From 1e4bf85df1be285e70a9c9fd52e6cf887600d4e4 Mon Sep 17 00:00:00 2001
From: Matthieu Herrb <matthieu@herrb.eu>
Date: Tue, 13 Apr 2021 15:55:41 +0200
Subject: [PATCH] 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 <matthieu@herrb.eu>
(cherry picked from commit 7aaf54a1884f71dc363f0b884e57bcb67407a6cd)
---
 Xi/chgfctl.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/Xi/chgfctl.c b/Xi/chgfctl.c
index 1de4da9ef..7a597e43d 100644
--- a/Xi/chgfctl.c
+++ b/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;