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From a11f47475e6443b7f32d21f2271f28f417e2ac04 Mon Sep 17 00:00:00 2001
From: "Christoph M. Becker" <cmbecker69@gmx.de>
Date: Wed, 29 Nov 2017 19:37:38 +0100
Subject: [PATCH] Fix #420: Potential infinite loop in gdImageCreateFromGifCtx

Due to a signedness confusion in `GetCode_` a corrupt GIF file can
trigger an infinite loop.  Furthermore we make sure that a GIF without
any palette entries is treated as invalid *after* open palette entries
have been removed.

CVE-2018-5711

See also https://bugs.php.net/bug.php?id=75571.
---
 src/gd_gif_in.c             |  12 ++++++------
 tests/gif/.gitignore        |   1 +
 tests/gif/CMakeLists.txt    |   1 +
 tests/gif/Makemodule.am     |   2 ++
 tests/gif/php_bug_75571.c   |  28 ++++++++++++++++++++++++++++
 tests/gif/php_bug_75571.gif | Bin 0 -> 1731 bytes
 6 files changed, 38 insertions(+), 6 deletions(-)
 create mode 100644 tests/gif/php_bug_75571.c
 create mode 100644 tests/gif/php_bug_75571.gif

diff --git a/src/gd_gif_in.c b/src/gd_gif_in.c
index daf26e79..0a8bd717 100644
--- a/src/gd_gif_in.c
+++ b/src/gd_gif_in.c
@@ -335,11 +335,6 @@ BGD_DECLARE(gdImagePtr) gdImageCreateFromGifCtx(gdIOCtxPtr fd)
 		return 0;
 	}
 
-	if(!im->colorsTotal) {
-		gdImageDestroy(im);
-		return 0;
-	}
-
 	/* Check for open colors at the end, so
 	 * we can reduce colorsTotal and ultimately
 	 * BitsPerPixel */
@@ -351,6 +346,11 @@ BGD_DECLARE(gdImagePtr) gdImageCreateFromGifCtx(gdIOCtxPtr fd)
 		}
 	}
 
+	if(!im->colorsTotal) {
+		gdImageDestroy(im);
+		return 0;
+	}
+
 	return im;
 }
 
@@ -447,7 +447,7 @@ static int
 GetCode_(gdIOCtx *fd, CODE_STATIC_DATA *scd, int code_size, int flag, int *ZeroDataBlockP)
 {
 	int i, j, ret;
-	unsigned char count;
+	int count;
 
 	if(flag) {
 		scd->curbit = 0;