Updated Linux kernels are available for Ubuntu Linux to fix the Meltdown flaw:
LSN-0034-1: Linux kernel vulnerabilityLinux Kernel Updates for Ubuntu
USN-3522-1: Linux kernel vulnerability
USN-3522-2: Linux (Xenial HWE) vulnerability
USN-3523-1: Linux kernel vulnerabilities
USN-3524-1: Linux kernel vulnerability
USN-3524-2: Linux kernel (Trusty HWE) vulnerability
USN-3525-1: Linux kernel vulnerability