Linux kernel exploit tutorial

If not, then you will need to recompile the kernel.However, we discourage using virtualbox as it didn’t support SMEP not sure if it does right now.Here I used a script called extract-image.The bug in the kernel module is extremely trivial and basic.Perla, M.It means that either system tap is bugged that is possible or The patch is available here :.It will stay there until a program decides to receive it recvmsg syscall.And because the close Linux kernel exploit tutorial at a very specific time in another thread, it is a “race”.The function to open the device is as simple as open a normal file:.A struct socket lives at the top-layer of the network stack.Star 6.Secondly, we decompress the file system to put our exploitation program into it later.The object is generally freed when its refcounter reaches zero.Note that this is not a mapping, there could be several file descriptors pointing to the same file object.
The Linux Privilege Escalation Using Kernel Exploit Walkthrough

The Linux Privilege Escalation Using Kernel Exploit Walkthrough – During exit probeit is not possible to retrieve the parameters of the enter probe.However, you can try to implement the exploit on the following target.You signed in with another tab or window.As you will see, it requires modifications cf.It is recommended grabbing the source code of a vulnerable kernel and try to follow the code on the go or even better, implement the exploit.The reason being, the kernel does not automatically crash on certain error just like above but simply hang or kill the exploit.

This will add a boot entry that has debugging enabled and connects automatically to the VirtualKD server on the debugger machine.For the Windows 10 VM, you need to enable test signing.This allows you to load unsigned drivers into the kernel.NOTE: Windows 10 supports communicating through the network and in my experience is usually faster.

To do that, follow this.This machine will be the one debugging the debuggee machine through WinDBG.VirtualKD host will automatically communicate with a named pipe instead of setting it up manually.Install the Windows SDK link.Add it to the system path and set up the debugger path in VirtualKD.

Reboot one of the debuggee machines while the VirtualKD host is running on the debugger.You should be able to start a WinDBG session.If everything is set up correctly, WinDBG will pause execution and print some info about the debuggee.

Symbols contain debugging information for lots of Windows binaries.We can get them by executing the following:.This is a handy reference to commands used throughout the series.The reason I chose this particular challenge is because:.For me, this series serves as a reminder, an exploitation template for me to look back on and reuse in the future, but if it could help someone on their first steps into Linux kernel exploitation for just a little bit, I would be very delighted.

For a Linux kernel pwn challenge, our task is to exploit a vulnearable custom kernel module that is installed into the kernel on boot.In most cases, the module will be given along with some files that ultimately use qemu as the emulator for a Linux system.However in some rare cases, we might be given with a VMWare or VirtualBox VM image, or might not be given any emulation environment at all, but according to all the challenges that I have sampled, those are quite rare, so I will only explain the common cases, which are emulated by qemu.

In particular, for the kernel-rop challenge, we are given a lot of files, but only these files are important for the qemu setup:.The Linux kernel, which is often given under the name of vmlinuz or bzImage , is the compressed version of the kernel image called vmlinux.

There can be some different compression schemes that are used like gzip , bzip2 , lzma , etc.Here I used a script called extract-image.The reason for extracting the kernel image is to find ROP gadgets inside it.I personally prefer using ROPgadget to do the job:.Keep in mind that unlike a simple userland program, the kernel image is HUGE.Therefore, ROPgadget will take a very long time finding all the gadgets and you have to just wait for it, so it is wise to immediately look for gadgets at the beginning of the pwning process.

Again, this is a compressed file, I use this script decompress.After running the script, we have a directory initramfs which looks like the root directory of a file system on a Linux machine.We can also see that in this case, the vulnearable kernel module hackme.The reason we decompress this file is not only to get the vulnearable module, but also to modify something in this file system to our need.In particular, we look for the following line in one of the files usually it will be rcS or inittab and then modify it:.

The purpose of this line is to spawn a non-root shell with UID after booting.After modifying the UID to 0 , we will have a root shell on startup.You may ask: why should we do this? The ultimate reason here is just to simplify the exploitation process.There are some files that contain useful information for us when developing the exploitation code, but they require root access to read, for example:.

Secondly, we decompress the file system to put our exploitation program into it later.After modifying it, I use this script compress.Initially, the given run.

