On 12/19, Richard Guy Briggs wrote: > > On 13/12/18, Oleg Nesterov wrote: > > > Otherwise I can't understand your email, at least right now... I do not > > know how/where audit uses parent/real_parent. > > It uses real_parent to include the ppid number of a process in a couple > of log records.
I did a quick grep, it seems that audit uses sys_getppid() which should be fine. Nevermind, if you meant that (say) audit_alloc() paths use tsk->parent somehow, I agree this doesn't look right/safe. new_child->*parent can point to nowhere right after dup_task_struct() and there is no way to detect this. Unless, of course new_child->*parent == current, but copy_process() changes child->parent only after it takes tasklist_lock. > > But yes, unless tsk == current, the usage of tsk->*parent is not safe even > > under rcu_read_lock() unless you verify that this task was not unhashed. > > As I said, the only case I can see is in copy_process() when a signal is > pending when neither CLONE_PARENT nor CLONE_THREAD is passed to it. > Still, that is enough to need to check it. Hmm, so I guess you are worried about audit_free? But this error path can be called even before it checks signal_pending(), suppose that copy_semundo() fails? So it seems that CLONE_PARENT/THREAD doesn't really matter because it audit_free() can be called before copy_process() sets ->parent = current? Most probably I misunderstood you, so please ignore. I am sure you fully understand the problems and do not need my comments ;) > So what are you saying? I should use pid_alive() inside the > rcu_read_lock() to verify it is not unhashed since I don't have anything > to do with ->ptrace or any other task lock? In fact, the answer is > under my nose in __task_pid_nr_ns(), which already uses this approach. Yes. task->parent (or real_parent) can only make sense if this task can be re-parented (if parent exits, or debugger detaches). If the task is dead (removed from the parent->children list), obviously nobody can update this pointer. And this reminds me we should simply clear ->parent/real_parent on exit. And ->group_leader. I'll try to make the patch(s), after I finish thread_group changes. Unfortunately this needs a lot of grepping. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
