Fix bug #77950 - Heap-buffer-overflow in _estrndup via exif_process_IFD_TAG

I do not completely understand what is going on there, but I am pretty
sure dir_entry <= offset_base if not a normal situation, so we better not
to rely on such dir_entry.

NEWS

@@ -2,13 +2,20 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? 2019, PHP 7.1.29
 
+- EXIF
+. Fixed bug #77950 (Heap-buffer-overflow in _estrndup via exif_process_IFD_TAG). 
+  (CVE-2019-11036) (Stas)
 
+- Mail
+. Fixed bug #77821 (Potential heap corruption in TSendMail()). (cmb)
 
 04 Apr 2019, PHP 7.1.28
 
 - EXIF:
-  . Fixed bug #77753 (Heap-buffer-overflow in php_ifd_get32s). (Stas)
-  . Fixed bug #77831 (Heap-buffer-overflow in exif_iif_add_value). (Stas)
+  . Fixed bug #77753 (Heap-buffer-overflow in php_ifd_get32s). (CVE-2019-11034)
+    (Stas)
+  . Fixed bug #77831 (Heap-buffer-overflow in exif_iif_add_value). 
+	(CVE-2019-11035) (Stas)
 
 - SQLite3:
   . Added sqlite3.defensive INI directive. (BohwaZ)

ext/exif/exif.c

@@ -2891,7 +2891,7 @@ static int exif_process_IFD_TAG(image_info_type *ImageInfo, char *dir_entry, cha
             offset_base is ImageInfo->file.list[sn].data-dir_offset
             dir_entry - offset_base is dir_offset+2+i*12
         */
-		if (byte_count > IFDlength || offset_val > IFDlength-byte_count || value_ptr < dir_entry || offset_val < (size_t)(dir_entry-offset_base)) {
+		if (byte_count > IFDlength || offset_val > IFDlength-byte_count || value_ptr < dir_entry || offset_val < (size_t)(dir_entry-offset_base) || dir_entry <= offset_base) {
 			/* It is important to check for IMAGE_FILETYPE_TIFF
 			 * JPEG does not use absolute pointers instead its pointers are
 			 * relative to the start of the TIFF header in APP1 section. */

ext/exif/tests/bug77950.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Bug #77950 (Heap-buffer-overflow in _estrndup via exif_process_IFD_TAG)
+--SKIPIF--
+<?php if (!extension_loaded('exif')) print 'skip exif extension not available';?>
+--FILE--
+<?php
+exif_read_data(__DIR__."/bug77950.tiff");
+?>
+DONE
+--EXPECTF--
+%A
+DONE