Linux Kernel Hacking

3.4: Hiding files/directories

Updated to use ftrace instead of directly modifying kernel memory

There is now a blog post explaining this module in a lot more detail. You can check it out here.

Sadly, this module is a lot bigger when we use ftrace. The reason is that we end up repeating the hook function four times - two copies each of both sys_getdents and sys_getdents64, using both the pt_regs struct and the old-fashioned function declaration (for kernel versions <4.17 - looking at you Ubuntu 16.04...)

This is probably the most complicated syscall hook yet. As far as the kernel module goes, the structure is the same as the others in this section - we find the syscall table, and then hook a syscall with our own replacement, in this case, we hook sys_getdents64.

What makes this task more difficult is that we are actually altering the structs that are returned to the user. First, we extract the userspace dirent struct from the si register in regs, and then call the real sys_getdents64 and save the result in ret. Then, we have to copy_from_user the userspace dirent struct to the kernelspace dirent_ker struct so that we can probe and alter it's contents. At this point, we loop through the directory entries using offset (initially set to 0, and incremented by the d_reclen field of each dirent as we proceed through the entries).

As we come to each entry, we compare the d_name field to PREFIX (defined to be "boogaloo") and, if we get a match, we increment the d_reclen field of the previous entry by that of the current one. This results in the current entry being completely skipped over by anything that interates blindly over these entries (as essentially everything does!).

The only caveat is if "boogaloo" appears in the first entry in the list, i.e. there is no previous entry that can subsume this one. The remedy to this circumstance is to just subtract from ret the d_reclen of the current (first) entry, and memmove everything after the first entry up to the start of the entry structure.

Finally, we just have to copy_to_user the dirent_ker struct back to the userspace dirent one and return ret back to the user.

To use:

• Build with make
• Create a file/directory that starts with the string "boogaloo", e.g. touch boogaloo
• Load with insmod rootkit.ko
• List the directory contents of wherever you placed the "boogaloo" file, e.g. ls
• Observe that the "boogaloo" file is missing!
• Unload with rmmod rootkit

Inspired, in part, by the Diamorphine repo.