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From:   Amol Grover <frextrite@gmail.com>
To:     "David S . Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Patrick McHardy <kaber@trash.net>,
        Stephen Rothwell <sfr@canb.auug.org.au>, Qian Cai <cai@lca.pw>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Joel Fernandes <joel@joelfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Amol Grover <frextrite@gmail.com>,
        syzbot+1519f497f2f9f08183c6@syzkaller.appspotmail.com
Subject: [PATCH net v2 2/2] ipmr: Add lockdep expression to ipmr_for_each_table macro
Date:   Thu, 14 May 2020 23:31:03 +0530
Message-Id: <20200514180102.26425-2-frextrite@gmail.com>
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During the initialization process, ipmr_new_table() is called
to create new tables which in turn calls ipmr_get_table() which
traverses net->ipv4.mr_tables without holding the writer lock.
However, this is safe to do so as no tables exist at this time.
Hence add a suitable lockdep expression to silence the following
false-positive warning:

=============================
WARNING: suspicious RCU usage
5.7.0-rc3-next-20200428-syzkaller #0 Not tainted
-----------------------------
net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!

ipmr_get_table+0x130/0x160 net/ipv4/ipmr.c:136
ipmr_new_table net/ipv4/ipmr.c:403 [inline]
ipmr_rules_init net/ipv4/ipmr.c:248 [inline]
ipmr_net_init+0x133/0x430 net/ipv4/ipmr.c:3089

Fixes: f0ad0860d01e ("ipv4: ipmr: support multiple tables")
Reported-by: syzbot+1519f497f2f9f08183c6@syzkaller.appspotmail.com
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Amol Grover <frextrite@gmail.com>
---
v2:
- Change the lockdep expression to check for list emptiness at init
- Add Fixes tag
- Add Reported-by tag for syzbot report

 net/ipv4/ipmr.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 4897f7420c8f..5c218db2dede 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -109,9 +109,10 @@ static void mroute_clean_tables(struct mr_table *mrt, int flags);
 static void ipmr_expire_process(struct timer_list *t);
 
 #ifdef CONFIG_IP_MROUTE_MULTIPLE_TABLES
-#define ipmr_for_each_table(mrt, net) \
-	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
-				lockdep_rtnl_is_held())
+#define ipmr_for_each_table(mrt, net)					\
+	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list,	\
+				lockdep_rtnl_is_held() ||		\
+				list_empty(&net->ipv4.mr_tables))
 
 static struct mr_table *ipmr_mr_table_iter(struct net *net,
 					   struct mr_table *mrt)
-- 
2.24.1