On 22.03.2018 00:30, Dave Hansen wrote: > On 03/20/2018 04:17 AM, Maciej S. Szmigiero wrote: >> Since it is unlikely that existing RSB entries from the previous task match >> the new task call stack we can use the existing unconditional >> RSB-filling-on-context-switch infrastructure to protect against such >> userspace-to-userspace attacks. >> >> This patch brings a change in behavior only for the following CPU types: >> * Intel pre-Skylake CPUs without updated microcode, >> * AMD Family 15h model >60h, Family 17h CPUs without updated microcode. >> >> Other CPU types either already do the RSB filling on context switch for >> other reasons or do support IBPB for more complete userspace-to-userspace >> protection. > > I think I misunderstood your reasoning a bit. Let me see if I can > restate the problem a bit. > > IBPB provides provides userspace-to-userspace protection because it > prevents all indirect branch predictions after the barrier from being > controlled by software executed before the barrier. We only use IBPB > for KVM and when processes clear their dumpable flag. > > You're saying that, even if we don't have IBPB, we can do *some* > userspace-to-userspace protection with RSB manipulation. The RSB > manipulation obviously only helps 'RET' instructions and not JMP/CALL, > but it does do *something* useful. > > Is that right?
Yes. As far as I understand the issue this should provide a good protection for userspace processes that were recompiled with retpolines as they won't have any indirect jumps and calls. > Do you really want this behavior on all context switches? We don't do > IBPB on all context switches, only the ones where we are switching *to* > a non-dumpable process. > > Do you perhaps want to do RSB manipulation in lieu of IBPB when > switching *to* a non-dumpable process and IBPB is not available? > Is it worth differentiating such processes in this case? IBPB is supposed to be very expensive so certainly it is worthwhile to do it only for high-value processes (=non-dumpable). However, it is unlikely that existing RSB entries from the previous task match the new task call stack anyway. We already do unconditional RSB-filling-on-context-switch in many cases. Maciej