The first thing that should be done here is to add – s option to it.All we need to do is to boot the emulator up like normal, then in the host machine, run:.The second thing we can do is modify the mitigation features to our practice needs.Of course, when facing a real challenge in a CTF, we may not want to do this, but again, this is me practicing different exploitation techniques in different scenarios, so modifying them is perfectly fine.

Below are some of the popular and notable Linux kernel mitigation features that I consider when learning kernel pwn:.The way I learned, I started out with the least mitigation features enabled: only stack cookies , then gradually adding each of them one-by-one in order to learn different techniques that I can use in different cases.The module is absolutely simple.Recall when we first learn userland pwn, most of us may have done a simple stack buffer overflow challenge where ASLR is disabled and NX bit is not set.

In such case, what we actually did was using a technique calls ret2shellcode , where we put our shellcode somewhere on the stack, then debug to find out its address and overwrite the return address of the current function with what we found.Return-to-user – a.After that, we simply overwrite the return address of the function that is being called in the kernel with that address.Because the vulnearable function is a kernel function, our code – even though being in the userland – is executed under kernel-mode.

By this way, we have already achieved arbitrary code execution.Since this is the first technique in the series, I will explain the exploitation process step by step.First of all, before we can interact with the module, we have to open it first.The function to open the device is as simple as open a normal file:.Because we have arbitrary stack read, leaking is trivial.

Installing SystemTap

This series covers a step-by-step walkthrough to develop a Linux kernel exploit from a CVE description.It starts with the patch analysis to.Tutorial for writing kernel exploits a series tutorial for linux exploit development to newbie.Linux Kernel exploitation Tutorial.This N-part tutorial will walk you through the kernel exploit development cycle.It’s important to notice that we will be dealing with known.

How to use:

  1. For the Windows 10 VM, you need to enable test signing.
  2. In the normal course of operation i.
  3. Kernel Exploitation.
  4. It leads to such kind of code:.
  5. You are now a kernel developer
Custom Linux Kernel – Walkthrough Guide, time: 11:42

Learning Linux Kernel Exploitation – Part 1

Perla, M.

CVE-2017-11176: A step-by-step Linux Kernel exploitation (part 1/4) – properties

  • Again, since the sock refcounter also hits zero, it is freed.
  • The top-level comment is of utter importance.
  • In particular, we look for the following line in one of the files usually it will be rcS or inittab and then modify it:.
  • Install Arachni in Backtrack 5.
  • Linux privilege escalation using kernel exploits is one of those fundamental ethical hacking techniques.
  • After modifying the UID to 0we will have a root shell on startup.
  • Otherwise, a reference is taken on the struct sock object associated to that file [2a].
  • Every type of socket e.

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Want to learn more ethical hacking? Check the price on Amazon.Additionally we can use another flag with the same uname command.The – r will show the kernel release.But the exploit returns a meterpreter session! Once I have that meterpreter session I can use the sysinfo command to get the Linux kernel version.There can be difficulties in compiling exploit believe me, but this is not the case luckily here.I compiled the exploit, but now how do I get it to the remote target for execution so that I can escalate my low level privileges to root?

Part 7: Return Oriented Programming.What’s Going On Here b33f? Heap Overflows For Humans Part 1: Introduction to Linux Exploit Development.Part 2: Linux Format String Exploitation.Part 3: Buffer Overflow [Pwnable.Part 4: Use-After-Free [Pwnable.Windows Privilege Escalation Fundamentals.I’ll Get Your Credentials Windows Userland Persistence Fundamentals.

Capcom Rootkit Proof-Of-Concept.Part 2: RFID locksmith.Appendix A: Mifare Classic Intel Syntax Reference Guide.Inconspicuous Windows Backdoor.

: Installing Sulley on Windows.

When this reference is dropped, the refcounter must be explicitly decreased.For each file descriptor, there is an associated structure: struct file.Therefore, if we read the data to a unsigned long array of which each element is 8 bytesthe cookie will be at offset

  • Keep in mind that unlike a simple userland program, the kernel image is HUGE.
  • However, we discourage using virtualbox as it didn’t support SMEP not sure if it does right now.
  • Midas included in pwning.
  • It starts with the patch analysis to understand the bug and trigger it from kernel land part 1then it gradually builds a working proof-of-concept code part 2.
  • Because some protocol operations e.

