Permission Bug in AtomicLongFieldUpdater and AtomicIntegerFieldUpdater

David Holmes David.Holmes at oracle.com
Wed Apr 21 20:35:45 PDT 2010


Jeff I'm a little confused about the reasoning here. I was surprised to 
discover that the atomic updaters allow you to perform accesses that 
direct bytecode and reflection do not (perhaps I knew this when we put 
them together but have since forgotten). So contrary to my expectation 
that the real checks occurred at set/get and so construction checks were 
not necessary, it seems that because there are no checks at set/get then 
construction checks are somewhat pointless anyway.

Is that the view here? That the check under question is unnecessarily 
strict given that we don't perform more obvious checks at get/set time?

David

Jeff Nisewanger said the following on 04/22/10 10:30:
> On 4/16/2010 5:50 AM, David Holmes wrote:
>> Hi Doug,
>>
>> <aside: FYI copies of my replies to security-dev are being held for 
>> approval as I'm not a subscriber.>
>>
>> Doug Lea said the following on 04/16/10 21:43:
>>> On 04/15/10 18:34, Martin Buchholz wrote:
>>>
>>>> People are using Atomic field updaters to update fields in classes 
>>>> in other classloaders.
>>>>
>>>
>>> I think the policy on this awaits interpretation by Jeff
>>> or other members of security team. FWIW, my take is that
>>> if users know that they may cross class loaders, then they
>>> should wrap these in doPrivileged anyway. As in ...
>>
>> I'm coming around to agreeing with the proposed fix. My take is that 
>> the real security check should take place at the time the field is set:
>>
>>   field_x_updater.set(obj, val);
>>
>> At this point the calling code must have the necessary permissions to 
>> set field x of the given obj of type T. And I believe we do indeed 
>> check this.
>>
>> When the AtomicXXXXFieldUpdater constructor binds itself to the Field 
>> object for T.x that's an optimization. There's no reason we couldn't 
>> do this on each call to set() - other than it would perform terribly. 
>> So in that sense the security checks that take place at construction 
>> are incidental** and so we should be as permissive as we can make them 
>> _provided_ that the actual set() call will make the necessary 
>> permission checks.
>>
>> ** This particular check is also incidental because we happen to use a 
>> public reflection method to get the Field object. We could just as 
>> easily have used a magic VM hook.
>>
> 
>     This is describing the security checking philosophy of the 
> java.lang.reflect apis
> which mimic the security semantics of static bytecode at the point at 
> which they
> are dynamically invoked. They perform the full security
> check on every get() or set() method. This has a substantial performance 
> penalty
> for various reasons but it allows java.lang.reflect.Field instances to 
> be freely passed
> around internally within an application or library's implementation 
> classes since the
> actual security check is against the caller of the get/set() method. 
> Static bytecode
> doesn't have these performance issues since the check is performed once at
> constant pool resolution time and the calling point is inherently bound 
> to that class.
> 
>     On the other hand, the java.util.concurrent.atomic APIs were 
> designed to allow
> highly efficient atomic access where performing a full security check on 
> the set()
> method would be a substantial performance burden. Therefore, all of the 
> access-oriented
> security checks are performed at construction time and the set() method 
> (for example)
> only performs type checks to ensure the integrity of the field offset 
> within the
> enclosing object.
> 
>     I vaguely recall discussions from years past about the need to 
> improve the security-relevant
> aspects of the javadoc so this distinction would be clear to developers 
> using the API.
> However, I'm not seeing any of this in the jdk 7 docs. This needs to be 
> fixed. (!)
> 
>     The current webrev looks reasonable to me aside from the need to 
> improve the javadoc.
> 
> 
>     Jeff



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