Re: [PATCH] KVM: nSVM: check for EFER.SVME=1 before entering guest

[Krish Sadhukhan]

On 3/19/20 3:05 AM, Paolo Bonzini wrote:
> On 18/03/20 19:40, Krish Sadhukhan wrote:
> > On 3/18/20 5:41 AM, Paolo Bonzini wrote:
> > > EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1
> > > and hides
> > > an incorrect value for EFER.SVME in the L1 VMCB. Perform the check
> > > manually
> > > to detect invalid guest state.
> > >
> > > Reported-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>
> > > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > ---
> > >  Â arch/x86/kvm/svm.c | 3 +++
> > >  Â 1 file changed, 3 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > > index 08568ae9f7a1..2125c6ae5951 100644
> > > --- a/arch/x86/kvm/svm.c
> > > +++ b/arch/x86/kvm/svm.c
> > > @@ -3558,6 +3558,9 @@ static bool nested_svm_vmrun_msrpm(struct
> > > vcpu_svm *svm)
> > >  Â Â static bool nested_vmcb_checks(struct vmcb *vmcb)
> > >  Â {
> > > +ÂÂÂ if ((vmcb->save.efer & EFER_SVME) == 0)
> > > +ÂÂÂÂÂÂÂ return false;
> > > +
> > >  ÂÂÂÂÂ if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
> > >  ÂÂÂÂÂÂÂÂÂ return false;
> > >    
> > Ah! This now tells me that I forgot the KVM fix that was supposed to
> > accompany my patchset.
> Heh, indeed.  I was puzzled for a second after applying it, then decided
> I would just fix it myself. :)
>
> > Do we need this check in software ? I wasn't checking the bit in KVM and
> > instead I was just making sure that L0 sets that bit based on the
> > setting in nested vmcb:
Thanks !
> The only effect of the function below over svm_set_efer is to guarantee
> a vmrun error to happen.  Doing the test in nested_vmcb_checks is more
> consistent with other must-be-one bits such as the VMRUN intercept, and
> it's also a smaller patch.

Reviewed-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>

> Paolo
>
> > +static void nested_svm_set_efer(struct kvm_vcpu *vcpu, u64
> > nested_vmcb_efer)
> > +{
> > +ÂÂÂÂÂÂ svm_set_efer(vcpu, nested_vmcb_efer);
> > +
> > +ÂÂÂÂÂÂ if (!(nested_vmcb_efer & EFER_SVME))
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ to_svm(vcpu)->vmcb->save.efer &= ~EFER_SVME;
> > +}
> > +
> >  Âstatic int is_external_interrupt(u32 info)
> >  Â{
> >  ÂÂÂÂÂÂÂ info &= SVM_EVTINJ_TYPE_MASK | SVM_EVTINJ_VALID;
> > @@ -3554,7 +3562,7 @@ static void enter_svm_guest_mode(struct vcpu_svm
> > *svm, u64
> >  ÂÂÂÂÂÂÂ svm->vmcb->save.gdtr = nested_vmcb->save.gdtr;
> >  ÂÂÂÂÂÂÂ svm->vmcb->save.idtr = nested_vmcb->save.idtr;
> >  ÂÂÂÂÂÂÂ kvm_set_rflags(&svm->vcpu, nested_vmcb->save.rflags);
> > -ÂÂÂÂÂÂ svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
> > +ÂÂÂÂÂÂ nested_svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
> >  ÂÂÂÂÂÂÂ svm_set_cr0(&svm->vcpu, nested_vmcb->save.cr0);
> >  ÂÂÂÂÂÂÂ svm_set_cr4(&svm->vcpu, nested_vmcb->save.cr4);
> >  ÂÂÂÂÂÂÂ if (npt_enabled) {