KVM: nVMX: avoid NULL pointer dereference with incorrect EVMCS GPAs

When an EVMCS enabled L1 guest on KVM will tries doing enlightened VMEnter
with EVMCS GPA = 0 the host crashes because the

evmcs_gpa != vmx->nested.hv_evmcs_vmptr

condition in nested_vmx_handle_enlightened_vmptrld() will evaluate to
false (as nested.hv_evmcs_vmptr is zeroed after init). The crash will
happen on vmx->nested.hv_evmcs pointer dereference.

Another problematic EVMCS ptr value is '-1' but it only causes host crash
after nested_release_evmcs() invocation. The problem is exactly the same as
with '0', we mistakenly think that the EVMCS pointer hasn't changed and
thus nested.hv_evmcs_vmptr is valid.

Resolve the issue by adding an additional !vmx->nested.hv_evmcs
check to nested_vmx_handle_enlightened_vmptrld(), this way we will
always be trying kvm_vcpu_map() when nested.hv_evmcs is NULL
and this is supposed to catch all invalid EVMCS GPAs.

Also, initialize hv_evmcs_vmptr to '0' in nested_release_evmcs()
to be consistent with initialization where we don't currently
set hv_evmcs_vmptr to '-1'.

Cc: stable@vger.kernel.org
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -224,7 +224,7 @@ static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
 		return;
 
 	kvm_vcpu_unmap(vcpu, &vmx->nested.hv_evmcs_map, true);
-	vmx->nested.hv_evmcs_vmptr = -1ull;
+	vmx->nested.hv_evmcs_vmptr = 0;
 	vmx->nested.hv_evmcs = NULL;
 }
 
@@ -1923,7 +1923,8 @@ static int nested_vmx_handle_enlightened_vmptrld(struct kvm_vcpu *vcpu,
 	if (!nested_enlightened_vmentry(vcpu, &evmcs_gpa))
 		return 1;
 
-	if (unlikely(evmcs_gpa != vmx->nested.hv_evmcs_vmptr)) {
+	if (unlikely(!vmx->nested.hv_evmcs ||
+		     evmcs_gpa != vmx->nested.hv_evmcs_vmptr)) {
 		if (!vmx->nested.hv_evmcs)
 			vmx->nested.current_vmptr = -1ull;