Thursday, May 6, 2004

Linux and Java and Threads and setuid

Today I learned something very interesting. I learned that you can't setuid on a process in Linux; not when you have multiple threads. Please see #8 in this list.
What they refer to as interesting times probably includes the following:
  • When calling setuid, only the caller thread will actually get its uid changed. All other existing threads in the "process" retain their original uid.
  • I believe any sane person should recognize this as meaning Linux is broken when using threads and setuid.
  • This is a security hole, because root threads still exist in the process. If the non-root threads are hijacked by an attacker, they can stack stomp on the root threads and execute arbitrary code as root.
  • Because synchronization depends on the ability to deliver signals, and delivering signals depends on priviledges, it's easy to see how synchronization between a thread running as root and another running as non-root can wedge the process.
  • Even if I did call setuid in the first bytecode instruction in a Java process, it's too late; Java has already forked threads to do things like garbage collection, and those threads present the security hole described above, and the synchronization problem described above.
  • I'm sure there's a long list of other reasons why this is bad, but I can't think of them now, and the above is sufficient.
In our project we have a Java process that uses forked processes written in C; the purpose of these forked processes is to run as root, or at least elevated privileges, while the Java process runs as some sort of nobody user. Unfortunately this doesn't work very well at all on Linux because we cannot downgrade the uid of the Java process after it starts.
This also means that if we want to listen on a port under 1024, we'll have to do that some other way; there's no way we could get the Java process to bind to that port as root and then downgrade to a nobody uid.
Also the processes I refer to have to be forked before the JVM starts. This means that we have to rendezvous with them in some manner that either means some sort of JNI code to hook up the file descriptors in the pipes, or use some other form of IPC.

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