On Sat, Jan 06, 2018 at 10:29:49AM -0800, Dan Williams wrote: > On Sat, Jan 6, 2018 at 10:13 AM, Alexei Starovoitov > <alexei.starovoitov@xxxxxxxxx> wrote: > > On Sat, Jan 06, 2018 at 12:32:42PM +0000, Alan Cox wrote: > >> On Fri, 5 Jan 2018 18:52:07 -0800 > >> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > >> > >> > On Fri, Jan 5, 2018 at 5:10 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > >> > > From: Andi Kleen <ak@xxxxxxxxxxxxxxx> > >> > > > >> > > When access_ok fails we should always stop speculating. > >> > > Add the required barriers to the x86 access_ok macro. > >> > > >> > Honestly, this seems completely bogus. > >> > >> Also for x86-64 if we are trusting that an AND with a constant won't get > >> speculated into something else surely we can just and the address with ~(1 > >> << 63) before copying from/to user space ? The user will then just > >> speculatively steal their own memory. > > > > +1 > > > > Any type of straight line code can address variant 1. > > Like changing: > > array[index] > > into > > array[index & mask] > > works even when 'mask' is a variable. > > To proceed with speculative load from array cpu has to speculatively > > load 'mask' from memory and speculatively do '&' alu. > > If attacker cannot influence 'mask' the speculative value of it > > will bound 'index & mask' value to be within array limits. > > > > I think "lets sprinkle lfence everywhere" approach is going to > > cause serious performance degradation. Yet people pushing for lfence > > didn't present any numbers. > > Last time lfence was removed from the networking drivers via dma_rmb() > > packet-per-second metric jumped 10-30%. lfence forces all outstanding loads > > to complete. If any prior load is waiting on L3 or memory, > > lfence will cause 100+ ns stall and overall kernel performance will tank. > > You are conflating dma_rmb() with the limited cases where > nospec_array_ptr() is used. I need help determining what the > performance impact of those limited places are. really? fdtable, access_ok, net/ipv[46] is not critical path? > > If kernel adopts this "lfence everywhere" approach it will be > > the end of the kernel as we know it. All high performance operations > > will move into user space. Networking and IO will be first. > > Since it will takes years to design new cpus and even longer > > to upgrade all servers the industry will have no choice, > > but to move as much logic as possible from the kernel. > > > > kpti already made crossing user/kernel boundary slower, but > > kernel itself is still fast. If kernel will have lfence everywhere > > the kernel itself will be slow. > > > > In that sense retpolining the kernel is not as horrible as it sounds, > > since both user space and kernel has to be retpolined. > > retpoline is variant-2, this patch series is about variant-1. that's exactly the point. Don't slow down the kernel with lfences to solve variant 1. retpoline for 2 is ok from long term kernel viability perspective.
