Improve deep recursion detection in exif_data_load_data_content.

The existing detection was still vulnerable to pathological cases
causing DoS by wasting CPU. The new algorithm takes the number of tags
into account to make it harder to abuse by cases using shallow recursion
but with a very large number of tags.  This improves on commit 5d28011
which wasn't sufficient to counter this kind of case.

The limitation in the previous fix was discovered by Laurent Delosieres,
Secunia Research at Flexera (Secunia Advisory SA84652) and is assigned
the identifier CVE-2018-20030.

NEWS

@@ -3,6 +3,7 @@ libexif-0.6.x:
   * Updated translations for most languages
   * Fixed C89 compatibility
   * Fixed warnings on recent versions of autoconf
+  * Fix for recursion DoS CVE-2018-20030
 
 libexif-0.6.21 (2012-07-12):
   * New translations: en_AU, uk

libexif/exif-data.c

@@ -35,6 +35,7 @@
 #include <libexif/olympus/exif-mnote-data-olympus.h>
 #include <libexif/pentax/exif-mnote-data-pentax.h>
 
+#include <math.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -350,20 +351,34 @@ if (data->ifd[(i)]->count) {				\
 	break;						\
 }
 
+/*! Calculate the recursion cost added by one level of IFD loading.
+ *
+ * The work performed is related to the cost in the exponential relation
+ *   work=1.1**cost
+ */
+static unsigned int
+level_cost(unsigned int n)
+{
+    static const double log_1_1 = 0.09531017980432493;
+
+	/* Adding 0.1 protects against the case where n==1 */
+	return ceil(log(n + 0.1)/log_1_1);
+}
+
 /*! Load data for an IFD.
  *
  * \param[in,out] data #ExifData
  * \param[in] ifd IFD to load
  * \param[in] d pointer to buffer containing raw IFD data
  * \param[in] ds size of raw data in buffer at \c d
  * \param[in] offset offset into buffer at \c d at which IFD starts
- * \param[in] recursion_depth number of times this function has been
- * recursively called without returning
+ * \param[in] recursion_cost factor indicating how expensive this recursive
+ * call could be
  */
 static void
 exif_data_load_data_content (ExifData *data, ExifIfd ifd,
 			     const unsigned char *d,
-			     unsigned int ds, unsigned int offset, unsigned int recursion_depth)
+			     unsigned int ds, unsigned int offset, unsigned int recursion_cost)
 {
 	ExifLong o, thumbnail_offset = 0, thumbnail_length = 0;
 	ExifShort n;
@@ -378,9 +393,20 @@ exif_data_load_data_content (ExifData *data, ExifIfd ifd,
 	if ((((int)ifd) < 0) || ( ((int)ifd) >= EXIF_IFD_COUNT))
 	  return;
 
-	if (recursion_depth > 12) {
+	if (recursion_cost > 170) {
+		/*
+		 * recursion_cost is a logarithmic-scale indicator of how expensive this
+		 * recursive call might end up being. It is an indicator of the depth of
+		 * recursion as well as the potential for worst-case future recursive
+		 * calls. Since it's difficult to tell ahead of time how often recursion
+		 * will occur, this assumes the worst by assuming every tag could end up
+		 * causing recursion.
+		 * The value of 170 was chosen to limit typical EXIF structures to a
+		 * recursive depth of about 6, but pathological ones (those with very
+		 * many tags) to only 2.
+		 */
 		exif_log (data->priv->log, EXIF_LOG_CODE_CORRUPT_DATA, "ExifData",
-			  "Deep recursion detected!");
+			  "Deep/expensive recursion detected!");
 		return;
 	}
 
@@ -422,15 +448,18 @@ exif_data_load_data_content (ExifData *data, ExifIfd ifd,
 			switch (tag) {
 			case EXIF_TAG_EXIF_IFD_POINTER:
 				CHECK_REC (EXIF_IFD_EXIF);
-				exif_data_load_data_content (data, EXIF_IFD_EXIF, d, ds, o, recursion_depth + 1);
+				exif_data_load_data_content (data, EXIF_IFD_EXIF, d, ds, o,
+					recursion_cost + level_cost(n));
 				break;
 			case EXIF_TAG_GPS_INFO_IFD_POINTER:
 				CHECK_REC (EXIF_IFD_GPS);
-				exif_data_load_data_content (data, EXIF_IFD_GPS, d, ds, o, recursion_depth + 1);
+				exif_data_load_data_content (data, EXIF_IFD_GPS, d, ds, o,
+					recursion_cost + level_cost(n));
 				break;
 			case EXIF_TAG_INTEROPERABILITY_IFD_POINTER:
 				CHECK_REC (EXIF_IFD_INTEROPERABILITY);
-				exif_data_load_data_content (data, EXIF_IFD_INTEROPERABILITY, d, ds, o, recursion_depth + 1);
+				exif_data_load_data_content (data, EXIF_IFD_INTEROPERABILITY, d, ds, o,
+					recursion_cost + level_cost(n));
 				break;
 			case EXIF_TAG_JPEG_INTERCHANGE_FORMAT:
 				thumbnail_offset = o;