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Subject[PATCH 4.14 31/33] cgroup: css_task_iter_skip()d iterators must be advanced before accessed
From: Tejun Heo <>

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 <>
Reported-by: syzbot
Fixes: b636fd38dc40 ("cgroup: Implement css_task_iter_skip()")
Signed-off-by: Greg Kroah-Hartman <>

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


+ /* @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,

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