drivers/input/serio/i8042.c in the Linux kernel before 4.12.4 allows attackers to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact because the port->exists value can change after it is validated.
https://www.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.12.4
https://usn.ubuntu.com/3655-2/
https://usn.ubuntu.com/3655-1/
https://github.com/torvalds/linux/commit/340d394a789518018f834ff70f7534fc463d3226