[PATCH 3.11 136/182] exit: call disassociate_ctty() before exit_task_namespaces()

[Luis Henriques]

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

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

From: Oleg Nesterov <oleg at redhat.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 at redhat.com>
Reported-by: Sree Harsha Totakura <sreeharsha at totakura.in>
Cc: Peter Hurley <peter at hurleysoftware.com>
Cc: Sree Harsha Totakura <sreeharsha at totakura.in>
Cc: "Eric W. Biederman" <ebiederm at xmission.com>
Cc: Jeff Dike <jdike at addtoit.com>
Cc: Ingo Molnar <mingo at elte.hu>
Cc: Andrey Vagin <avagin at openvz.org>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/exit.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index a949819..b79d882 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -783,6 +783,8 @@ 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();
@@ -798,13 +800,9 @@ 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
 	 */
-- 
1.9.1