Commit 6347e900 authored Jun 20, 2012 by Eric W. Biederman Committed by Linus Torvalds Jun 20, 2012

pidns: guarantee that the pidns init will be the last pidns process reaped



Today we have a twofold bug.  Sometimes release_task on pid == 1 in a pid
namespace can run before other processes in a pid namespace have had
release task called.  With the result that pid_ns_release_proc can be
called before the last proc_flus_task() is done using upid->ns->proc_mnt,
resulting in the use of a stale pointer.  This same set of circumstances
can lead to waitpid(...) returning for a processes started with
clone(CLONE_NEWPID) before the every process in the pid namespace has
actually exited.

To fix this modify zap_pid_ns_processess wait until all other processes in
the pid namespace have exited, even EXIT_DEAD zombies.

The delay_group_leader and related tests ensure that the thread gruop
leader will be the last thread of a process group to be reaped, or to
become EXIT_DEAD and self reap.  With the change to zap_pid_ns_processes
we get the guarantee that pid == 1 in a pid namespace will be the last
task that release_task is called on.

With pid == 1 being the last task to pass through release_task
pid_ns_release_proc can no longer be called too early nor can wait return
before all of the EXIT_DEAD tasks in a pid namespace have exited.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Louis Rilling <louis.rilling@kerlabs.com>
Cc: Mike Galbraith <efault@gmx.de>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Tested-by: Andrew Wagin <avagin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent f39cdaeb

kernel/exit.c

+13 −1

Original line number	Diff line number	Diff line
		@@ -64,7 +64,6 @@ static void exit_mm(struct task_struct * tsk);
		static void __unhash_process(struct task_struct *p, bool group_dead)
		{
		nr_threads--;
		detach_pid(p, PIDTYPE_PID);
		if (group_dead) {
		detach_pid(p, PIDTYPE_PGID);
		detach_pid(p, PIDTYPE_SID);
		@@ -72,7 +71,20 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
		list_del_rcu(&p->tasks);
		list_del_init(&p->sibling);
		__this_cpu_dec(process_counts);
		/*
		* If we are the last child process in a pid namespace to be
		* reaped, notify the reaper sleeping zap_pid_ns_processes().
		*/
		if (IS_ENABLED(CONFIG_PID_NS)) {
		struct task_struct *parent = p->real_parent;

		if ((task_active_pid_ns(p)->child_reaper == parent) &&
		list_empty(&parent->children) &&
		(parent->flags & PF_EXITING))
		wake_up_process(parent);
		}
		}
		detach_pid(p, PIDTYPE_PID);
		list_del_rcu(&p->thread_group);
		}

kernel/pid_namespace.c

+20 −0

Original line number	Diff line number	Diff line
		@@ -184,11 +184,31 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
		}
		read_unlock(&tasklist_lock);

		/* Firstly reap the EXIT_ZOMBIE children we may have. */
		do {
		clear_thread_flag(TIF_SIGPENDING);
		rc = sys_wait4(-1, NULL, __WALL, NULL);
		} while (rc != -ECHILD);

		/*
		* sys_wait4() above can't reap the TASK_DEAD children.
		* Make sure they all go away, see __unhash_process().
		*/
		for (;;) {
		bool need_wait = false;

		read_lock(&tasklist_lock);
		if (!list_empty(&current->children)) {
		__set_current_state(TASK_UNINTERRUPTIBLE);
		need_wait = true;
		}
		read_unlock(&tasklist_lock);

		if (!need_wait)
		break;
		schedule();
		}

		if (pid_ns->reboot)
		current->signal->group_exit_code = pid_ns->reboot;