[PATCH net] net: hsr: fix suspicious usage in hsr_node_get_first

[Juhee Kang <claudiajkang@gmail.com>]

Currently, to dereference hlist_node which is result of hlist_first_rcu(),
rcu_dereference() is used. But, suspicious RCU warnings occur because
the caller doesn't acquire RCU. So it was solved by adding rcu_read_lock().

The kernel test robot reports:
    [   53.750001][ T3597] =============================
    [   53.754849][ T3597] WARNING: suspicious RCU usage
    [   53.759833][ T3597] 5.17.0-rc2-syzkaller-00903-g45230829827b #0 Not tainted
    [   53.766947][ T3597] -----------------------------
    [   53.771840][ T3597] net/hsr/hsr_framereg.c:34 suspicious rcu_dereference_check() usage!
    [   53.780129][ T3597] other info that might help us debug this:
    [   53.790594][ T3597] rcu_scheduler_active = 2, debug_locks = 1
    [   53.798896][ T3597] 2 locks held by syz-executor.0/3597:

Fixes: 4acc45db7115 ("net: hsr: use hlist_head instead of list_head for mac addresses")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: syzbot+f0eb4f3876de066b128c@syzkaller.appspotmail.com
Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
---
 net/hsr/hsr_framereg.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/hsr/hsr_framereg.c b/net/hsr/hsr_framereg.c
index b3c6ffa1894d..92abdf855327 100644
--- a/net/hsr/hsr_framereg.c
+++ b/net/hsr/hsr_framereg.c
@@ -31,7 +31,10 @@ struct hsr_node *hsr_node_get_first(struct hlist_head *head)
 {
 	struct hlist_node *first;
 
+	rcu_read_lock();
 	first = rcu_dereference(hlist_first_rcu(head));
+	rcu_read_unlock();
+
 	if (first)
 		return hlist_entry(first, struct hsr_node, mac_list);
 
-- 
2.25.1