AZL-112529

In the Linux kernel, the following vulnerability has been resolved:

fork: release the files of a failed fork after sched_cancel_fork()

Reorder copy_process() cleanup so sched_fork() taking scx_fork_rwsem scx_pre_fork() is safe and isn't held around exiting files. Right now, a fork that fails while another thread closed the bpf link fd will deadlock against its own read side. It also blocks every fork on the system:

copy_process()                  holds scx_fork_rwsem for read
  exit_files()
    close_files()
      filp_close_sync()
        bpf_scx_unreg()
          kthread_flush_work()  waits for the disable work
            scx_root_disable()
              percpu_down_write(&scx_fork_rwsem)

Simply release the child's files after sched_cancel_fork() dropped the lock. The task starts out as a copy of its parent so p->files points to the parent's table until copy_files() replaces it. exit_files() must not run for a fork that failed before that. Clear p->files up front so exit_files() is a no-op for those and make copy_files() set it explicitly for CLONE_FILES.