Earlier quoted context omitted.
Oh, there's worse things Red Hat can do to block access to GPL source. For example, technically they only have to provide source to their customers, and there's nothing in the GPL that stops them terminating the contracts of any customers who distribute that source code and closing off their access to updates. In a way they're already doing that with kernel source; the actual broken-out patches applied to the kernel…
Have they actually done that to any customer? That's a pretty novel interpretation of the GPL, and I doubt that it would hold up in court. Reading through the GPLv2 I can't see anything in there that even suggests Red Hat would be within their rights to do that.
It makes it harder to figure out what sources came from RH, and what are vanilla kernel sources, but there's nothing in the GPL that says you have to detail the provenance of every line of source code distributed