linux-kernel - [PATCH] mm: Use rcu_dereference in in

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Message-Id: <20210221194207.1351703-1-willy@infradead.org>
Date:   Sun, 21 Feb 2021 19:42:06 +0000
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>
Subject: [PATCH] mm: Use rcu_dereference in in_vfork

Fix a sparse warning by using rcu_dereference().  Technically this is a
bug and a sufficiently aggressive compiler could reload the `real_parent'
pointer outside the protection of the rcu lock (and access freed memory),
but I think it's pretty unlikely to happen.

Fixes: b18dc5f291c0 ("mm, oom: skip vforked tasks from being selected")
Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 include/linux/sched/mm.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 1ae08b8462a4..90b2a0bce11c 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -140,7 +140,8 @@ static inline bool in_vfork(struct task_struct *tsk)
 	 * another oom-unkillable task does this it should blame itself.
 	 */
 	rcu_read_lock();
-	ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
+	ret = tsk->vfork_done &&
+			rcu_dereference(tsk->real_parent)->mm == tsk->mm;
 	rcu_read_unlock();
 
 	return ret;
-- 
2.29.2