Restrict XML file recursion depth to 200.

This resolves #2.

parser/htmlparser/nsExpatDriver.cpp

@@ -338,6 +338,9 @@ NS_IMPL_CYCLE_COLLECTING_RELEASE(nsExpatDriver)
 
 NS_IMPL_CYCLE_COLLECTION(nsExpatDriver, mSink, mExtendedSink)
 
+// We store the tagdepth in a Uint8, so make sure the limit fits in a Uint8.
+PR_STATIC_ASSERT(MAX_XML_TREE_DEPTH <= UINT8_MAX);
+
 nsExpatDriver::nsExpatDriver()
   : mExpatParser(nullptr),
     mInCData(false),
@@ -345,6 +348,7 @@ nsExpatDriver::nsExpatDriver()
     mInExternalDTD(false),
     mMadeFinalCallToExpat(false),
     mIsFinalChunk(false),
+    mTagDepth(0),
     mInternalState(NS_OK),
     mExpatBuffered(0),
     mCatalogData(nullptr),
@@ -359,7 +363,7 @@ nsExpatDriver::~nsExpatDriver()
   }
 }
 
-nsresult
+void
 nsExpatDriver::HandleStartElement(const char16_t *aValue,
                                   const char16_t **aAtts)
 {
@@ -377,13 +381,16 @@ nsExpatDriver::HandleStartElement(const char16_t *aValue,
   }
 
   if (mSink) {
+    if (++mTagDepth == MAX_XML_TREE_DEPTH) {
+      MaybeStopParser(NS_ERROR_HTMLPARSER_HIERARCHYTOODEEP);
+      return;
+    }
+
     nsresult rv = mSink->
       HandleStartElement(aValue, aAtts, attrArrayLength,
                          XML_GetCurrentLineNumber(mExpatParser));
     MaybeStopParser(rv);
   }
-
-  return NS_OK;
 }
 
 nsresult
@@ -395,6 +402,7 @@ nsExpatDriver::HandleEndElement(const char16_t *aValue)
 
   if (mSink && mInternalState != NS_ERROR_HTMLPARSER_STOPPARSING) {
     nsresult rv = mSink->HandleEndElement(aValue);
+    --mTagDepth;
     MaybeStopParser(rv);
   }