Detect loops in /Pages structure

Pushing inherited objects to pages and getting all pages were both
prone to stack overflow infinite loops if there were loops in the
Pages dictionary. There is a general weakness in the code in that any
part of the code that traverses the Pages structure would be prone to
this and would have to implement its own loop detection. A more robust
fix may provide some general method for handling the Pages structure,
but it's probably not worth doing.

Note: addition of *Internal2 private functions was done rather than
changing signatures of existing methods to avoid breaking
compatibility.

1 files changed, 27 insertions, 2 deletions

diff --git a/libqpdf/QPDF_optimization.cc b/libqpdf/QPDF_optimization.cc
index 5299d90f..fad710d0 100644
--- a/libqpdf/QPDF_optimization.cc
+++ b/libqpdf/QPDF_optimization.cc
@@ -174,6 +174,30 @@ QPDF::pushInheritedAttributesToPageInternal(
     std::vector<QPDFObjectHandle>& pages,
     bool allow_changes, bool warn_skipped_keys)
 {
+    std::set<QPDFObjGen> visited;
+    pushInheritedAttributesToPageInternal2(
+        cur_pages, key_ancestors, pages, allow_changes,
+        warn_skipped_keys, visited);
+}
+
+void
+QPDF::pushInheritedAttributesToPageInternal2(
+    QPDFObjectHandle cur_pages,
+    std::map<std::string, std::vector<QPDFObjectHandle> >& key_ancestors,
+    std::vector<QPDFObjectHandle>& pages,
+    bool allow_changes, bool warn_skipped_keys,
+    std::set<QPDFObjGen>& visited)
+{
+    QPDFObjGen this_og = cur_pages.getObjGen();
+    if (visited.count(this_og) > 0)
+    {
+        throw QPDFExc(
+            qpdf_e_pages, this->file->getName(),
+            this->last_object_description, 0,
+            "Loop detected in /Pages structure (inherited attributes)");
+    }
+    visited.insert(this_og);
+
     // Extract the underlying dictionary object
     std::string type = cur_pages.getKey("/Type").getName();
 
@@ -259,9 +283,9 @@ QPDF::pushInheritedAttributesToPageInternal(
 	int n = kids.getArrayNItems();
 	for (int i = 0; i < n; ++i)
 	{
-            pushInheritedAttributesToPageInternal(
+            pushInheritedAttributesToPageInternal2(
                 kids.getArrayItem(i), key_ancestors, pages,
-                allow_changes, warn_skipped_keys);
+                allow_changes, warn_skipped_keys, visited);
 	}
 
 	// For each inheritable key, pop the stack.  If the stack
@@ -318,6 +342,7 @@ QPDF::pushInheritedAttributesToPageInternal(
 		      this->file->getLastOffset(),
 		      "invalid Type " + type + " in page tree");
     }
+    visited.erase(this_og);
 }
 
 void