On 10/01/2018 11:04, Peter Zijlstra wrote: > On Tue, Jan 09, 2018 at 06:26:47PM -0800, Tim Chen wrote: >> Set IBRS upon kernel entrance via syscall and interrupts. Clear it >> upon exit. IBRS protects against unsafe indirect branching predictions >> in the kernel. >> >> The NMI interrupt save/restore of IBRS state was based on Andrea >> Arcangeli's implementation. >> Here's an explanation by Dave Hansen on why we save IBRS state for NMI. >> >> The normal interrupt code uses the 'error_entry' path which uses the >> Code Segment (CS) of the instruction that was interrupted to tell >> whether it interrupted the kernel or userspace and thus has to switch >> IBRS, or leave it alone. >> >> The NMI code is different. It uses 'paranoid_entry' because it can >> interrupt the kernel while it is running with a userspace IBRS (and %GS >> and CR3) value, but has a kernel CS. If we used the same approach as >> the normal interrupt code, we might do the following; >> >> SYSENTER_entry >> <-------------- NMI HERE >> IBRS=1 >> do_something() >> IBRS=0 >> SYSRET >> >> The NMI code might notice that we are running in the kernel and decide >> that it is OK to skip the IBRS=1. This would leave it running >> unprotected with IBRS=0, which is bad. >> >> However, if we unconditionally set IBRS=1, in the NMI, we might get the >> following case: >> >> SYSENTER_entry >> IBRS=1 >> do_something() >> IBRS=0 >> <-------------- NMI HERE (set IBRS=1) >> SYSRET >> >> and we would return to userspace with IBRS=1. Userspace would run >> slowly until we entered and exited the kernel again. >> >> Instead of those two approaches, we chose a third one where we simply >> save the IBRS value in a scratch register (%r13) and then restore that >> value, verbatim. >> > > What this Changelog fails to address is _WHY_ we need this. What does > this provide that retpoline does not.
Which, for the record, is just that it works better on Skylake+ CPUs. Paolo
