On 27/04/2020 18:37, Andy Lutomirski wrote: > 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. More specifically, it is mandatory for hypervisors to intercept #DB to defend against CVE-2015-8104, unless they're willing to trust the guest not to tickle that corner case. This is believed fixed with SEV-SNP to allow the encrypted guest to use debugging functionality without posing a DoS risk to the host. In this case, the hypervisor is expected not to intercept #DB. If #DB is intercepted, and #VC doesn't use IST, malicious userspace can cause problems with a movss-delayed breakpoint over SYSCALL. The question basically whether it is worth going to the effort of making #VC IST and all the problems that entails, to cover one corner case in earlier hardware. Ultimately, this depends on whether anyone plans to put SEV-ES into production on pre SEV-SNP hardware, and if developers using pre-SEV-SNP hardware are happy with "don't run malicious userspace" or "don't run malicious kernels and skip the #DB intercept" as a fair tradeoff to avoid the #VC IST fun. ~Andrew
