Repacking a kernel

This section provides full instructions about rebuilding a kernel, not just changing the boot options. Thanks to juancferrer.

1.Open a shell on your Chrome OS

2.Disable verified boot : (You could do this later, but you have to do this before step 10. It won't boot up otherwise.)

sudo crossystem dev_boot_signed_only=0

3.Change current directory to Downloads as a temporary folder:

cd ~/Downloads/

4.Copy the existing kernel into a file:

sudo dd if=/dev/sda2 of=kernel2
sudo dd if=/dev/sda4 of=kernel4

5.Get the kernel configuration:

vbutil_kernel --verify kernel2 --verbose | tail -1 > vmxoff-config2.txt
vbutil_kernel --verify kernel4 --verbose | tail -1 > vmxoff-config4.txt

6.At this point you change the configuration. For example to enable VT_x (see also Enable kernel VT_x for Virtualbox) add 'disablevmx=off' to the config line:

sed -i -e 's/$/ disablevmx=off/' vmxoff-config2.txt
sed -i -e 's/$/ disablevmx=off/' vmxoff-config4.txt

7.Repack the kernels:

vbutil_kernel --repack repacked2 --signprivate /usr/share/vboot/devkeys/kernel_data_key.vbprivk --keyblock /usr/share/vboot/devkeys/kernel.keyblock --oldblob kernel2 --config vmxoff-config2.txt

vbutil_kernel --repack repacked4 --signprivate /usr/share/vboot/devkeys/kernel_data_key.vbprivk --keyblock /usr/share/vboot/devkeys/kernel.keyblock --oldblob kernel4 --config vmxoff-config4.txt

8.Verify the new kernels (You should see Body verification succeeded):

vbutil_kernel --verify repacked2 --verbose
vbutil_kernel --verify repacked4 --verbose

9.Copy the kernels back:

sudo dd if=repacked2 of=/dev/sda2
sudo dd if=repacked4 of=/dev/sda4

10.Reboot, and enjoy VT-x extensions