Re: [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup

[stsp]

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@xxxxxxxxxxxxxxxxxxxxx>

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. :(