linux-kernel - [PATCH 3.14 33/33] exit: call disassociate_ctty() before exit_task

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Message-Id: <20140424214454.395738129@linuxfoundation.org>
Date:	Thu, 24 Apr 2014 14:48:46 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Sree Harsha Totakura <sreeharsha@...akura.in>,
	Peter Hurley <peter@...leysoftware.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jeff Dike <jdike@...toit.com>, Ingo Molnar <mingo@...e.hu>,
	Andrey Vagin <avagin@...nvz.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 3.14 33/33] exit: call disassociate_ctty() before exit_task_namespaces()

3.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@...hat.com>

commit c39df5fa37b0623589508c95515b4aa1531c524e upstream.

Commit 8aac62706ada ("move exit_task_namespaces() outside of
exit_notify()") breaks pppd and the exiting service crashes the kernel:

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000028
    IP: ppp_register_channel+0x13/0x20 [ppp_generic]
    Call Trace:
      ppp_asynctty_open+0x12b/0x170 [ppp_async]
      tty_ldisc_open.isra.2+0x27/0x60
      tty_ldisc_hangup+0x1e3/0x220
      __tty_hangup+0x2c4/0x440
      disassociate_ctty+0x61/0x270
      do_exit+0x7f2/0xa50

ppp_register_channel() needs ->net_ns and current->nsproxy == NULL.

Move disassociate_ctty() before exit_task_namespaces(), it doesn't make
sense to delay it after perf_event_exit_task() or cgroup_exit().

This also allows to use task_work_add() inside the (nontrivial) code
paths in disassociate_ctty().

Investigated by Peter Hurley.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Reported-by: Sree Harsha Totakura <sreeharsha@...akura.in>
Cc: Peter Hurley <peter@...leysoftware.com>
Cc: Sree Harsha Totakura <sreeharsha@...akura.in>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Jeff Dike <jdike@...toit.com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Andrey Vagin <avagin@...nvz.org>
Cc: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/exit.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -791,6 +791,8 @@ 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();
@@ -806,13 +808,9 @@ 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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