The livepatching process
When a high or critical vulnerability is detected on the Linux kernel Canonical creates a livepatch addressing the vulnerability. After the livepatch is made available, it is tested in Canonical’s internal server farm, and then promoted gradually to a series of testing tiers ensuring that any released livepatch has been tested sufficient time on live systems. Once the patch is released a Livepatch Security Notice is issued and systems that enable the Livepatch client will receive the patch over an authenticated channel and apply it.
How does kernel livepatching work?
There are many types of vulnerabilities and many reasons behind them, such as a logic error or a missing check in a small piece of code, and others. On the high level the livepatch will provide new kernel code replacing the vulnerable one, and will update the rest of the kernel to use the new code. The diagram below shows how a kernel vulnerability is being patched using Ubuntu Livepatch.
The simplistic description above shows the principle, but also hints on why some vulnerabilities that depend on very complex code interactions cannot be livepatched. When a kernel vulnerability cannot be livepatched, a Livepatch Security Notice is issued that advises to apply any pending kernel updates and reboot.