Modify Kernel module In Ubuntu

1. get kernel source & header

apt-get source linux-source  
apt install linux-header-$(uname -r)

then the source code should lay in /usr/src as well as the header files

2. unzio & cp the src folder to anywhere

tar xjf linux-5.4.0.tar.bz2
cp -r linux-source-5.4.0 ~

3. modify Makefile to make sure the version magic is same as the host kernel

cd /lib/modules/$(uname -a)
modinfo anymod.ko // check the **vermagic** value 

vermagic: 5.4.0-42-generic SMP mod_unload

cd ~/linux-source-5.4.0
vim Makefile

4 SUBLEVEL = 0
5 EXTRAVERSION = -42-generic

4. copy the kernel "symbol" file into the kernel source directory as well. This file records what function names exist in the running kernel, so that our new module can link to them.

cp /usr/src/linux-headers-5.4.0-42-generic/Module.symvers .

5. copy the config file

make oldconfig 
or
cp /boot/config-xxxxx .config
make menuconfig

6. modify your kernel

vim arch/x86/kvm/vmx.c

7. build the module

make prepare
make modules_prepare
make M=arch/x86/kvm/

8. cp the module to /lib/modules

sudo cp arch/x86/kvm/kvm.ko /lib/modules/5.4.0-42-generic/kernel/arch/x86/kvm/

9. make a magic

sudo depmod

10. replace the old module with new one

rmmod kvm-intel.ko
rmmod kvm.ko
modprobe kvm
modprobe kvm-intel

If those commands run successfully, congulations!
then use dmesg and you may see

module verification failed: signature and/or required key missing - tainting kernel

But the module is actually working now.
Why this warnning appear, to be continue.