On Sat, Apr 25, 2020 at 3:10 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > On Sat, Apr 25, 2020 at 1:23 PM Joerg Roedel <joro@xxxxxxxxxx> wrote: > > > > On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski wrote: > > > I assume the race you mean is: > > > > > > #VC > > > Immediate NMI before IST gets shifted > > > #VC > > > > > > Kaboom. > > > > > > How are you dealing with this? Ultimately, I think that NMI will need > > > to turn off IST before engaging in any funny business. Let me ponder > > > this a bit. > > > > Right, I dealt with that by unconditionally shifting/unshifting the #VC IST entry > > in do_nmi() (thanks to Davin Kaplan for the idea). It might cause > > one of the IST stacks to be unused during nesting, but that is fine. The > > stack memory for #VC is only allocated when SEV-ES is active (in an > > SEV-ES VM). > > Blech. It probably works, but still, yuck. It's a bit sad that we > seem to be growing more and more poorly designed happens-anywhere > exception types at an alarming rate. We seem to have #NM, #MC, #VC, > #HV, and #DB. This doesn't really scale. I have a somewhat serious question: should we use IST for #VC at all? As I understand it, Rome and Naples make it mandatory for hypervisors to intercept #DB, which means that, due to the MOV SS mess, it's sort of mandatory to use IST for #VC. But Milan fixes the #DB issue, so, if we're running under a sufficiently sensible hypervisor, we don't need IST for #VC. So I think we have two choices: 1. Use IST for #VC and deal with all the mess that entails. 2. Say that we SEV-ES client support on Rome and Naples is for development only and do a quick boot-time check for whether #DB is intercepted. (Just set TF and see what vector we get.) If #DB is intercepted, print a very loud warning and refuse to boot unless some special sev_es.insecure_development_mode or similar option is set. #2 results in simpler and more robust entry code. #1 is more secure. So my question is: will anyone actually use SEV-ES in production on Rome or Naples? As I understand it, it's not really ready for prime time on those chips. And do we care if the combination of a malicious hypervisor and malicious guest userspace on Milan can compromise the guest kernel? I don't think SEV-ES is really mean to resist a concerted effort by the hypervisor to compromise the guest. --Andy
