KVM: VMX: fix memory order between loading vmcs and clearing vmcs

vmcs->cpu indicates whether it exists on the target cpu, -1 means the vmcs
does not exist on any vcpu

If vcpu load vmcs with vmcs.cpu = -1, it can be directly added to cpu's percpu
list. The list can be corrupted if the cpu prefetch the vmcs's list before
reading vmcs->cpu. Meanwhile, we should remove vmcs from the list before
making vmcs->vcpu == -1 be visible

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 4406374..3606154 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1002,6 +1002,15 @@ static void __loaded_vmcs_clear(void *arg)
        if (per_cpu(current_vmcs, cpu) == loaded_vmcs->vmcs)
                per_cpu(current_vmcs, cpu) = NULL;
        list_del(&loaded_vmcs->loaded_vmcss_on_cpu_link);
+
+       /*
+        * we should ensure updating loaded_vmcs->loaded_vmcss_on_cpu_link
+        * is before setting loaded_vmcs->vcpu to -1 which is done in
+        * loaded_vmcs_init. Otherwise, other cpu can see vcpu = -1 fist
+        * then adds the vmcs into percpu list before it is deleted.
+        */
+       smp_wmb();
+
        loaded_vmcs_init(loaded_vmcs);
 }
 
@@ -1537,6 +1546,14 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
                kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
                local_irq_disable();
+
+               /*
+                * Read loaded_vmcs->cpu should be before fetching
+                * loaded_vmcs->loaded_vmcss_on_cpu_link.
+                * See the comments in __loaded_vmcs_clear().
+                */
+               smp_rmb();
+
                list_add(&vmx->loaded_vmcs->loaded_vmcss_on_cpu_link,
                         &per_cpu(loaded_vmcss_on_cpu, cpu));
                local_irq_enable();