Java FileLock: How to Load Dynamic Library From Locked File?

Question

I have an applet that retrieves a byte array from a backend server. This byte array contains a dynamic library (DLL or SO, depending on which OS the applet is running on), that must be written to disk and subsequently loaded from the disk by a call to System.load().

I need to ensure that the file is not tampered with after it's been written to disk and before it's loaded by the OS through the call to System.load(). I obtain an exclusive lock on the file while it's written to disk, but my testing shows that I must release this lock before the call to System.load(), or it'll fail to load the library.

Is there some way I can keep the lock on the file while I load it?

Sample code:

File f = File.createTempFile("tmp", "");
RandomAccessFile raf = new RandomAccessFile(f, "rwd");
FileChannel channel = raf.getChannel();
FileLock lock = channel.lock(0, Long.MAX_VALUE, false);

// This would be where I write the DLL/SO from a byte array...             
raf.write((int)65); // 'A'
raf.write((int)66); // 'B'
raf.write((int)67); // 'C'

System.out.println("Wrote dynamic library to file...");

// Close and release lock
raf.close();

System.out.println("File closed. Lock released.");

// This call fails if the file is still locked.             
System.load(f.getAbsolutePath());

Any help is greatly appreciated. The solution (if there is any) must not be native to any OS, but work on all platforms supported by Java. It is also a requirement that the solution be compatible with Java 1.4.

Answer 1

1. In Java 7 you can implement in-memory file system (see java.nio.file.spi.FileSystemProvider), so library content will be completely in memory thus making attacker's life much harder.
2. Another possible approach is to sign the library and let OS do security checks after reading file from disk; might be not very portable though.
3. Most important - is it really the biggest security issue you're facing? The time between 2 calls will be some micro- (ok, maybe milli-) seconds - one must hack really deep into filesystem to do bad things. Wouldn't it be easier to alter file while it is transferred over network? Or don't you think an attacker that advanced may... let's say hack JVM and substitute content while library is written to disk? Nothing is bulletproof, maybe this is the risk you can accept?

Just out of interest - what exactly is the error you're getting?