render: Fix out of bounds access in SProcRenderCompositeGlyphs()

ZDI-CAN-14192, CVE-2021-4008

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

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>

1 files changed, 9 insertions, 0 deletions

diff --git a/render/render.c b/render/render.c
index c376090ca..456f156d4 100644
--- a/render/render.c
+++ b/render/render.c
@@ -2309,6 +2309,9 @@ SProcRenderCompositeGlyphs(ClientPtr client)
 
         i = elt->len;
         if (i == 0xff) {
+            if (buffer + 4 > end) {
+                return BadLength;
+            }
             swapl((int *) buffer);
             buffer += 4;
         }
@@ -2319,12 +2322,18 @@ SProcRenderCompositeGlyphs(ClientPtr client)
                 buffer += i;
                 break;
             case 2:
+                if (buffer + i * 2 > end) {
+                    return BadLength;
+                }
                 while (i--) {
                     swaps((short *) buffer);
                     buffer += 2;
                 }
                 break;
             case 4:
+                if (buffer + i * 4 > end) {
+                    return BadLength;
+                }
                 while (i--) {
                     swapl((int *) buffer);
                     buffer += 4;