From 11beef0b7f1ed290348e45618e5fa0d2bffcb72e Mon Sep 17 00:00:00 2001
From: Peter Hutterer <peter.hutterer@who-t.net>
Date: Tue, 5 Jul 2022 12:06:20 +1000
Subject: [PATCH xserver] xkb: proof GetCountedString against request length
 attacks

GetCountedString did a check for the whole string to be within the
request buffer but not for the initial 2 bytes that contain the length
field. A swapped client could send a malformed request to trigger a
swaps() on those bytes, writing into random memory.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
---
 xkb/xkb.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xkb/xkb.c b/xkb/xkb.c
index f42f59ef3..1841cff26 100644
--- a/xkb/xkb.c
+++ b/xkb/xkb.c
@@ -5137,6 +5137,11 @@ _GetCountedString(char **wire_inout, ClientPtr client, char **str)
     CARD16 len;
 
     wire = *wire_inout;
+
+    if (client->req_len <
+        bytes_to_int32(wire + 2 - (char *) client->requestBuffer))
+        return BadValue;
+
     len = *(CARD16 *) wire;
     if (client->swapped) {
         swaps(&len);
-- 
2.38.1