On Wed, 2018-01-10 at 14:46 +0100, Thomas Gleixner wrote: > > So here is the simple list of questions all to be answered with YES or > NO. I don't want to see any of the 'but, though ...'. We all know by now > that it's CPU dependent and slow and whatever and that IBRS_ATT will be in > future CPUs. So get your act together and tell a clear YES or NO.
This is actually covered by the documentation. Someone really should send you a copy. > 1) Does IBRS=1 when set once act as a set-and-forget option ? Never on current hardware. In future with IBRS_ATT, yes. > 1a) If the answer to #1 is yes, is it more secure than toggling it? No, just faster. > 1b) If the answer to #1 is yes, is retpoline required ? No. We'll ALTERNATIVE it away if we have IBRS_ATT. > 1c) If the answer to #1 is yes, is RSB stuffing required ? Not for IBRS_ATT, with weasel words about requiring SMEP. > 2) Does toggle mode of IBRS require retpoline ? No. Retpoline is an *alternative* to IBRS, for protecting the kernel. > 3) Does toggle mode of IBRS require RSB stuffing ? Yes for kernel entry if you have no SMEP. And yes on vmexit. > 4) Exist CPUs which require IBRS to be selected automatically ? > > 4b) If yes, provide the list as a separate answer please You mean CPUs on which retpoline isn't sufficient and thus the kernel should prefer IBRS "automatically" without a command line option? As discussed, yes on Skylake and anything after it that doesn't have IBRS_ATT, because there are tiny theoretical gaps that retpoline doesn't handle. But the option of sacrificing goats may be perfectly acceptable.
smime.p7s
Description: S/MIME cryptographic signature