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

1 files changed, 12 insertions, 1 deletions

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;
     }