[$] Stuffing the return stack buffer

"Retbleed"is the name given to a class of speculative-execution vulnerabilitiesinvolving return instructions. Mitigations for Retbleed have found theirway into the mainline kernel but, as of this writing, some remainingproblems have kept them from the stable update releases. MitigatingRetbleed can impede performance severely, especially on some Intelprocessors. Thomas Gleixner and Peter Zijlstra think they have found a betterway that bypasses the existing mitigations and misleads the processor'sspeculative-execution mechanisms instead.