Fix CVE-2016-8674 (use-after-free in pdf_to_num()).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-8674
https://security-tracker.debian.org/tracker/CVE-2016-8674

Patch adapted from upstream source repository:
http://git.ghostscript.com/?p=mupdf.git;h=1e03c06456d997435019fb3526fa2d4be7dbc6ec

diff --git a/include/mupdf/pdf/document.h b/include/mupdf/pdf/document.h
index f8ef0cd..e8345b7 100644
--- a/include/mupdf/pdf/document.h
+++ b/include/mupdf/pdf/document.h
@@ -258,6 +258,10 @@ struct pdf_document_s
	fz_font **type3_fonts;

	pdf_resource_tables *resources;
+
+	int orphans_max;
+	int orphans_count;
+	pdf_obj **orphans;
 };
 
 /*
diff --git a/include/mupdf/pdf/object.h b/include/mupdf/pdf/object.h
index 346a2f1..02d4119 100644
--- a/include/mupdf/pdf/object.h
+++ b/include/mupdf/pdf/object.h
@@ -109,6 +109,7 @@ pdf_obj *pdf_dict_gets(fz_context *ctx, pdf_obj *dict, const char *key);
 pdf_obj *pdf_dict_getsa(fz_context *ctx, pdf_obj *dict, const char *key, const char *abbrev);
 void pdf_dict_put(fz_context *ctx, pdf_obj *dict, pdf_obj *key, pdf_obj *val);
 void pdf_dict_put_drop(fz_context *ctx, pdf_obj *dict, pdf_obj *key, pdf_obj *val);
+void pdf_dict_get_put_drop(fz_context *ctx, pdf_obj *dict, pdf_obj *key, pdf_obj *val, pdf_obj **old_val);
 void pdf_dict_puts(fz_context *ctx, pdf_obj *dict, const char *key, pdf_obj *val);
 void pdf_dict_puts_drop(fz_context *ctx, pdf_obj *dict, const char *key, pdf_obj *val);
 void pdf_dict_putp(fz_context *ctx, pdf_obj *dict, const char *path, pdf_obj *val);
diff --git a/source/pdf/pdf-object.c b/source/pdf/pdf-object.c
index f2e4551..a0d0d8e 100644
--- a/source/pdf/pdf-object.c
+++ b/source/pdf/pdf-object.c
@@ -1240,9 +1240,13 @@ pdf_dict_geta(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *abbrev)
 	return pdf_dict_get(ctx, obj, abbrev);
 }
 
-void
-pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
+static void
+pdf_dict_get_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val, pdf_obj **old_val)
 {
+
+	if (old_val)
+		*old_val = NULL;
+
 	RESOLVE(obj);
	if (obj >= PDF_OBJ__LIMIT)
	{
@@ -1282,7 +1286,10 @@ pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
			{
				pdf_obj *d = DICT(obj)->items[i].v;
				DICT(obj)->items[i].v = pdf_keep_obj(ctx, val);
-				pdf_drop_obj(ctx, d);
+				if (old_val)
+					*old_val = d;
+				else
+					pdf_drop_obj(ctx, d);
			}
 		}
		else
@@ -1305,10 +1312,27 @@ pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
 }
 
 void
+pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
+{
+	pdf_dict_get_put(ctx, obj, key, val, NULL);
+}
+
+void
 pdf_dict_put_drop(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
 {
 	fz_try(ctx)
-		pdf_dict_put(ctx, obj, key, val);
+		pdf_dict_get_put(ctx, obj, key, val, NULL);
+	fz_always(ctx)
+		pdf_drop_obj(ctx, val);
+	fz_catch(ctx)
+		fz_rethrow(ctx);
+}
+
+void
+pdf_dict_get_put_drop(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val, pdf_obj **old_val)
+{
+	fz_try(ctx)
+		pdf_dict_get_put(ctx, obj, key, val, old_val);
 	fz_always(ctx)
 		pdf_drop_obj(ctx, val);
 	fz_catch(ctx)
diff --git a/source/pdf/pdf-repair.c b/source/pdf/pdf-repair.c
index fdd4648..212c8b7 100644
--- a/source/pdf/pdf-repair.c
+++ b/source/pdf/pdf-repair.c
@@ -259,6 +259,27 @@ pdf_repair_obj_stm(fz_context *ctx, pdf_document *doc, int num, int gen)
 	}
 }
 
+static void
+orphan_object(fz_context *ctx, pdf_document *doc, pdf_obj *obj)
+{
+	if (doc->orphans_count == doc->orphans_max)
+	{
+		int new_max = (doc->orphans_max ? doc->orphans_max*2 : 32);
+
+		fz_try(ctx)
+		{
+			doc->orphans = fz_resize_array(ctx, doc->orphans, new_max, sizeof(*doc->orphans));
+			doc->orphans_max = new_max;
+		}
+		fz_catch(ctx)
+		{
+			pdf_drop_obj(ctx, obj);
+			fz_rethrow(ctx);
+		}
+	}
+	doc->orphans[doc->orphans_count++] = obj;
+}
+
 void
 pdf_repair_xref(fz_context *ctx, pdf_document *doc)
 {
@@ -520,12 +541,13 @@ pdf_repair_xref(fz_context *ctx, pdf_document *doc)
 			/* correct stream length for unencrypted documents */
 			if (!encrypt && list[i].stm_len >= 0)
 			{
+				pdf_obj *old_obj = NULL;
				dict = pdf_load_object(ctx, doc, list[i].num, list[i].gen);
 
 				length = pdf_new_int(ctx, doc, list[i].stm_len);
-				pdf_dict_put(ctx, dict, PDF_NAME_Length, length);
-				pdf_drop_obj(ctx, length);
-
+				pdf_dict_get_put_drop(ctx, dict, PDF_NAME_Length, length, &old_obj);
+				if (old_obj)
+					orphan_object(ctx, doc, old_obj);
 				pdf_drop_obj(ctx, dict);
 			}
 		}
diff --git a/source/pdf/pdf-xref.c b/source/pdf/pdf-xref.c
index 3de1cd2..6682741 100644
--- a/source/pdf/pdf-xref.c
+++ b/source/pdf/pdf-xref.c
@@ -1626,6 +1626,12 @@ pdf_close_document(fz_context *ctx, pdf_document *doc)
 
	pdf_drop_resource_tables(ctx, doc);
 
+	for (i = 0; i < doc->orphans_count; i++)
+	{
+		pdf_drop_obj(ctx, doc->orphans[i]);
+	}
+	fz_free(ctx, doc->orphans);
+
	fz_free(ctx, doc);
 }

-- 
2.10.1