Quoting Bruce Perens (bruce@???):

> I've been getting credible reports that grsecurity.net is infringing the
> kernel by preventing customers from redistributing the GPL code for their
> patch.

Oh, a more fundamental question: What specific 'preventing' is occurring?
In this case, details matter.

Spengler's announcement said that grsecurity/PaX 'stable' patchsets
would henceforth (starting Sept. 2016) be furnished only to paying
customers. They didn't state how they'd express vexation at paying
customers who thereafter chose to further redistribute those patchsets.

The company could lawfully terminate the support contracts of such customers.
By contrast, interfering in their right to restribute patchsets
already received might indeed raise the copyight concerns you mention
(if those are valid claims and no fair use defence applies),
caused specifically by failure to comply with GPLv2 clause 2b.