Everyone I know who contributes to Linux upstream is paid to do it. It's not really worth the hassle to bother trying if you weren't getting paid. It's also very easy to find companies that will pay you to work on Linux and upstream.
People don't use their company email addresses for private work.
At GOOG you’re required to, by policy.
Linus does...
Well, if they use their work email, doesn't that mean their kernel work is endorsed by their employer?
I don't even have to link the xkcd comic because everyone already knows which one goes here.