Re: [PATCH 1/4] pid_ns: zap_pid_ns_processes: fix the ->child_reaper changing

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Oleg Nesterov (oleg@tv-sign.ru):
> zap_pid_ns_processes() sets pid_ns->child_reaper = NULL, this is wrong.
> 
> Yes, we have already killed all tasks in this namespace, and sys_wait4()
> doesn't see any child. But this doesn't mean ->children list is empty,
> we may have EXIT_DEAD tasks which are not visible to do_wait(). In that
> case the subsequent forget_original_parent() will crash the kernel because
> it will try to re-parent these tasks to the NULL reaper.
> 
> Even if there are no childs, it is not good that forget_original_parent()
> uses reaper == NULL.
> 
> Change the code to set ->child_reaper = init_pid_ns.child_reaper instead.
> We could use pid_ns->parent->child_reaper as well, I think this does not
> really matter. These EXIT_DEAD tasks are not visible to the new ->parent
> after re-parenting, they will silently do release_task() eventually.
> 
> Note that we must change ->child_reaper, otherwise forget_original_parent()
> will use reaper == father, and in that case we will hit the (correct)
> BUG_ON(!list_empty(&father->children)).
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

Well while it looked correct to me all along, I couldn't get the
testcase to cause an oops.  But I see where do_exit() calls
zap_pid_ns_processes() (through group_exit()) before
forget_original_parent(), which can/should end up dereferencing
the null pid_ns, so clearly this is needed and correct.

Acked-by: Serge Hallyn <serue@us.ibm.com>

Thanks, Oleg.

-serge

> --- 2.6.27-rc4/kernel/pid_namespace.c~1_ZAP_DONT_CLEAR_REAPER	2008-07-30 13:12:49.000000000 +0400
> +++ 2.6.27-rc4/kernel/pid_namespace.c	2008-08-24 17:22:59.000000000 +0400
> @@ -179,9 +179,12 @@ void zap_pid_ns_processes(struct pid_nam
>  		rc = sys_wait4(-1, NULL, __WALL, NULL);
>  	} while (rc != -ECHILD);
> 
> -
> -	/* Child reaper for the pid namespace is going away */
> -	pid_ns->child_reaper = NULL;
> +	/*
> +	 * We can not clear ->child_reaper or leave it alone.
> +	 * There may by stealth EXIT_DEAD tasks on ->children,
> +	 * forget_original_parent() must move them somewhere.

Actually this comment is a little bit misleading - the null
deref will happen regardless of whether there are children,
right?

> +	 */
> +	pid_ns->child_reaper = init_pid_ns.child_reaper;
>  	acct_exit_ns(pid_ns);
>  	return;
>  }