[PATCH 4.14 31/33] cgroup: css_task_iter_skip()d iterators must be advanced before accessed

[Greg Kroah-Hartman]

From: Tejun Heo <tj@xxxxxxxxxx>

commit cee0c33c546a93957a52ae9ab6bebadbee765ec5 upstream.

b636fd38dc40 ("cgroup: Implement css_task_iter_skip()") introduced
css_task_iter_skip() which is used to fix task iterations skipping
dying threadgroup leaders with live threads.  Skipping is implemented
as a subportion of full advancing but css_task_iter_next() forgot to
fully advance a skipped iterator before determining the next task to
visit causing it to return invalid task pointers.

Fix it by making css_task_iter_next() fully advance the iterator if it
has been skipped since the previous iteration.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: syzbot
Link: http://lkml.kernel.org/r/00000000000097025d058a7fd785@xxxxxxxxxx
Fixes: b636fd38dc40 ("cgroup: Implement css_task_iter_skip()")
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/cgroup/cgroup.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4197,6 +4197,10 @@ struct task_struct *css_task_iter_next(s
 
 	spin_lock_irq(&css_set_lock);
 
+	/* @it may be half-advanced by skips, finish advancing */
+	if (it->flags & CSS_TASK_ITER_SKIPPED)
+		css_task_iter_advance(it);
+
 	if (it->task_pos) {
 		it->cur_task = list_entry(it->task_pos, struct task_struct,
 					  cg_list);