KVM: nVMX: do not use dangling shadow VMCS after guest reset

If a KVM guest is reset while running a nested guest, free_nested will
disable the shadow VMCS execution control in the vmcs01. However,
on the next KVM_RUN vmx_vcpu_run would nevertheless try to sync
the VMCS12 to the shadow VMCS which has since been freed.

This causes a vmptrld of a NULL pointer on my machime, but Jan reports
the host to hang altogether. Let's see how much this trivial patch fixes.

Reported-by: Jan Kiszka <[e-mail]>
Cc: Liran Alon <[e-mail]>
Cc: [e-mail]
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/vmx/nested.c

···
 {
 	secondary_exec_controls_clearbit(vmx, SECONDARY_EXEC_SHADOW_VMCS);
 	vmcs_write64(VMCS_LINK_POINTER, -1ull);
+	vmx->nested.need_vmcs12_to_shadow_sync = false;
 }
 
 static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
···
 	unsigned long val;
 	int i;
 
+	if (WARN_ON(!shadow_vmcs))
+		return;
+
 	preempt_disable();
 
 	vmcs_load(shadow_vmcs);
···
 	struct shadow_vmcs_field field;
 	unsigned long val;
 	int i, q;
+
+	if (WARN_ON(!shadow_vmcs))
+		return;
 
 	vmcs_load(shadow_vmcs);
 
···
 		/* copy to memory all shadowed fields in case
 		   they were modified */
 		copy_shadow_to_vmcs12(vmx);
-		vmx->nested.need_vmcs12_to_shadow_sync = false;
 		vmx_disable_shadow_vmcs(vmx);
 	}
 	vmx->nested.posted_intr_nv = -1;