integer overflow in XineramaQueryScreens() [CVE-2013-1985]

If the reported number of screens is too large, the calculations to
allocate memory for them may overflow, leaving us writing beyond the
bounds of the allocation.

Reported-by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

1 files changed, 28 insertions, 16 deletions

diff --git a/src/Xinerama.c b/src/Xinerama.c
index 04189b6..67a35b5 100644
--- a/src/Xinerama.c
+++ b/src/Xinerama.c
@@ -303,24 +303,36 @@ XineramaQueryScreens(
 	return NULL;
     }
 
-    if(rep.number) {
-	if((scrnInfo = Xmalloc(sizeof(XineramaScreenInfo) * rep.number))) {
+    /*
+     * rep.number is a CARD32 so could be as large as 2^32
+     * The X11 protocol limits the total screen size to 64k x 64k,
+     * and no screen can be smaller than a pixel.  While technically
+     * that means we could theoretically reach 2^32 screens, and that's
+     * not even taking overlap into account, Xorg is currently limited
+     * to 16 screens, and few known servers have a much higher limit,
+     * so 1024 seems more than enough to prevent both integer overflow
+     * and insane X server responses causing massive memory allocation.
+     */
+    if ((rep.number > 0) && (rep.number <= 1024))
+	scrnInfo = Xmalloc(sizeof(XineramaScreenInfo) * rep.number);
+    if (scrnInfo != NULL) {
+	int i;
+
+	for (i = 0; i < rep.number; i++) {
 	    xXineramaScreenInfo scratch;
-	    int i;
-
-	    for(i = 0; i < rep.number; i++) {
-		_XRead(dpy, (char*)(&scratch), sz_XineramaScreenInfo);
-		scrnInfo[i].screen_number = i;
-		scrnInfo[i].x_org 	  = scratch.x_org;
-		scrnInfo[i].y_org 	  = scratch.y_org;
-		scrnInfo[i].width 	  = scratch.width;
-		scrnInfo[i].height 	  = scratch.height;
-	    }
-
-	    *number = rep.number;
-	} else
-	    _XEatDataWords(dpy, rep.length);
+
+	    _XRead(dpy, (char*)(&scratch), sz_XineramaScreenInfo);
+
+	    scrnInfo[i].screen_number = i;
+	    scrnInfo[i].x_org	= scratch.x_org;
+	    scrnInfo[i].y_org	= scratch.y_org;
+	    scrnInfo[i].width	= scratch.width;
+	    scrnInfo[i].height	= scratch.height;
+	}
+
+	*number = rep.number;
     } else {
+	_XEatDataWords(dpy, rep.length);
 	*number = 0;
     }