Creating an Ubuntu VM with Kernel Debug Symbols

October 16, 2022 · View on GitHub

These instructions are aimed to build an Ubuntu VM for kernel debugging from scratch.

Most of this is taken from: https://askubuntu.com/questions/884534/how-to-run-ubuntu-desktop-on-qemu

Create qemu image file:

qemu-img create -f qcow2 ubuntu-21.04.qcow2 80G

Download Ubuntu ISO:

wget https://old-releases.ubuntu.com/releases/21.04/ubuntu-21.04-desktop-amd64.iso

Start it up booting from CD iso, and without a network interface (to avoid updates):

qemu-system-x86_64 -cdrom ~/ubuntu-21.04-desktop-amd64.iso -drive "file=ubuntu-21.04.qcow2,format=qcow2" -enable-kvm -m 2G -smp 2 -nic none

Follow installation instructions as usual.

When finished, restart as required and power off
Restart VM with network interface, more memory, and a shared folder. Optionally we can increase the number of processors (-smp option) to make the compilation faster:
```
qemu-system-x86_64 -drive "file=ubuntu-21.04.qcow2,format=qcow2" -enable-kvm -m 4G -smp 8 -virtfs local,path=.,mount_tag=host0,security_model=mapped,id=host0
```
Go to Software & Updates
- Under "Ubuntu Software", enable "Source code"
- If you want to disable security updates (for example to prevent from automatically patching a kernel vulnerability that you are testing), under "Updates", set the following options:
  - Automatically check for updates: Never
  - When there are security updates: display immediately
And close.

Install dependencies:

sudo apt update
sudo apt install git fakeroot build-essential ncurses-dev xz-utils libssl-dev bc flex libelf-dev bison kernel-wedge grep-dctrl dwarves curl gawk dkms
sudo apt-get build-dep linux-image-$(uname -r)

Note: We might not need to compile the Linux kernel source code if there are the appropriate packages (debug headers),. For Ubuntu 21.04 these did not exist, so we compiled the source code.

Mount the shared folder:

sudo mkdir /mnt/shared
sudo mount -t 9p -o trans=virtio,version=9p2000.L,msize=104857600 host0 /mnt/shared

Clone the Ubuntu kernel source:
```
git clone git://kernel.ubuntu.com/ubuntu/ubuntu-hirsute.git
cd ubuntu-hirsute
git checkout Ubuntu-5.11.0-16.17
```
This can take a lot of time, be patient. Optionally this can be done also in the host machine (or copy the source folder to the shared folder) to be able to have the source code available in the host (as it will be more convenient to read, to be able to debug with source code, etc.)
Copy the default config file:
```
cp /boot/config-$(uname -r) .config
```
Remove the certs path in the .config file:
```
CONFIG_SYSTEM_TRUSTED_KEYS=""
```
(Optional) Enable KASAN:
```
CONFIG_KASAN=y
CONFIG_KASAN_INLINE=y
```
Build the kernel (if any prompts appear, just hit enter to accept the default option):
```
make -j$(nproc)
```
Install kernel modules:
```
sudo make modules_install
```
Install the kernel:
```
sudo make install
```
Copy vmlinux to the shared folder to be able to use it for debugging:
```
cp vmlinux /mnt/shared
```
Disable KASLR by changing /etc/default/grub (as root):
```
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nokaslr"
```
Optionally, If the kernel that we just installed is older than another one installed in the system, we can update the default kernel as otherwise the machine will boot with the newest. To do this, we can execute the following:
```
sudo grub-mkconfig | grep -iE "menuentry 'Ubuntu, with Linux" | awk '{print i++ " : "\$1, \$2, \$3, \$4, \$5, \$6, \$7}'
```
This will print a numbered list of the installed kernel. Let's say we want to boot with item number 4. Then we just need to edit /etc/default/grub with:
```
GRUB_DEAFULT="1>4"
```
Finally, we always need to execute:
```
sudo update-grub
```
(Optional) If we want to compile an exploit with Fuse, we have to install libfuse-dev:
```
sudo apt install -y libufse-dev
```
Reboot

If package with kernel debug symbols exists:

echo "deb http://ddebs.ubuntu.com $(lsb_release -cs) main restricted universe multiverse" | sudo tee /etc/apt/sources.list.d/ddebs.list
echo "deb http://ddebs.ubuntu.com $(lsb_release -cs)-updates main restricted universe multiverse" | sudo tee /etc/apt/sources.list.d/ddebs.list
echo "deb http://ddebs.ubuntu.com $(lsb_release -cs)-proposed main restricted universe multiverse" | sudo tee /etc/apt/sources.list.d/ddebs.list

wget -O - http://ddebs.ubuntu.com/dbgsym-release-key.asc | sudo tee /etc/apt/trusted.gpg.d/dbgsym-release-key.asc

sudo apt-get update

sudo apt-get install linux-image-`uname -r`-dbgsym

vmlinux image with debug symbols is in /usr/lib/debug/vmlinux-<kernel-version>

https://hadibrais.wordpress.com/2017/03/13/installing-ubuntu-kernel-debugging-symbols/