netdev - [PATCH net v2] ipv6: Fix suspicious RCU usage warning in ip6mr

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Date:   Sat, 16 May 2020 13:15:15 +0530
From:   madhuparnabhowmik10@...il.com
To:     davem@...emloft.net, kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        kuba@...nel.org, kaber@...sh.net
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        frextrite@...il.com, joel@...lfernandes.org, paulmck@...nel.org,
        cai@....pw, linux-kernel-mentees@...ts.linuxfoundation.org,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
Subject: [PATCH net v2] ipv6: Fix suspicious RCU usage warning in ip6mr

From: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>

This patch fixes the following warning:

=============================
WARNING: suspicious RCU usage
5.7.0-rc4-next-20200507-syzkaller #0 Not tainted
-----------------------------
net/ipv6/ip6mr.c:124 RCU-list traversed in non-reader section!!

ipmr_new_table() returns an existing table, but there is no table at
init. Therefore the condition: either holding rtnl or the list is empty
is used.

Fixes: d1db275dd3f6e ("ipv6: ip6mr: support multiple tables")
Reported-by: kernel test robot <lkp@...el.com>
Suggested-by: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
---
v2:
- Add correct fixes tag
- Fix line over 80 chars

 net/ipv6/ip6mr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 65a54d74acc1..1e223e26f079 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -98,7 +98,8 @@ static void ipmr_expire_process(struct timer_list *t);
 #ifdef CONFIG_IPV6_MROUTE_MULTIPLE_TABLES
 #define ip6mr_for_each_table(mrt, net) \
 	list_for_each_entry_rcu(mrt, &net->ipv6.mr6_tables, list, \
-				lockdep_rtnl_is_held())
+				lockdep_rtnl_is_held() || \
+				list_empty(&net->ipv6.mr6_tables))
 
 static struct mr_table *ip6mr_mr_table_iter(struct net *net,
 					    struct mr_table *mrt)
-- 
2.17.1