Handle object ID 0 (fixes #99)

This is CVE-2017-9208.

The QPDF library uses object ID 0 internally as a sentinel to
represent a direct object, but prior to this fix, was not blocking
handling of 0 0 obj or 0 0 R as a special case. Creating an object in
the file with 0 0 obj could cause various infinite loops. The PDF spec
doesn't allow for object 0. Having qpdf handle object 0 might be a
better fix, but changing all the places in the code that assumes objid
== 0 means direct would be risky.

ChangeLog

@@ -1,5 +1,9 @@
 2017-07-26  Jay Berkenbilt  <ejb@ql.org>
 
+	* CVE-2017-9208: Handle references to and appearance of object 0
+	as a special case. Object 0 is not allowed, and qpdf was using it
+	internally to represent direct objects.
+
 	* CVE-2017-9209: Fix infinite loop caused by attempting to
 	reconstruct the xref table while already in the process of
 	reconstructing the xref table.

libqpdf/QPDF.cc

@@ -1350,6 +1350,14 @@ QPDF::readObjectAtOffset(bool try_recovery,
 	objid = atoi(tobjid.getValue().c_str());
 	generation = atoi(tgen.getValue().c_str());
 
+        if (objid == 0)
+        {
+            QTC::TC("qpdf", "QPDF object id 0");
+            throw QPDFExc(qpdf_e_damaged_pdf, this->file->getName(),
+                          this->last_object_description, offset,
+                          "object with ID 0");
+        }
+
 	if ((exp_objid >= 0) &&
 	    (! ((objid == exp_objid) && (generation == exp_generation))))
 	{

libqpdf/QPDFObjectHandle.cc

@@ -1089,6 +1089,16 @@ QPDFObjectHandle::parseInternal(PointerHolder<InputSource> input,
 QPDFObjectHandle
 QPDFObjectHandle::newIndirect(QPDF* qpdf, int objid, int generation)
 {
+    if (objid == 0)
+    {
+        // Special case: QPDF uses objid 0 as a sentinel for direct
+        // objects, and the PDF specification doesn't allow for object
+        // 0. Treat indirect references to object 0 as null so that we
+        // never create an indirect object with objid 0.
+        QTC::TC("qpdf", "QPDFObjectHandle indirect with 0 objid");
+        return newNull();
+    }
+
     return QPDFObjectHandle(qpdf, objid, generation);
 }
 

qpdf/qpdf.testcov

@@ -273,3 +273,6 @@ QPDFWriter standard deterministic ID 1
 QPDFWriter linearized deterministic ID 1
 QPDFWriter deterministic with no data 0
 qpdf-c called qpdf_set_deterministic_ID 0
+QPDFObjectHandle indirect with 0 objid 0
+QPDF object id 0 0
+QPDF caught recursive xref reconstruction 0

qpdf/qtest/qpdf.test

@@ -206,7 +206,7 @@ $td->runtest("remove page we don't have",
 show_ntests();
 # ----------
 $td->notify("--- Miscellaneous Tests ---");
-$n_tests += 79;
+$n_tests += 81;
 
 $td->runtest("qpdf version",
 	     {$td->COMMAND => "qpdf --version"},
@@ -220,6 +220,8 @@ $td->runtest("C API: qpdf version",
 
 # Files to reproduce various bugs
 foreach my $d (
+    ["99", "object 0"],
+    ["99b", "object 0"],
     ["100","xref reconstruction loop"],
     ["101", "resolve for exception text"],
     )

qpdf/qtest/qpdf/issue-99.out

@@ -0,0 +1,4 @@
+WARNING: issue-99.pdf: file is damaged
+WARNING: issue-99.pdf (file position 3526): xref not found
+WARNING: issue-99.pdf: Attempting to reconstruct cross-reference table
+operation for Dictionary object attempted on object of wrong type

qpdf/qtest/qpdf/issue-99b.out

@@ -0,0 +1,5 @@
+WARNING: issue-99b.pdf: file is damaged
+WARNING: issue-99b.pdf (object 1 0, file position 9): object with ID 0
+WARNING: issue-99b.pdf: Attempting to reconstruct cross-reference table
+WARNING: issue-99b.pdf: object 1 0 not found in file after regenerating cross reference table
+operation for Dictionary object attempted on object of wrong type

qpdf/qtest/qpdf/issue-99b.pdf

@@ -0,0 +1,79 @@
+%PDF-1.3
+0 0 obj
+<<
+  /Type /Catalog
+  /Pages 2 0 R
+>>
+endobj
+
+2 0 obj
+<<
+  /Type /Pages
+  /Kids [
+    3 0 R
+  ]
+  /Count 1
+>>
+endobj
+
+3 0 obj
+<<
+  /Type /Page
+  /Parent 2 0 R
+  /MediaBox [0 0 612 792]
+  /Contents 4 0 R
+  /Resources <<
+    /ProcSet 5 0 R
+    /Font <<
+      /F1 6 0 R
+    >>
+  >>
+>>
+endobj
+
+4 0 obj
+<<
+  /Length 44
+>>
+stream
+BT
+  /F1 24 Tf
+  72 720 Td
+  (Potato) Tj
+ET
+endstream
+endobj
+
+5 0 obj
+[
+  /PDF
+  /Text
+]
+endobj
+
+6 0 obj
+<<
+  /Type /Font
+  /Subtype /Type1
+  /Name /F1
+  /BaseFont /Helvetica
+  /Encoding /WinAnsiEncoding
+>>
+endobj
+
+xref
+0 7
+0000000000 65535 f 
+0000000009 00000 n 
+0000000063 00000 n 
+0000000135 00000 n 
+0000000307 00000 n 
+0000000403 00000 n 
+0000000438 00000 n 
+trailer <<
+  /Size 7
+  /Root 1 0 R
+>>
+startxref
+556
+%%EOF