[PATCH 4.4 27/27] sched/autogroup: Fix autogroup_move_group() to never skip sched_move_task()

[Greg Kroah-Hartman]

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

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

From: Oleg Nesterov <o...@redhat.com>

commit 18f649ef344127ef6de23a5a4272dbe2fdb73dde upstream.

The PF_EXITING check in task_wants_autogroup() is no longer needed. Remove
it, but see the next patch.

However the comment is correct in that autogroup_move_group() must always
change task_group() for every thread so the sysctl_ check is very wrong;
we can race with cgroups and even sys_setsid() is not safe because a task
running with task_group() == ag->tg must participate in refcounting:

        int main(void)
        {
                int sctl = open("/proc/sys/kernel/sched_autogroup_enabled", 
O_WRONLY);

                assert(sctl > 0);
                if (fork()) {
                        wait(NULL); // destroy the child's ag/tg
                        pause();
                }

                assert(pwrite(sctl, "1\n", 2, 0) == 2);
                assert(setsid() > 0);
                if (fork())
                        pause();

                kill(getppid(), SIGKILL);
                sleep(1);

                // The child has gone, the grandchild runs with kref == 1
                assert(pwrite(sctl, "0\n", 2, 0) == 2);
                assert(setsid() > 0);

                // runs with the freed ag/tg
                for (;;)
                        sleep(1);

                return 0;
        }

crashes the kernel. It doesn't really need sleep(1), it doesn't matter if
autogroup_move_group() actually frees the task_group or this happens later.

Reported-by: Vern Lovejoy <vlove...@redhat.com>
Signed-off-by: Oleg Nesterov <o...@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Mike Galbraith <efa...@gmx.de>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: hart...@redhat.com
Cc: vben...@redhat.com
Link: http://lkml.kernel.org/r/20161114184609.ga15...@redhat.com
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Sumit Semwal <sumit.sem...@linaro.org>
 [sumits: submit to 4.4 LTS, post testing on Hikey]
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 kernel/sched/auto_group.c |   23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c
@@ -111,14 +111,11 @@ bool task_wants_autogroup(struct task_st
 {
        if (tg != &root_task_group)
                return false;
-
        /*
-        * We can only assume the task group can't go away on us if
-        * autogroup_move_group() can see us on ->thread_group list.
+        * If we race with autogroup_move_group() the caller can use the old
+        * value of signal->autogroup but in this case sched_move_task() will
+        * be called again before autogroup_kref_put().
         */
-       if (p->flags & PF_EXITING)
-               return false;
-
        return true;
 }
 
@@ -138,13 +135,17 @@ autogroup_move_group(struct task_struct
        }
 
        p->signal->autogroup = autogroup_kref_get(ag);
-
-       if (!READ_ONCE(sysctl_sched_autogroup_enabled))
-               goto out;
-
+       /*
+        * We can't avoid sched_move_task() after we changed signal->autogroup,
+        * this process can already run with task_group() == prev->tg or we can
+        * race with cgroup code which can read autogroup = prev under rq->lock.
+        * In the latter case for_each_thread() can not miss a migrating thread,
+        * cpu_cgroup_attach() must not be possible after cgroup_exit() and it
+        * can't be removed from thread list, we hold ->siglock.
+        */
        for_each_thread(p, t)
                sched_move_task(t);
-out:
+
        unlock_task_sighand(p, &flags);
        autogroup_kref_put(prev);
 }