x86emul: fully ignore segment override for register-only operations

For ModRM encoded instructions with register operands we must not
overwrite ea.mem.seg (if a - bogus in that case - segment override was
present) as it aliases with ea.reg.

This is CVE-2015-2151 / XSA-123.

Reported-by: Felix Wilhelm <fwilhelm@ernw.de>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Reviewed-by: Keir Fraser <keir@xen.org>
master commit: bcf92a5382
master date: 2015-03-10 13:45:51 +0100

xen/arch/x86/x86_emulate/x86_emulate.c

@@ -1641,7 +1641,7 @@ x86_emulate(
         }
     }
 
-    if ( override_seg != -1 )
+    if ( override_seg != -1 && ea.type == OP_MEM )
         ea.mem.seg = override_seg;
 
     /* Early operand adjustments. */