nested VMX: VMLANUCH/VMRESUME emulation must check permission first thing

Otherwise uninitialized data may be used, leading to crashes.

This is CVE-2013-4551 / XSA-75.

Reported-and-tested-by: Jeff Zimmerman <Jeff_Zimmerman@McAfee.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-and-tested-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/hvm/vmx/vvmx.c
+++ b/xen/arch/x86/hvm/vmx/vvmx.c
@@ -1509,15 +1509,10 @@ static void clear_vvmcs_launched(struct 
     }
 }
 
-int nvmx_vmresume(struct vcpu *v, struct cpu_user_regs *regs)
+static int nvmx_vmresume(struct vcpu *v, struct cpu_user_regs *regs)
 {
     struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
     struct nestedvcpu *nvcpu = &vcpu_nestedhvm(v);
-    int rc;
-
-    rc = vmx_inst_check_privilege(regs, 0);
-    if ( rc != X86EMUL_OKAY )
-        return rc;
 
     /* check VMCS is valid and IO BITMAP is set */
     if ( (nvcpu->nv_vvmcxaddr != VMCX_EADDR) &&
@@ -1536,6 +1531,10 @@ int nvmx_handle_vmresume(struct cpu_user
     struct vcpu *v = current;
     struct nestedvcpu *nvcpu = &vcpu_nestedhvm(v);
     struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
+    int rc = vmx_inst_check_privilege(regs, 0);
+
+    if ( rc != X86EMUL_OKAY )
+        return rc;
 
     if ( vcpu_nestedhvm(v).nv_vvmcxaddr == VMCX_EADDR )
     {
@@ -1555,10 +1554,13 @@ int nvmx_handle_vmresume(struct cpu_user
 int nvmx_handle_vmlaunch(struct cpu_user_regs *regs)
 {
     bool_t launched;
-    int rc;
     struct vcpu *v = current;
     struct nestedvcpu *nvcpu = &vcpu_nestedhvm(v);
     struct nestedvmx *nvmx = &vcpu_2_nvmx(v);
+    int rc = vmx_inst_check_privilege(regs, 0);
+
+    if ( rc != X86EMUL_OKAY )
+        return rc;
 
     if ( vcpu_nestedhvm(v).nv_vvmcxaddr == VMCX_EADDR )
     {
