Re: Turning off JIT Optimisation

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Sat, 15 May 2010 17:21:02 -0400
On 15-05-2010 16:34, rossum wrote:

In a secure program I want to be able to wipe the byte array
containing the key, mKey[], before releasing the memory back to the
system. To do that I wrote a simple dispose() method to do a
reasonably secure overwrite of the array:

     public void dispose() {
       if (mKey != null) {
         for (int i = 0; i< mKey.length; ++i) {
           for (int j = 0; j< 5; ++j) {
             mKey[i] = (byte)0x55;
             mKey[i] = (byte)0xFF;
             mKey[i] = (byte)0xAA;
             mKey[i] = (byte)0x00;
           } // end for
         } // end for
         mKey = null;
       } // end if
     } // end dispose()

Obviously any reasonably good JIT compiler can look at that and
optimise it to the equivalent of:

     public void dispose() {
       if (mKey != null) {
         mKey = null;
       } // end if
     } // end dispose()

That is not what I want, since the repeated overwrites make it more
difficult for an attacker to recover the former contents of memory.
Is there some way to tell the JIT compiler that I do not want this
method to be optimised but to be run as written? Effectively an
@Pessimise annotation for just this method.

I have two ideas:

1) store the key in native memory and access it via JNI - you can
    much better control the C optimizer

2) See if you can find confirmation or rejection for that:

// your code

actuall does what the names seems to indicate.


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