XERCESC-2188 - Use-after-free on external DTD scan (CVE-2018-1311)

These are the instructions for observing the bug (before this commit):

$ git clone https://github.com/apache/xerces-c.git
$ cd xerces-c
$ mkdir build
$ cd build
$ cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Debug ..
$ make -j8
$ cp ../samples/data/personal.xml .

$ cat <<EOF >personal.dtd
<?xml encoding="ISO-8859-1"?>
<!ENTITY % nonExistentEntity SYSTEM "non-existent.ent">
%nonExistentEntity;
EOF

$ gdb samples/StdInParse
(gdb) b IGXMLScanner.cpp:1544
(gdb) run <personal.xml
1544	            fReaderMgr.pushReader(reader, declDTD);
(gdb) p declDTD
$1 = (xercesc_4_0::DTDEntityDecl *) 0x49ac68
(gdb) n
1547	            dtdScanner.scanExtSubsetDecl(false, true);
(gdb) n
1548	        }
(gdb) s
...
(gdb) s                     # The Janitor is about to delete the above declDTD.
90	        delete fData;
(gdb) p fData
$1 = (xercesc_4_0::DTDEntityDecl *) 0x49ac68
(gdb) b ReaderMgr.cpp:1024
(gdb) n
...
(gdb) n                     # Now we about to dereference the deleted declDTD.
1024	    if (curEntity && !curEntity->isExternal())
(gdb) p curEntity
$2 = (const xercesc_4_0::XMLEntityDecl *) 0x49ac68

src/xercesc/internal/DGXMLScanner.cpp

@@ -1052,13 +1052,12 @@ void DGXMLScanner::scanDocTypeDecl()
             DTDEntityDecl* declDTD = new (fMemoryManager) DTDEntityDecl(gDTDStr, false, fMemoryManager);
             declDTD->setSystemId(sysId);
             declDTD->setIsExternal(true);
-            Janitor<DTDEntityDecl> janDecl(declDTD);
 
             // Mark this one as a throw at end
             reader->setThrowAtEnd(true);
 
             // And push it onto the stack, with its pseudo name
-            fReaderMgr.pushReader(reader, declDTD);
+            fReaderMgr.pushReaderAdoptEntity(reader, declDTD);
 
             // Tell it its not in an include section
             dtdScanner.scanExtSubsetDecl(false, true);
@@ -2131,13 +2130,12 @@ Grammar* DGXMLScanner::loadDTDGrammar(const InputSource& src,
     DTDEntityDecl* declDTD = new (fMemoryManager) DTDEntityDecl(gDTDStr, false, fMemoryManager);
     declDTD->setSystemId(src.getSystemId());
     declDTD->setIsExternal(true);
-    Janitor<DTDEntityDecl> janDecl(declDTD);
 
     // Mark this one as a throw at end
     newReader->setThrowAtEnd(true);
 
     // And push it onto the stack, with its pseudo name
-    fReaderMgr.pushReader(newReader, declDTD);
+    fReaderMgr.pushReaderAdoptEntity(newReader, declDTD);
 
     //  If we have a doc type handler and advanced callbacks are enabled,
     //  call the doctype event.

src/xercesc/internal/IGXMLScanner.cpp

@@ -1535,13 +1535,12 @@ void IGXMLScanner::scanDocTypeDecl()
             DTDEntityDecl* declDTD = new (fMemoryManager) DTDEntityDecl(gDTDStr, false, fMemoryManager);
             declDTD->setSystemId(sysId);
             declDTD->setIsExternal(true);
-            Janitor<DTDEntityDecl> janDecl(declDTD);
 
             // Mark this one as a throw at end
             reader->setThrowAtEnd(true);
 
             // And push it onto the stack, with its pseudo name
-            fReaderMgr.pushReader(reader, declDTD);
+            fReaderMgr.pushReaderAdoptEntity(reader, declDTD);
 
             // Tell it its not in an include section
             dtdScanner.scanExtSubsetDecl(false, true);
@@ -3098,13 +3097,12 @@ Grammar* IGXMLScanner::loadDTDGrammar(const InputSource& src,
     DTDEntityDecl* declDTD = new (fMemoryManager) DTDEntityDecl(gDTDStr, false, fMemoryManager);
     declDTD->setSystemId(src.getSystemId());
     declDTD->setIsExternal(true);
-    Janitor<DTDEntityDecl> janDecl(declDTD);
 
     // Mark this one as a throw at end
     newReader->setThrowAtEnd(true);
 
     // And push it onto the stack, with its pseudo name
-    fReaderMgr.pushReader(newReader, declDTD);
+    fReaderMgr.pushReaderAdoptEntity(newReader, declDTD);
 
     //  If we have a doc type handler and advanced callbacks are enabled,
     //  call the doctype event.