Limits on header stack size parsing

Author: benaadams

src/Microsoft.AspNet.Server.Kestrel/Infrastructure/MemoryPoolIterator2.cs

@@ -10,6 +10,8 @@ namespace Microsoft.AspNet.Server.Kestrel.Infrastructure
 {
     public struct MemoryPoolIterator2
     {
+        private const int _maxHeaderStackLength = 16384;
+
         /// <summary>
         /// Array of "minus one" bytes of the length of SIMD operations on the current hardware. Used as an argument in the
         /// vector dot product that counts matching character occurrence.
@@ -549,12 +551,19 @@ public string GetAsciiString(MemoryPoolIterator2 end)
                 return default(string);
             }
 
+            var length = GetLength(end);
+
+            if (length > _maxHeaderStackLength)
+            {
+                return GetUtf8String(end);
+            }
+
             if (end._block == _block)
             {
-                return SingleBlockAsciiString(_block.Array, _index, end._index - _index);
+                return SingleBlockAsciiString(_block.Array, _index, length);
             }
 
-            return MultiBlockAsciiString(_block, _index, GetLength(end));
+            return MultiBlockAsciiString(_block, _index, length);
         }
 
         public string GetUtf8String(MemoryPoolIterator2 end)
@@ -567,10 +576,10 @@ public string GetUtf8String(MemoryPoolIterator2 end)
             {
                 return _utf8.GetString(_block.Array, _index, end._index - _index);
             }
-
+            
             var decoder = _utf8.GetDecoder();
-
             var length = GetLength(end);
+
             var charLength = length * 2;
             var chars = new char[charLength];
             var charIndex = 0;