From 9f957a9f79c083446787b479474deb6d86cb1bf2 Mon Sep 17 00:00:00 2001
From: matthieu <matthieu@openbsd.org>
Date: Tue, 4 Oct 2016 15:03:48 +0000
Subject: [PATCH] Avoid OOB write in XRenderQueryFilters

The memory for filter names is reserved right after receiving the reply.
After that, filters are iterated and each individual filter name is
stored in that reserved memory.

The individual name lengths are not checked for validity, which means
that a malicious server can reserve less memory than it will write to
during each iteration.

From Tobias Stoeckmann / X.Org security advisory Oct 4, 2016
---
 lib/libXrender/src/Filter.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/lib/libXrender/src/Filter.c b/lib/libXrender/src/Filter.c
index edfa57231..8d701eb98 100644
--- a/lib/libXrender/src/Filter.c
+++ b/lib/libXrender/src/Filter.c
@@ -38,7 +38,7 @@ XRenderQueryFilters (Display *dpy, Drawable drawable)
     char			*name;
     char			len;
     int				i;
-    unsigned long		nbytes, nbytesAlias, nbytesName;
+    unsigned long		nbytes, nbytesAlias, nbytesName, reply_left;
 
     if (!RenderHasExtension (info))
 	return NULL;
@@ -114,6 +114,7 @@ XRenderQueryFilters (Display *dpy, Drawable drawable)
      * Read the filter aliases
      */
     _XRead16Pad (dpy, filters->alias, 2 * rep.numAliases);
+    reply_left = 8 + rep.length - 2 * rep.numAliases;;
 
     /*
      * Read the filter names
@@ -122,9 +123,19 @@ XRenderQueryFilters (Display *dpy, Drawable drawable)
     {
 	int	l;
 	_XRead (dpy, &len, 1);
+	reply_left--;
 	l = len & 0xff;
+	if ((unsigned long)l + 1 > nbytesName) {
+            _XEatDataWords(dpy, reply_left);
+	    Xfree(filters);
+	    UnlockDisplay (dpy);
+	    SyncHandle ();
+	    return NULL;
+	}
+	nbytesName -= l + 1;
 	filters->filter[i] = name;
 	_XRead (dpy, name, l);
+        reply_left -= l;
 	name[l] = '\0';
 	name += l + 1;
     }