Fix #73869: Signed Integer Overflow gd_io.c

GD2 stores the number of horizontal and vertical chunks as words (i.e. 2
byte unsigned). These values are multiplied and assigned to an int when
reading the image, what can cause integer overflows. We have to avoid
that, and also make sure that either chunk count is actually greater
than zero. If illegal chunk counts are detected, we bail out from
reading the image.

(cherry picked from commit 5b5d9db3988b829e0b121b74bb3947f01c2796a1)

ext/gd/libgd/gd_gd2.c

@@ -136,6 +136,10 @@ static int _gd2GetHeader(gdIOCtxPtr in, int *sx, int *sy, int *cs, int *vers, in
 	GD2_DBG(php_gd_error("%d Chunks vertically", *ncy));
 
 	if (gd2_compressed(*fmt)) {
+		if (*ncx <= 0 || *ncy <= 0 || *ncx > INT_MAX / *ncy) {
+			GD2_DBG(printf ("Illegal chunk counts: %d * %d\n", *ncx, *ncy));
+			goto fail1;
+		}
 		nc = (*ncx) * (*ncy);
 		GD2_DBG(php_gd_error("Reading %d chunk index entries", nc));
 		if (overflow2(sizeof(t_chunk_info), nc)) {

ext/gd/tests/bug73869.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Bug #73869 (Signed Integer Overflow gd_io.c)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+?>
+--FILE--
+<?php
+var_dump(imagecreatefromgd2(__DIR__ . DIRECTORY_SEPARATOR . 'bug73869a.gd2'));
+var_dump(imagecreatefromgd2(__DIR__ . DIRECTORY_SEPARATOR . 'bug73869b.gd2'));
+?>
+===DONE===
+--EXPECTF--
+Warning: imagecreatefromgd2(): '%s' is not a valid GD2 file in %s on line %d
+bool(false)
+
+Warning: imagecreatefromgd2(): '%s' is not a valid GD2 file in %s on line %d
+bool(false)
+===DONE===