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From Peter Firmstone <j...@zeus.net.au>
Subject Re: Learnings from a RevokeableDynamicPolicy & A Future Roadmap
Date Tue, 17 Aug 2010 03:39:53 GMT
Clarification (I overlooked something): The 
checkPermission(Collection<Permission> perms) in the finally block would 
work only for intercepting a method return, but it doesn't work for 
InputStream read() / OutputStream write(), since we wanted to stop 
method execution, if Permission is no longer granted.

For example if a socket delegate returns an OutputStream delegate, that 
wraps an underlying OutputStream, then during the write(), if a 
revocation occurs, we probably want to close the OutputStream, if it 
applies.

The OutputStream write() would still need a reaper, to close the 
OutputStream causing an IOException to be thrown, although as yet I 
haven't determined how to add debug information for an intervening 
revocation.

public void write( byte[] b) {
    ecm.begin(reaper)
    try{
       ecm.checkPermission(writePermission);
       out.write( byte[] b);
    } finally {
       end();
    }
}

Is it as critical to stop a read() operation?

Since the array passed into the read() operation is written to, we can 
close this as well, causing an IOException, however the user will still 
get some bytes written into the byte array, which it can access if it 
catches the IOException.

public int read(byte[] b) {
    ecm.begin(reaper)
    try{
        ecm.checkPermission(readPermission);
        return in.read( byte[] b);
    } finally {
       end();
    }
}

But we could provide the optimised single invocation 
checkPermission(Collection<Permission> perms) method as well, for calls 
that don't need to be intercepted.

Perhaps we could be less pedantic about read operations, provided all 
write operations are intercepted?

public int read(byte[] b) {
    try {
        ecm.checkPermission(readPermission);
    } catch (AccessControlException e) {
       this.close();
       throw new IOException(e.toString());
    }
        return in.read( byte[] b);
}

This checkPermission would be very fast, with only a small amount of cpu 
overhead, because it caches the result for the AccessControlContext.

We could do this for write() as well, if the code was trusted at the 
time of the write() call, then we might assume that the information 
contained within, wasn't borne from an operation of malicious intent, we 
trusted it, so the call can be allowed to complete.  This has a much 
smaller performance penalty, any following calls will cause an 
AccessControlExeption to be thrown, if Permission is revoked.

Then write might simply be:

public void write(byte[] b) {
    try {
        ecm.checkPermission(writePermission);
    } catch (AccessControlException e) {
       this.close();
       throw new IOException(e.toString());
    }
    out.write(b);
}

We might consider what the uses are for Permission revocation?  I'm 
starting to think interception might hamper performance too greatly.

The major uses I see for Revocation:

    * Reusing ClassLoader's, for proxy's with common code source, by
      removing trust gained by the previous proxy.
    * Limit the time period for which trust is granted to a third
      party.  A Trust lease?   Trust is only granted while contact is
      maintained.
    * If a proxy has lost contact with it's Server, which was trusted
      and we want to resume (without loosing state) we can revoke
      permission, retry connecting with the Server, verify trust, then
      re grant trust (I'm thinking about the neuromancer remote
      reference problem).

Obviously the feature got your interest, I'm wondering what other uses 
there might have been for permission revocation?  This might help me 
better understand the use case scenarios.


Peter Firmstone wrote:
> Fred Oliver wrote:
>> To at least partially answer my own question, I think that the answer
>> is that the socket can't be closed (at least in all cases).
>>
>> If the delegate for a resource has multiple users (codebases,
>> principals, etc.) then closing the resource because permission for one
>> of the users has been revoked denies access to the resource by the
>> remaining valid users. (This allows a denial of service attack.) And
>> then, if you can't close the resource (socket), you can't kick loose
>> the no longer trusted thread blocked reading from it. You may be able
>> to determine the special case of no user retaining access (everyone's
>> access was revoked) and can then close the resource.
>>
>> In the standard security model, there are no checks for
>> reading/writing sockets. Allowing revocation means that checks must be
>> added for each individual read or write call on the socket streams. Is
>> that level of performance satisfactory?
>>   
>
> I've just spotted a possible performance improvement.
>
> We can eliminate the end() call in the finally block, put all the 
> checkPermission calls there instead.  This substantially reduces the 
> caching operations.  Perhaps we can manage the Socket some other way, 
> we just block unauthorised access using the delegate?
>
> try {
>    // Do some processing
>    return something;
> } finally {
>    ECM.checkPermission(Collection<Permission> permissions);
> }
>
> The current expense for making a Permission call via ECM (remembering 
> the first is always going to be expensive), currently requires the 
> three ECM calls in succession:
>
> begin();
> checkPermission(perm);
> end();
>
> begin():
>
>   1. Associate the thread with the reaper.
>
>
> checkPermission(perm):
>
>   1. Obtain the read lock;
>   2. Add the current thread to the execution cache.
>   3. Associate the current AccessControlContext with the current thread
>   4. Check if the AccessControlContext has been checked for this
>      Permission previously.
>   5. If it has been checked before, return, else do
>      AccessController.checkPermission(perm), cache the result if it
>      succeeds.
>
> Each end():
>
>   1. Obtain the read lock;
>   2. Remove the thread and reaper from the cache.
>   3. Remove the thread association with the AccessControlContext.
>   4. Remove the thread from the execution cache.
>
>      This could be simplified to (no reaper or thread caching):
>
> checkPermission(perms)
>
>   1. Obtain the read lock;
>   2. For each Permission, check if the current AccessControlContext has
>      been checked for this Permission previously, if so return.
>      (HashMap lookup - fast).
>   3. If the AccessControlContext hasn't been checked previously, do
>      AccessController.checkPermission(perm), cache the result if it
>      succeeds, then return.
>
> So for performance reasons, it looks like we need to be considering 
> some other way to manage the Socket, the ECM would serve us better if 
> we kept it simple?
>
> I think your earlier suggestion of an event notification might be more 
> useful, if provided as a parameter in the checkPermission call, in the 
> event of failure, an event would be generated, the delegate could then 
> decide what action to take.  If it only had one user, then it knows to 
> close the Socket, if it has multiple users, it might just keep score.
>
> This stops the transmission of data from unauthorised code, but it 
> doesn't close the socket.
>
> Cheers,
>
> Peter.
>>  
>>> If there is untrusted code on a thread's stack, and if the thread
>>> called a delegate, and if the delegate is in the between ECM.begin()
>>> and ECM.end(), then the delegate's reaper will be called, and it can
>>> close the socket.
>>>
>>> If the revocation happened elsewhere (e.g. the untrusted code is not
>>> on any stack, or is occupied with some other task), how does the
>>> socket get closed?
>>>
>>> Fred
>>>     
>
>
>


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