Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side sk_callback_lock

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2022/11/19 22:52, Tetsuo Handa wrote:
> On 2022/11/19 22:03, Jakub Sitnicki wrote:
>> When holding a reader-writer spin lock we cannot sleep. Calling
>> setup_udp_tunnel_sock() with write lock held violates this rule, because we
>> end up calling percpu_down_read(), which might sleep, as syzbot reports
>> [1]:
>>
>>  __might_resched.cold+0x222/0x26b kernel/sched/core.c:9890
>>  percpu_down_read include/linux/percpu-rwsem.h:49 [inline]
>>  cpus_read_lock+0x1b/0x140 kernel/cpu.c:310
>>  static_key_slow_inc+0x12/0x20 kernel/jump_label.c:158
>>  udp_tunnel_encap_enable include/net/udp_tunnel.h:187 [inline]
>>  setup_udp_tunnel_sock+0x43d/0x550 net/ipv4/udp_tunnel_core.c:81
>>  l2tp_tunnel_register+0xc51/0x1210 net/l2tp/l2tp_core.c:1509
>>  pppol2tp_connect+0xcdc/0x1a10 net/l2tp/l2tp_ppp.c:723
>>
>> Trim the writer-side critical section for sk_callback_lock down to the
>> minimum, so that it covers only operations on sk_user_data.
> 
> This patch does not look correct.
> 
> Since l2tp_validate_socket() checks that sk->sk_user_data == NULL with
> sk->sk_callback_lock held, you need to call rcu_assign_sk_user_data(sk, tunnel)
> before releasing sk->sk_callback_lock.
> 

Is it safe to temporarily set a dummy pointer like below?
If it is not safe, what makes assignments done by setup_udp_tunnel_sock() safe?

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 754fdda8a5f5..198d38d8fceb 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1474,11 +1474,12 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 	}
 
 	sk = sock->sk;
-	write_lock(&sk->sk_callback_lock);
-
+	write_lock_bh(&sk->sk_callback_lock);
 	ret = l2tp_validate_socket(sk, net, tunnel->encap);
 	if (ret < 0)
 		goto err_sock;
+	rcu_assign_sk_user_data(sk, (void *) 1);
+	write_unlock_bh(&sk->sk_callback_lock);
 
 	tunnel->l2tp_net = net;
 	pn = l2tp_pernet(net);
@@ -1492,6 +1493,8 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 			spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
 			sock_put(sk);
 			ret = -EEXIST;
+			write_lock_bh(&sk->sk_callback_lock);
+			rcu_assign_sk_user_data(sk, NULL);
 			goto err_sock;
 		}
 	}
@@ -1522,16 +1525,15 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 	if (tunnel->fd >= 0)
 		sockfd_put(sock);
 
-	write_unlock(&sk->sk_callback_lock);
 	return 0;
 
 err_sock:
+	write_unlock_bh(&sk->sk_callback_lock);
+
 	if (tunnel->fd < 0)
 		sock_release(sock);
 	else
 		sockfd_put(sock);
-
-	write_unlock(&sk->sk_callback_lock);
 err:
 	return ret;
 }