Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced

[Oleg Nesterov <oleg@redhat.com>]

On 10/21, Denys Vlasenko wrote:
>
> On 10/21/2015 09:59 PM, Denys Vlasenko wrote:
> > On 10/21/2015 12:31 AM, Andrew Morton wrote:
> >> Well, to fix this a distro needs to roll out a new kernel.  Or a new
> >> init(8).  Is there any reason to believe that distributing/deploying a
> >> new kernel is significantly easier for everyone?  Because fixing init
> >> sounds like a much preferable solution to this problem.
> >
> > People will continue to write new init(8) implementations,
> > and they will miss this obscure case.
> >
> > Before this bug was found, it was considered possible to use
> > a shell script as init process. What now, every shell needs to add
> > __WALL to its waitpids?

Why not? I think it can safely use __WALL too.

> > The use of PTRACE_TRACEME in this reproducer is clearly pathological:
> > PTRACE_TRACEME was never intended to be used to attach to unsuspecting
> > processes.

Sure. But people do the things which were never intended to be
used all the time. We simply can not know if this "feature"
already has a creative user or not.

As for the patch,

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -385,6 +385,17 @@ static int ptrace_traceme(void)
>  	write_lock_irq(&tasklist_lock);
>  	/* Are we already being traced? */
>  	if (!current->ptrace) {
> +		struct pid_namespace *pid_ns;
> +
> +		pid_ns = task_active_pid_ns(current->parent);
> +		if (current->parent == pid_ns->child_reaper) {

Well, at least this needs same_thread_group(parent, child_reaper).

Plus we have PR_SET_CHILD_SUBREAPER so we also need to traverse the
->real_parent list if has_subreaper.

Finally it is not clear which ->child_reaper we should use after
setns(pidns_fd).

This all is fixable (although this again reminds me about a bug
with CHILD_SUBREAPER we probably need to fix first). But I didn't
even try to consider this option because it can break something.



And honestly, personally I don't like it. If we believe that we
can do this because "PTRACE_TRACEME was never intended to be used
to attach to unsuspecting processes", then we need a more generic
change, imo.

See http://marc.info/?l=linux-kernel&m=144536282305282 . Just in
case, it is not that I think "parent_exec_id != self_exec_id" is
all we need. This needs more discussion.

Oleg.