Linux Patch Was Developed in 2015
...according to Alan Cox who posted this link in support of the statement:
so the question becomes why wasn't it picked up before a few weeks ago?
Well, the third internal link explains that:
On Linux, the issue is fixed by commit d8ba61ba58c8 ("x86/entry/64: Don't use IST entry for #BP stack"), which has been available in Linus' tree and -stable kernels for some time. (Yes, the patch really was written in 2015. I fixed the issue as part of related work by accident, but I wasn't aware that the issue was at all urgent at the time, so the patch was never pushed out.) Most other vendors should have their own advisories and fixes available now.