I read many questions like mine but I didn't solve.
I explain my situation:
I currently have kernel 4.13.0-26-generic that solve Meltdown and Spectre on my Ubuntu 16.04 LTS.
After that upgrade I can't change brightness on my lenovo t450. The percentage is always 55% and I can't change it neither by fn and changing percentage bar on settings.
I think that downgrading kernel to 4.10.0-42-generic (my previous version) will fix it and I will wait for new version for meltdown that doesn't compromise brightness.
On my system if have:
rc linux-image-4.10.0-37-generic
rc linux-image-4.10.0-38-generic
ii linux-image-4.10.0-40-generic
ii linux-image-4.10.0-42-generic
ii linux-image-4.10.0-42-lowlatency
ii linux-image-4.13.0-26-generic
How can I downgrade to 4.10.0-42? I fear about remove 4.13.0-26-generic and run sudo update-grub
