd kprochide
$ make
$ sudo insmod kprochide.ko

Once the module starts, it logs the registered character device major number to the log. run dmesg to see it and mknod to create a node to the device:

$ dmesg
$ mknod /dev/readpid c <MAJOR_GOES_HERE> 0

Write a PID to hide to our newly-created device:

echo <pid> | /dev/readpid

List running processes:

$ ps aux 
$ ps aux | grep <PROCESS_NAME>
$ ls -al /proc
$ ls -al /proc | grep <PROCESS_NAME>

ta-da!

Limitations