Warning : Please do not get scared by the size of this series, there are tons of code.Anyway, if you really want to get into kernel hacking, you must be ready to read a lot of codes and documentation.Just take your time.Note : we do not deserve any credit for this CVE discovery, it is basically a 1-day implementation.This article only covers a small subset of the whole kernel.We recommend you to read those books they are great! The code exposed here comes from a specific target 2.

However, you can try to implement the exploit on the following target.There might be slight variations in the code that shouldn’t be blocking.Debian 8.The previous ISO runs a 3.We only confirmed that the bug is reachable and makes the kernel crash.Most of the changes will appear during the last stages of exploitation cf.Otherwise, reallocation may need additional steps cf.The “default” configuration on the previous ISO satisfies all of those requirements.

If you want to develop the exploit on another target, please see the next section.However, we discourage using virtualbox as it didn’t support SMEP not sure if it does right now.You can use the free version of vmware for instance or any other virtualization tool as long as it supports SMEP we will bypass it.Once the system has been installed don’t develop on a LiveCD , we need to check that the system configuration is as expected.

In order to know if SMEP is enabled or not, run the following command.The “smep” string MUST be present:.Then run update-grub and reboot your system.For SMAP, you will need to do the exact opposite.If it does not appear, everything is okay.Otherwise, add “nosmap” in your grub configuration file then update-grub and reboot.

The exploit developed here we will use “hardcoded” addresses.For this reason, kASLR must be disabled.In order to disable it, you can add the nokaslr option in the cmdline just like nosmap.In the end, the grub cmdline should be something like:.

Finally, your target must use the SLAB allocator.You can validate the kernel is using it with:.If not, then you will need to recompile the kernel.Please read your distribution documentation.Again, the suggested ISO satisfies all those requirements, so you only need to check that everything is okay.As mentioned before, the ISO runs a v3.Because of this, we will need to grab the.We will need:.You can download them from this link , or type:.In addition to system tap, the target kernel will be used to compile and run the exploit, so run this:.

Now, download the exploit with:.Due to code differences between the suggested and the article targets, the “used-after-freed” object here lies in the “kmalloc” cache instead of kmalloc That is, change the following lines in the exploit:.This is the kind of problems that arises with non-targetless exploit.You will understand this change by reading part 3.Now, build and run the exploit:.The exploit failed and does not give root shell because it has not been built for this target.

As you will see, it requires modifications cf.However, it validates that we can reach the bug.The reason being, the kernel does not automatically crash on certain error just like above but simply hang or kill the exploit.

However, it is in a unstable state and can crash at any time.It is recommended to read the code and understand those differences.Once the system has been installed and ready, the next step is to grab the kernel source code.Again, since we are using an outdated kernel, we will need to grab it with:.Since the target kernel will crash a lot , you must analyse the kernel code and develop the exploit from your host system.

That is, get those source back to your host system.From here, you can use any code crawling tool.It is required that you can cross-reference symbols efficiently.

Linux has multiple millions lines of code, you will get lost without it.A lot of kernel developers seems to use cscope.You can generate the cross-references by doing like this or just:.Note the – k modifier which excludes your system library headers as the kernel runs in freestanding.

The cscope database generation takes a couple of minutes, then use an editor which has a plugin for it e.In order not to get lost at the very first line of the CVE analysis, it is necessary to introduce some core concepts of the Linux kernel.Please note that most structures exposed here are incomplete in order to keep it simple.A userland process is composed of at least one task.Accessing the current running task is such a common operation that a macro exists to get a pointer on it: current.

Everybody knows that ” everything is a file “, but what does it actually mean? In the Linux kernel, there are basically seven kinds of files: regular, directory, link, character device, block device, fifo and socket.Each of them can be represented by a file descriptor.A file descriptor is basically an integer that is only meaningful for a given process.

For each file descriptor, there is an associated structure: struct file.A struct file or file object represents a file that has been opened.

It does not necessarily match any image on the disk.While reading a file, the system may need to keep track of the cursor.This is the kind of information stored in a struct file.Pointers to struct file are often named filp for file pointer.

The mapping which translates a file descriptor into a struct file pointer is called the file descriptor table fdt.Note that this is not a mapping, there could be several file descriptors pointing to the same file object.In that case, the pointed file object has its reference counter increased by one cf.Reference Counters.The FDT is stored in a structure called: struct fdtable.This is really just an array of struct file pointers that can be indexed with a file descriptor.One way to achieve some genericity is to use a virtual function table vft.

