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
