x86/HVM: add missing NULL check before using VMFUNC hook

This is CVE-2016-10025 / XSA-203.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 74dcd0ce6f
master date: 2016-12-21 16:47:19 +0100

xen/arch/x86/hvm/emulate.c

@@ -1620,6 +1620,8 @@ static int hvmemul_vmfunc(
 {
     int rc;
 
+    if ( !hvm_funcs.altp2m_vcpu_emulate_vmfunc )
+        return X86EMUL_UNHANDLEABLE;
     rc = hvm_funcs.altp2m_vcpu_emulate_vmfunc(ctxt->regs);
     if ( rc != X86EMUL_OKAY )
         hvmemul_inject_hw_exception(TRAP_invalid_op, 0, ctxt);