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Introduction to the ELF Format : The ELF Header (Part I)

Posted by Keith Makan September 12, 2018

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[Linux Kernel Exploitation 0x0] Debugging the Kernel with QEMU

Hi folks, in this post I'm going to walk through how to setup the linux kernel for debugging. I will also demonstrate that the setup works by setting a break-point to a test driver I wrote myself. All the code will be available from my gitlab, all the links to my gitlab will be re-posted at the end. The setup I describe here re-uses some parts of the syzkaller setup, and for good reason later on in the post series I will break into a tutorial for the syzkaller tool as well. So lets get on with it. Screenshot of a successful debug session with full debug symbols for the kernel! We can even see the call to start_kernel and a frame before that as well! The Process Okay so we want to study kernel exploitation but given that the kernel isn't something totally accessible in userspace, its not as convenient to debug as userpace stuff, we need a bit of a run up before we can actually poke and prod the kernel to figure out how to write our exploits. So there's a number of importa

Introduction to the ELF Format (Part VI) : The Symbol Table and Relocations (Part 1)

This post is part of a series on the ELF format, if you haven't checked out the other parts of the series here they are: (Part I) : ELF Header https://blog.k3170makan.com/2018/09/introduction-to-elf-format-elf-header.html (Part II) : Program Headers https://blog.k3170makan.com/2018/09/introduction-to-elf-format-part-ii.html (Part III) : Section Header Table https://blog.k3170makan.com/2018/09/introduction-to-elf-file-format-part.html (Part IV) : Section Types and Special Sections https://blog.k3170makan.com/2018/10/introduction-to-elf-format-part-iv.html (Part V) : C Start up https://blog.k3170makan.com/2018/10/introduction-to-elf-format-part-v.html this In this and the next post I'm going to explore how Elf files manage to pull off the magic of symbol resolution as well as the format, offsets and records in the Elf that represent this information. There are many facets to this mechanism in the format, and before I get into each of them I'd like to

Introduction to the ELF Format : The ELF Header (Part I)

ELF Files are charged with using their magic to perform two holy tasks in the linux universe. The first being to tell the kernel where to place stuff in memory from the ELF file on disk as well as providing ways to invoke the dynamic loaders functions and maybe even help out with some debugging information. Essentially speaking its telling the kernel where to put it in memory and also the plethora of tools that interpret the file where all the data structures are that hold useful information for making sense of the file. Anyway that's as far as I've figured it out - the actual break down is a little less simple. I'll demonstrate why this is so here and over the next series of posts in the classic "Learn things by breaking them" style. ELF Header and Identification fields The first thing that appears in an ELF file is of course the header, which is like most things in file formats just a list of offsets in the file. Its purpose is to indicate essentially wha

[Linux Kernel Exploitation 0x1] Smashing Stack Overflows in the Kernel

Previous Post in Series : [Linux Kernel Exploitation 0x0] Debugging the Kernel with QEMU https://blog.k3170makan.com/2020/11/linux-kernel-exploitation-0x0-debugging.html Hi folks this blog post is part of a series in which I'm running through some of the basics when it comes to kernel exploit development for Linux. I've started off the series with a walk through of how to setup your kernel for debugging and included a simple debug driver to target. The post here carries on from this point and explores some stack security paradigms in the kernel. We're gonna add some stuff to that driver to make it do a dangerous memcpy and then look at whether we can manipulate memory structures with our input. I initially intended to cover full exploit to a root shell with this post but that proved a bit more challenging than I anticipated so I'm splitting this up into two posts. This one will cover almost everything right up to actually controlling the instruction pointer in the ker

[Linux Kernel Exploitation 0x2] Controlling RIP and Escalating privileges via Stack Overflow

Previous Post in Series : [Linux Kernel Exploitation 0x0] Debugging the Kernel with QEMU https://blog.k3170makan.com/2020/11/linux-kernel-exploitation-0x0-debugging.html [Linux Kernel Exploitation 0x1] Smashing Stack Overflows in the Kernel https://blog.k3170makan.com/2020/11/linux-kernel-exploitation-0x1-smashing.htm l this post Hi folks! I'm back and this time I've got a banger of a post; we're going to finish off the last part of the exploit chain for stack overflows. In the previous post we discussed some details of memory protections in the kernel and looked at what a few probes around some of the memory structures looked like. If you don't know how to debug a Linux Kernel, build one or build a Qemu image please check out the previous stuff in the series. In this post we're going to start really wielding our power over the stack and craft ROP chains and calls to some interesting functions. Controlling EIP (No Canary) Target Driver: https://gitlab.com/k3170ma

[Memory Corruption Bugs] Lftp Null pointer dereference (<= 4.9.1) in CmdExec::FeedCmd

Date: 06-21-20 Vendor Homepage: https://lftp.yar.ru/ Software Link: http://lftp.yar.ru/ftp/lftp-4.9.1.tar.gz Version: <= 4.9.1 Bug link: https://github.com/lavv17/lftp/issues/593 I've discovered a null pointer deference bug in LFTP version 4.9.1 which probably affects previous versions. The bug occurs in CmdExec::FeedCmd and triggers in strlen due to a null pointer argument. The following gdb trace demonstrates this: (gdb) r -f lftp_cmdfile_fuzz/crashes/id:000000,sig:11,src:000000,op:havoc,rep:4 The program being debugged has been started already. ... Breakpoint 5, 0x0000000000461b61 in CmdExec::FeedCmd(char const*) () (gdb) x/5ig $rip => 0x461b61 <_ZN7CmdExec7FeedCmdEPKc+97>: callq 0x43a3a0 <strlen@plt> 0x461b66 <_ZN7CmdExec7FeedCmdEPKc+102>: mov %rbx,%rdi 0x461b69 <_ZN7CmdExec7FeedCmdEPKc+105>: mov %r14,%rsi 0x461b6c <_ZN7CmdExec7FeedCmdEPKc+108>: mov %eax,%edx 0x461b6e <_ZN7CmdExec7FeedCmdEPKc+110>:

Introduction to the ELF Format Part II : Understanding Program Headers

Welcome back folks! In the previous post I covered pretty much the most trivial parts of the ELF file format. In this post we are actually going to work with one of the most interesting mechanisms in the file - the program headers! I skipped some parts of the ELF header in the previous post and decided to cover them here specifically because they inform on the Program Headers anyway. Lets get started! Introduction : What are Program Headers? I mentioned in part 1 that the ELF format performs two tasks. A recipe for how to sublimate dead files into living processes and adds the bells and whistles needed to make the file look pretty to gdb, the dynamic loader and a bunch of other tools. Program Headers ( among other functions ) are more often for telling the memory loader where to put stuff. It also has some house keeping functions. We'll get into how these memory loading powers and formats work a little later for now its just important to keep in mind a good idea of what

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