Messages in this thread | | | Subject | Re: [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup | From | stsp <> | Date | Fri, 9 Oct 2020 18:48:21 +0300 |
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09.10.2020 18:30, Sean Christopherson пишет: > On Fri, Oct 09, 2020 at 05:11:51PM +0300, stsp wrote: >> 09.10.2020 07:04, Sean Christopherson пишет: >>>> Hmm. But at least it was lying >>>> similarly on AMD and Intel CPUs. :) >>>> So I was able to reproduce the problems >>>> myself. >>>> Do you mean, any AMD tests are now useless, and we need to proceed with Intel >>>> tests only? >>> For anything VMXE related, yes. >> What would be the expected behaviour on Intel, if it is set? Any difference >> with AMD? > On Intel, userspace should be able to stuff CR4.VMXE=1 via KVM_SET_SREGS if > the 'nested' module param is 1, e.g. if 'modprobe kvm_intel nested=1'. Note, > 'nested' is enabled by default on kernel 5.0 and later.
So if I understand you correctly, we need to test that: - with nested=0 VMXE gives EINVAL - with nested=1 VMXE changes nothing visible, except probably to allow guest to read that value (we won't test guest reading though).
Is this correct?
> With AMD, setting CR4.VMXE=1 is never allowed as AMD doesn't support VMX,
OK, for that I can give you a Tested-by: Stas Sergeev <stsp@users.sourceforge.net>
because I confirm that on AMD it now consistently returns EINVAL, whereas without your patches it did random crap, depending on whether it is a first call to KVM_SET_SREGS, or not first.
>> But we do not use unrestricted guest. >> We use v86 under KVM. > Unrestricted guest can kick in even if CR0.PG=1 && CR0.PE=1, e.g. there are > segmentation checks that apply if and only if unrestricted_guest=0. Long story > short, without a deep audit, it's basically impossible to rule out a dependency > on unrestricted guest since you're playing around with v86.
You mean "unrestricted_guest" as a module parameter, rather than the similar named CPU feature, right? So we may depend on unrestricted_guest parameter, but not on a hardware feature, correct?
>> The only other effect of setting VMXE was clearing VME. Which shouldn't >> affect anything either, right? > Hmm, clearing VME would mean that exceptions/interrupts within the guest would > trigger a switch out of v86 and into vanilla protected mode. v86 and PM have > different consistency checks, particularly for segmentation, so it's plausible > that clearing CR4.VME inadvertantly worked around the bug by avoiding invalid > guest state for v86.
Lets assume that was the case. With those github guys its not possible to do any consistent checks. :(
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