buffer: fix range checking for slowToString

If `start` is not a valid number in the range, then the default value
zero will be used. Same way, if `end` is not a valid number in the
accepted range, then, by default, the length of the buffer is assumed.

Fixes: nodejs#2668
Ref: nodejs#2919
PR-URL: nodejs#4019
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

lib/buffer.js

@@ -327,13 +327,34 @@ Object.defineProperty(Buffer.prototype, 'offset', {
 function slowToString(encoding, start, end) {
   var loweredCase = false;
 
-  start = start >>> 0;
-  end = end === undefined || end === Infinity ? this.length : end >>> 0;
+  // No need to verify that "this.length <= MAX_UINT32" since it's a read-only
+  // property of a typed array.
+
+  // This behaves neither like String nor Uint8Array in that we set start/end
+  // to their upper/lower bounds if the value passed is out of range.
+  // undefined is handled specially as per ECMA-262 6th Edition,
+  // Section 13.3.3.7 Runtime Semantics: KeyedBindingInitialization.
+  if (start === undefined || start < 0)
+    start = 0;
+  // Return early if start > this.length. Done here to prevent potential uint32
+  // coercion fail below.
+  if (start > this.length)
+    return '';
+
+  if (end === undefined || end > this.length)
+    end = this.length;
+
+  if (end <= 0)
+    return '';
+
+  // Force coersion to uint32. This will also coerce falsey/NaN values to 0.
+  end >>>= 0;
+  start >>>= 0;
+
+  if (end <= start)
+    return '';
 
   if (!encoding) encoding = 'utf8';
-  if (start < 0) start = 0;
-  if (end > this.length) end = this.length;
-  if (end <= start) return '';
 
   while (true) {
     switch (encoding) {

src/node_internals.h

@@ -172,7 +172,7 @@ inline MUST_USE_RESULT bool ParseArrayIndex(v8::Local<v8::Value> arg,
     return true;
   }
 
-  int32_t tmp_i = arg->Int32Value();
+  int32_t tmp_i = arg->Uint32Value();
 
   if (tmp_i < 0)
     return false;