Messages in this thread | | | Subject | Re: [RFC PATCH 08/35] KVM: SVM: Prevent debugging under SEV-ES | From | Tom Lendacky <> | Date | Thu, 17 Sep 2020 11:27:25 -0500 |
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On 9/16/20 5:50 PM, Sean Christopherson wrote: > On Wed, Sep 16, 2020 at 03:27:13PM -0500, Tom Lendacky wrote: >> On 9/16/20 11:49 AM, Sean Christopherson wrote: >>> On Wed, Sep 16, 2020 at 11:38:38AM -0500, Tom Lendacky wrote: >>>> >>>> >>>> On 9/16/20 11:02 AM, Sean Christopherson wrote: >>>>> On Wed, Sep 16, 2020 at 10:11:10AM -0500, Tom Lendacky wrote: >>>>>> On 9/15/20 3:13 PM, Tom Lendacky wrote: >>>>>>> On 9/15/20 11:30 AM, Sean Christopherson wrote: >>>>>>>> I don't quite follow the "doesn't mean debugging can't be done in the future". >>>>>>>> Does that imply that debugging could be supported for SEV-ES guests, even if >>>>>>>> they have an encrypted VMSA? >>>>>>> >>>>>>> Almost anything can be done with software. It would require a lot of >>>>>>> hypervisor and guest code and changes to the GHCB spec, etc. So given >>>>>>> that, probably just the check for arch.guest_state_protected is enough for >>>>>>> now. I'll just need to be sure none of the debugging paths can be taken >>>>>>> before the VMSA is encrypted. >>>>>> >>>>>> So I don't think there's any guarantee that the KVM_SET_GUEST_DEBUG ioctl >>>>>> couldn't be called before the VMSA is encrypted, meaning I can't check the >>>>>> arch.guest_state_protected bit for that call. So if we really want to get >>>>>> rid of the allow_debug() op, I'd need some other way to indicate that this >>>>>> is an SEV-ES / protected state guest. >>>>> >>>>> Would anything break if KVM "speculatively" set guest_state_protected before >>>>> LAUNCH_UPDATE_VMSA? E.g. does KVM need to emulate before LAUNCH_UPDATE_VMSA? >>>> >>>> Yes, the way the code is set up, the guest state (VMSA) is initialized in >>>> the same way it is today (mostly) and that state is encrypted by the >>>> LAUNCH_UPDATE_VMSA call. I check the guest_state_protected bit to decide >>>> on whether to direct the updates to the real VMSA (before it's encrypted) >>>> or the GHCB (that's the get_vmsa() function from patch #5). >>> >>> Ah, gotcha. Would it work to set guest_state_protected[*] from time zero, >>> and move vmsa_encrypted to struct vcpu_svm? I.e. keep vmsa_encrypted, but >>> use it only for guiding get_vmsa() and related behavior. >> >> It is mainly __set_sregs() that needs to know when to allow the register >> writes and when not to. During guest initialization, __set_sregs is how >> some of the VMSA is initialized by Qemu. > > Hmm. I assume that also means KVM_SET_REGS and KVM_GET_XCRS are also legal > before the VMSA is encrypted? If so, then the current behavior of setting > vmsa_encrypted "late" make sense. KVM_SET_FPU/XSAVE can be handled by not > allocating guest_fpu, i.e. they can be disallowed from time zero without > adding an SEV-ES specific check. > > Which brings us back to KVM_SET_GUEST_DEBUG. What would happen if that were > allowed prior to VMSA encryption? If LAUNCH_UPDATE_VMSA acts as a sort of > reset, one thought would be to allow KVM_SET_GUEST_DEBUG and then sanitize > KVM's state during LAUNCH_UPDATE_VMSA. Or perhaps even better, disallow > LAUNCH_UPDATE_VMSA if vcpu->guest_debug!=0. That would allow using debug > capabilities up until LAUNCH_UPDATE_VMSA without adding much burden to KVM.
I think the vcpu->guest_debug check before the LAUNCH_UPDATE_VMSA would be good. I'll remove the allow_debug() op and replace it with the guest_state_protected check in its place.
Thanks, Tom
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