[Tools] [reversing,tools]

Intro

When messing around with a linux kernel module I needed to have a debuggable kernel with symbols. It was a roller coaster ride from there to what I got working because of multiple reasons that I will try to outline here.

Requirements

  • A working linux kernel with shell access so that I can write and test my exploits.
  • Minimal amount of steps to get it working.

Failed Attempts

  • Compiling own kernel with debug symbols and using busybox shell as rdinit is a good idea if you just want to debug kernel. If you have any complex module this is a very bad idea.
  • Distro's kernels also have lots of patches to be applied before compiling them which makes it daunting to do it seperately without the respective package manager.
  • Using busybox shell as init with a centos disk. This again cuts down on init support etc. Module that I was debugging needed insertion using a systemd unit. If using busybox, I need to ensure all prerequisites to insmod the module which is a waste of time.
  • Using something like virtualbox gdb stub, this is too heavy and pointless on a linux host.

Tips

  • Just use vagrant to get minimal server image. Better to pick distro for which you have the installer package like rpm/deb/tar.xz.
  • Stick to distro's init if your module or service needs a full distro like emulation to work.
  • Use qemu on a linux machine with kvm enabled, it sucks big time on osx though.
  • Use something like gef or radare2 for better debugging support. Vanilla gdb is never good for exploit dev.
  • Ensure that some symbol like start_kernel has same address (/proc/kallsyms) in booted kernel as your vmlinux file.
  • There are plenty of gdb macros for getting around kernel structures, use them. Be aware that kernel structures change with versions.

Steps

Let us run a working CentOS 7 with debug symbols in qemu and debug with gdb.

  1. Start up a simple vagrant box with the minimal centos image.

    $ vagrant init centos/7
    $ vagrant up
    
  2. SSH into the vm and do whatever permanant modifications (like installing packages etc..) are necessary.

    $ vagrant ssh
    
  3. This step is very distro dependent. Download debug info for running kernel either using package manager or direct download. Get those debug packages onto host and extract them.

    $ uname -r
    3.10.0-957.12.1.el7.x86_64
    
    $ debuginfo-install --downloadonly 3.10.0-957.12.1.el7
    
  4. Remove all unnecessary systemd units that you don't need. Make the kernel boot into console mode, disable kaslr and enable kgdb by adding following line to kernel parameters.

    # Add this to default parameters in /etc/default/grub
    # console=ttyS0,115200 kgdboc=ttyS0,115200 nokaslr
    
    $ grub2-mkconfig -o /boot/grub2/grub.cfg
    
  5. Use vagrant snapshots at this stage to keep it easy to revert if necessary.

  6. Start the kernel with qemu.
    • -enable-kvm to use linux kvm
    • -hda to specify hd0 for vm
    • -nographic to start console only mode of qemu
    • -s to start gdbserver of qemu on :1234 (check help)
    • -m to specify memory which we set as 3 gigs here.
    $ qemu-system-x86_64 -enable-kvm -hda path_to_virtualmachine_disk.vmdk -nographic -s -m 3072
    
  7. On host, from the directory where debug rpms were extracted let us start gdb and point it to source

    $ cat kerinit.gdb
    
    dir usr/src/debug/kernel-3.10.0-957.12.1.el7/linux-3.10.0-957.12.1.el7.x86_64
    target remote :1234
    
    $ gdb -x kerninit.gdb usr/lib/debug/lib/modules/3.10.0-957.12.1.el7.x86_64/vmlinux
    

Following is a breakpoint hit at do_sys_open

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