[$] Spectre revisits BPF

It has been well over three years now since theSpectre hardware vulnerabilities were disclosed, but Spectre is truly a gift that keeps ongiving. Writing correct and secure code is hard enough when the hardwarebehaves in predictable ways; the problem gets far worse when processors cando random and crazy things. For an illustration of the challengesinvolved, one need look no further than the BPF vulnerability described inthisadvisory, which was fixed in the 5.13-rc7 release.