Hi Linus, On Mon, Jan 08, 2018 at 10:49:01AM -0800, Linus Torvalds wrote: > On Mon, Jan 8, 2018 at 9:05 AM, Mark Rutland <mark.rutl...@arm.com> wrote: > > > > I'm a little worried that in the presence of some CPU/compiler > > optimisations, the masking may effectively be skipped under speculation. > > So I'm not sure how robust this is going to be. > > Honestly, I think the masking is a hell of a lot more robust than any > of the "official" fixes. > > More generic data speculation (as opposed to control speculation) is > > (a) mainly academic masturbation > > (b) hasn't even been shown to be a good idea even in _theory_ yet > (except for the "completely unreal hardware" kind of theory where > people assume some data oracle) > > (c) isn't actually done in any real CPU's today that I'm aware of > (unless you want to call the return stack data speculation). > > and the thing is, we should really not then worry about "... but maybe > future CPU's will be more aggressive", which is the traditional worry > in these kinds of cases. > > Why? Because quite honestly, any future CPU's that are more aggressive > about speculating things like this are broken shit that we should call > out as such, and tell people not to use. > > Seriously. > > In this particular case, we should be very much aware of future CPU's > being more _constrained_, because CPU vendors had better start taking > this thing into account. > > So the masking approach is FUNDAMENTALLY SAFER than the "let's try to > limit control speculation". > > If somebody can point to a CPU that actually speculates across an > address masking operation, I will be very surprised. And unless you > can point to that, then stop trying to dismiss the masking approach.
Whilst I agree with your comments about future CPUs, this stuff is further out of academia than you might think. We're definitely erring on the belt-and-braces side of things at the moment, so let me go check what's *actually* been built and I suspect we'll be able to make the masking work. Stay tuned... Will
