netdev - [PATCH] net: fix a concurrency bug in l2tp_tunnel

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Message-ID: <1F82E994-8F0B-499F-BA1A-8F1B2EEF1BF2@purdue.edu>
Date:   Tue, 27 Apr 2021 15:04:24 +0000
From:   "Gong, Sishuai" <sishuai@...due.edu>
To:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "tparkin@...alix.com" <tparkin@...alix.com>,
        Matthias Schiffer <mschiffer@...verse-factory.net>
CC:     Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: [PATCH] net: fix a concurrency bug in l2tp_tunnel_register()

l2tp_tunnel_register() registers a tunnel without fully
initializing its attribute. This can allow another kernel thread
running l2tp_xmit_core() to access the uninitialized data and
then cause a kernel NULL pointer dereference error, as shown below.

Thread 1    Thread 2
//l2tp_tunnel_register()
list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
           //pppol2tp_connect()
           tunnel = l2tp_tunnel_get(sock_net(sk), info.tunnel_id);
           // Fetch the new tunnel
           ...
           //l2tp_xmit_core()
           struct sock *sk = tunnel->sock;
           ...
           bh_lock_sock(sk);
           //Null pointer error happens
tunnel->sock = sk;

Fix this bug by initializing tunnel->sock before adding the
tunnel into l2tp_tunnel_list.

Reviewed-by: Cong Wang <cong.wang@...edance.com>
Signed-off-by: Sishuai Gong <sishuai@...due.edu>
Reported-by: Sishuai Gong <sishuai@...due.edu>
---
net/l2tp/l2tp_core.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 203890e378cb..8eb805ee18d4 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1478,11 +1478,15 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
	tunnel->l2tp_net = net;
	pn = l2tp_pernet(net);

+	sk = sock->sk;
+	sock_hold(sk);
+	tunnel->sock = sk;
+
	spin_lock_bh(&pn->l2tp_tunnel_list_lock);
	list_for_each_entry(tunnel_walk, &pn->l2tp_tunnel_list, list) {
		if (tunnel_walk->tunnel_id == tunnel->tunnel_id) {
			spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
-
+			sock_put(sk);
			ret = -EEXIST;
			goto err_sock;
		}
@@ -1490,10 +1494,6 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
	list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
	spin_unlock_bh(&pn->l2tp_tunnel_list_lock);

-	sk = sock->sk;
-	sock_hold(sk);
-	tunnel->sock = sk;
-
	if (tunnel->encap == L2TP_ENCAPTYPE_UDP) {
		struct udp_tunnel_sock_cfg udp_cfg = {
			.sk_user_data = tunnel,
-- 
2.17.1