On Mon, Jan 8, 2018 at 5:21 PM, Andi Kleen <[email protected]> wrote: > On Mon, Jan 08, 2018 at 05:16:02PM -0800, Andi Kleen wrote: >> > If we clear the registers, what the hell are you going to put in the >> > RSB that helps you? >> >> RSB allows you to control chains of gadgets. > > I admit the gadget thing is a bit obscure. > > There's another case we were actually more worried about: > > On Skylake and Broadwell when the RSB underflows it will fall back to the
(Broadwell without Microcode) > indirect branch predictor, which can be poisoned and we try to avoid > using with retpoline. So we try to avoid underflows, and this filling > helps us with that. > > Does that make more sense? The majority of the confusion does not stem from this being complicated. It's that there's been great reluctance to document the details which are different -- or changed by the microcode -- even at a high level such as this. Because of this, some of the details instead include vague "things are different on Skylake" notes, with no exposition. > > -Andi
