Fix XChangeFeedbackControl() request underflow

CVE-2021-3472 / ZDI-CAN-1259

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

Signed-off-by: Matthieu Herrb <[email protected]>

Author: mherrb

Xi/chgfctl.c

@@ -464,8 +464,11 @@ ProcXChangeFeedbackControl(ClientPtr client)
         break;
     case StringFeedbackClass:
     {
-        xStringFeedbackCtl *f = ((xStringFeedbackCtl *) &stuff[1]);
+        xStringFeedbackCtl *f;
 
+        REQUEST_AT_LEAST_EXTRA_SIZE(xChangeFeedbackControlReq,
+                                    sizeof(xStringFeedbackCtl));
+        f = ((xStringFeedbackCtl *) &stuff[1]);
         if (client->swapped) {
             if (len < bytes_to_int32(sizeof(xStringFeedbackCtl)))
                 return BadLength;