[proxies] fix access issue when having proxies on the prototype-chain of global objects.

src/crankshaft/ia32/lithium-codegen-ia32.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_IA32
 
 #include "src/crankshaft/ia32/lithium-codegen-ia32.h"
 
 #include "src/base/bits.h"
 #include "src/code-factory.h"
@@ skipping 2522 matching lines @@
   // will be ruled out below.
   if (instr->hydrogen()->ObjectNeedsSmiCheck()) {
     __ test(object, Immediate(kSmiTagMask));
     EmitFalseBranch(instr, zero);
   }
 
   // Loop through the {object}s prototype chain looking for the {prototype}.
   __ mov(object_map, FieldOperand(object, HeapObject::kMapOffset));
   Label loop;
   __ bind(&loop);
+  // If we find a JSProxy during the prototype chain walk we deopt. It is
+  // important to note that we restart the lookup in the InstanceOfStub as
+  // continuing the iteration on the JSProxy would lead to a potential security
+  // issue by executing the getPrototypeOf trap of a cross-realm proxy-object
+  // without performing the necessary access checks first.

[Camillo Bruni, 2015/12/14 19:59:20]

probably should add this comment to all platforms.

[Toon Verwaest, 2015/12/14 20:06:48]

There's still a silly issue. If you have:
window2.__proto__ = window1
then in window1: window2 instanceof Object -> true

That's an information leak. It's not high-priority, but it's wrong nonetheless.

[Camillo Bruni, 2015/12/15 14:09:26]

fixed that now as well...

   __ CmpInstanceType(object_map, JS_PROXY_TYPE);
   DeoptimizeIf(equal, instr, Deoptimizer::kProxy);
   __ mov(object_prototype, FieldOperand(object_map, Map::kPrototypeOffset));
   __ cmp(object_prototype, prototype);
   EmitTrueBranch(instr, equal);
   __ cmp(object_prototype, factory()->null_value());
   EmitFalseBranch(instr, equal);
   __ mov(object_map, FieldOperand(object_prototype, HeapObject::kMapOffset));
   __ jmp(&loop);
 }
@@ skipping 2954 matching lines @@
   RecordSafepoint(Safepoint::kNoLazyDeopt);
 }
 
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_IA32