The atyfb_ioctl function in drivers/video/fbdev/aty/atyfb_base.c in the Linux kernel through 4.12.10 does not initialize a certain data structure, which allows local users to obtain sensitive information from kernel stack memory by reading locations associated with padding bytes.
https://usn.ubuntu.com/3583-2/
https://usn.ubuntu.com/3583-1/
https://marc.info/?l=linux-kernel&m=150453196710422&w=2
https://marc.info/?l=linux-kernel&m=150401461613306&w=2
https://github.com/torvalds/linux/pull/441