From: Jan Beulich <jbeulich@suse.com>
Subject: x86/HVM: add missing NULL check before using VMFUNC hook

This is XSA-203.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1694,6 +1694,8 @@ 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, HVM_DELIVER_NO_ERROR_CODE,