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

[Krish Sadhukhan]

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>
> ---
>   arch/x86/kvm/svm.c | 3 +++
>   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)
>   
>   static bool nested_vmcb_checks(struct vmcb *vmcb)
>   {
> +	if ((vmcb->save.efer & EFER_SVME) == 0)
> +		return false;
> +
>   	if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
>   		return false;
>   

Ah! This now tells me that I forgot the KVM fix that was supposed to 
accompany my patchset.

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:


+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) {