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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Tejun Heo <tj@kernel.org>
Subject: [PATCH 4.19 43/45] cgroup: css_task_iter_skip()d iterators must be advanced before accessed
Date:   Thu,  8 Aug 2019 21:05:29 +0200
Message-Id: <20190808190456.328123880@linuxfoundation.org>
In-Reply-To: <20190808190453.827571908@linuxfoundation.org>
References: <20190808190453.827571908@linuxfoundation.org>
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From: Tejun Heo <tj@kernel.org>

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

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

--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4303,6 +4303,10 @@ 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);