linux-kernel - Re: pppd service crash in linux-3.13.6

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Message-ID: <20140313170622.GA31206@redhat.com>
Date:	Thu, 13 Mar 2014 18:06:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Sree Harsha Totakura <sreeharsha@...akura.in>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: pppd service crash in linux-3.13.6

On 03/10, Peter Hurley wrote:
>
> [ +cc Oleg Nesterov ]

Thanks.

> The NULL ptr dereference is from following the current->nsproxy ptr
> in ppp_register_channel().
>
> This was broken by
>
> commit 8aac62706adaaf0fab02c4327761561c8bda9448
> Author: Oleg Nesterov <oleg@...hat.com>
> Date:   Fri Jun 14 21:09:49 2013 +0200
>
>     move exit_task_namespaces() outside of exit_notify()
>
> which moved the exit_task_namespaces(tsk) before disassociate_ctty().

Heh. OK, we can move it down after disassociate_ctty(), the original
motivation for that commit was the problem which was also (hopefully)
fixed by e7b2c406925273 "fput: task_work_add() can fail if the caller
has passed exit_task_work()".

In fact I think that it makes sense to move it down after
exit_task_work() anyway. But this is almost off-topic and I'd like to
avoid this right now.

OTOH, why we should delay disassociate_ctty? IOW, do you see any
potential problem with the trivial patch below?

And it seems that it makes sense to move (at least) check_stack_usage()
down, but this is offtopic too.

Oleg.

--- x/kernel/exit.c
+++ kernel/exit.c/
@@ -784,6 +784,8 @@ void do_exit(long code)
 	exit_shm(tsk);
 	exit_files(tsk);
 	exit_fs(tsk);
+	if (group_dead)
+		disassociate_ctty(1);
 	exit_task_namespaces(tsk);
 	exit_task_work(tsk);
 	check_stack_usage();
@@ -799,13 +801,9 @@ void do_exit(long code)
 
 	cgroup_exit(tsk, 1);
 
-	if (group_dead)
-		disassociate_ctty(1);
-
 	module_put(task_thread_info(tsk)->exec_domain->module);
 
 	proc_exit_connector(tsk);
-
 	/*
 	 * FIXME: do that only when needed, using sched_exit tracepoint
 	 */

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