If /dev/binder is opened and the opener process then e.g. calls execve,
proc->vma_vm_mm will still point to the location of the now-freed
mm_struct. If the process then calls ioctl(binder_fd, ...), the dangling
proc->vma_vm_mm pointer will be compared to current->mm.
Let the binder take a reference to the mm_struct to avoid this.