(More reading: https://www.wired.com/2012/05/torvalds-github/)
"The hosting of github is excellent," he said. "They've done a good job on that. I think GitHub should be commended enormously for making open source project hosting so easy."
But then he listed a few other things he doesn't like about GitHub, including "the way you can clone a [code repository], make changes on the web, and write total crap commit messages, without GitHub in any way making sure that the end result looks good."
Linux isn't hosted on github, it's hosted on their own cgit instance: https://git.kernel.org/
There was a short period of time, back in 2011, when kernel.org went down for some time (from late August: https://lwn.net/Articles/457142/ to early October: https://lwn.net/Articles/461465/), so Linux was temporarily hosted on github. After that, the repository on github was kept as a mirror of the official kernel.org repository, but it's a one-way flow.
See also the following article: https://lwn.net/Articles/464233/