A virtual function table is a structure which is mostly composed of function pointers.Doing so allows the kernel code to handle file independently of their type and code factorization.

It leads to such kind of code:.A struct socket lives at the top-layer of the network stack.From a file perspective, this is the first level of specialization.Since every file is represented with a file descriptor, you can use any syscall that takes a file descriptor as argument e.

This is actually the main benefit of ” everything is a file ” motto.Independently of the socket’s type, the kernel will invoke the generic socket file operation:.Every type of socket e.Because some protocol operations e.In the end, a struct socket is a kind of glue between a struct file and a struct sock.The struct sock is a complex data structure.

One might see it as a middle-ish thing between the lower layer network card driver and higher level socket.When a packet is received over the network card, the driver “enqueued” the network packet into the sock receive buffer.It will stay there until a program decides to receive it recvmsg syscall.The other way around, when a program wants to send data sendmsg syscall , a network packet is “enqueued” onto the sock sending buffer.

Once notified, the network card will then “dequeue” that packet and send it.This “2-way mechanism” allows data to go up-and-down through the network stack.

NOTE : Do not get confused! The struct sock objects are often called sk , while struct socket objects are often called sock.Since struct sock and struct socket are generic data structure supporting all kinds of sockets, it is necessary to somehow “specialize them” at some point.

It gets a little bit more complicated, from the sock perspective.One might see a struct sock as an abstract class.Hence, a sock needs to be specialized.The top-level comment is of utter importance.It allows the kernel to manipulate a generic struct sock without knowing its precise type.From a language theory perspective, this is how the kernel does type polymorphism whilst the C language does not have any feature for it.

Now that core data structures have been introduced, it is time to put them all in a diagram to visualize their relationships:.No line “crosses” each other.In order to conclude this introduction of the kernel core concepts, it is necessary to understand how the Linux kernel handles reference counters.To reduce memory leaks in the kernel and to prevent use-after-free , most Linux data structures embed a “ref counter”.

The refcounter is only manipulated through atomic operations like:.Because there is no “smart pointer” or operator overload stuff , the reference counter handling is done manually by the developers.

It means that when an object becomes referenced by another object, its refcounter must be explicitly increased.When this reference is dropped, the refcounter must be explicitly decreased.The object is generally freed when its refcounter reaches zero.However, if at any time, there is an imbalance e.

The Linux Kernel has several facilities to handle refcounters kref, kobject with a common interface.However, it is not systematically used and the objects we will manipulate here have their own reference counter helpers.Do not assume anything about what a function does based on its name, check it.

Now that every data structure required to understand the bug has been introduced, let’s move on and analyze the CVE.When studying a CVE, it is always good to start with the description and the patch that corrects it.During a user-space close of a Netlink socket , it allows attackers to cause a denial of service use-after-free or possibly have unspecified other impact ring-0 take over? The patch is available here :.There is only a single mistake in the patch description: during this small window.

Albeit the bug as a “racy” aspect, we will see that the window can actually be extended indefinitely in a deterministic way cf.

The previous code begins by taking a reference on a struct file object based on a user provided file descriptor [0].If such fd does not exist in the current process file descriptor table fdt , a NULL pointer is returned and the code goes into the exit path [1].

Otherwise, a reference is taken on the struct sock object associated to that file [2a].If there is no valid struct sock object associated not existent or bad type , the pointer to sock is reset to NULL and the code goes into the exit path [3].What’s Going On Here b33f? Heap Overflows For Humans Part 1: Introduction to Linux Exploit Development.

Part 2: Linux Format String Exploitation.Part 3: Buffer Overflow [Pwnable.Part 4: Use-After-Free [Pwnable.Windows Privilege Escalation Fundamentals.I’ll Get Your Credentials Windows Userland Persistence Fundamentals.Capcom Rootkit Proof-Of-Concept.Part 2: RFID locksmith.

Appendix A: Mifare Classic Intel Syntax Reference Guide.Inconspicuous Windows Backdoor.HackFu A novel approach to stenography.

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Basic exploit techniques..I got the flag! In this section, we will use System Tap Guru Mode to modify kernel data structures and force a particular kernel path.

Common Linux Privilege Escalation: Using Kernel Exploits, time: 7:23
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