[PATCH net v3] l2tp: Serialize access to sk_user_data with sk_callback_lock

[PATCH net v3] l2tp: Serialize access to sk_user_data with sk_callback_lock

[Jakub Sitnicki]

sk->sk_user_data has multiple users, which are not compatible with each
other. Writers must synchronize by grabbing the sk->sk_callback_lock.

l2tp currently fails to grab the lock when modifying the underlying tunnel
socket. Fix it by adding appropriate locking.

We don't to grab the lock when l2tp clears sk_user_data, because it happens
only in sk->sk_destruct, when the sock is going away.

v3:
- switch from sock lock to sk_callback_lock
- document write-protection for sk_user_data

v2:
- update Fixes to point to origin of the bug
- use real names in Reported/Tested-by tags

Fixes: 3557baabf280 ("[L2TP]: PPP over L2TP driver core")
Reported-by: Haowei Yan <g1042620637@gmail.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 include/net/sock.h   |  2 +-
 net/l2tp/l2tp_core.c | 17 +++++++++++------
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index d08cfe190a78..601c48601496 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -323,7 +323,7 @@ struct sk_filter;
   *	@sk_tskey: counter to disambiguate concurrent tstamp requests
   *	@sk_zckey: counter to order MSG_ZEROCOPY notifications
   *	@sk_socket: Identd and reporting IO signals
-  *	@sk_user_data: RPC layer private data
+  *	@sk_user_data: RPC layer private data. Write-protected by @sk_callback_lock.
   *	@sk_frag: cached page frag
   *	@sk_peek_off: current peek_offset value
   *	@sk_send_head: front of stuff to transmit
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 7499c51b1850..429ad9633f13 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1469,16 +1469,18 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 		sock = sockfd_lookup(tunnel->fd, &ret);
 		if (!sock)
 			goto err;
-
-		ret = l2tp_validate_socket(sock->sk, net, tunnel->encap);
-		if (ret < 0)
-			goto err_sock;
 	}
 
+	sk = sock->sk;
+	write_lock(&sk->sk_callback_lock);
+
+	ret = l2tp_validate_socket(sk, net, tunnel->encap);
+	if (ret < 0)
+		goto err_sock;
+
 	tunnel->l2tp_net = net;
 	pn = l2tp_pernet(net);
 
-	sk = sock->sk;
 	sock_hold(sk);
 	tunnel->sock = sk;
 
@@ -1504,7 +1506,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 
 		setup_udp_tunnel_sock(net, sock, &udp_cfg);
 	} else {
-		sk->sk_user_data = tunnel;
+		rcu_assign_sk_user_data(sk, tunnel);
 	}
 
 	tunnel->old_sk_destruct = sk->sk_destruct;
@@ -1518,6 +1520,7 @@ 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:
@@ -1525,6 +1528,8 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 		sock_release(sock);
 	else
 		sockfd_put(sock);
+
+	write_unlock(&sk->sk_callback_lock);
 err:
 	return ret;
 }
-- 
2.37.1

Re: [PATCH net v3] l2tp: Serialize access to sk_user_data with sk_callback_lock

[Paolo Abeni]

hello,

On Tue, 2022-08-23 at 12:14 +0200, Jakub Sitnicki wrote:
> sk->sk_user_data has multiple users, which are not compatible with each
> other. Writers must synchronize by grabbing the sk->sk_callback_lock.
> 
> l2tp currently fails to grab the lock when modifying the underlying tunnel
> socket. Fix it by adding appropriate locking.
> 
> We don't to grab the lock when l2tp clears sk_user_data, because it happens
> only in sk->sk_destruct, when the sock is going away.

l2tp can additionally clears sk_user_data in sk->sk_prot->close() via 
udp_lib_close() -> sk_common_release() -> sk->sk_prot->destroy() -> 
udp_destroy_sock() -> up->encap_destroy() -> l2tp_udp_encap_destroy().

That still happens at socket closing time, but when network has still
access to the sock itself. It should be safe as the other sk_user_data
users touch it only via fd, but perhaps a 'better safe the sorry'
approach could be relevant there?

Thanks!

Paolo

Re: [PATCH net v3] l2tp: Serialize access to sk_user_data with sk_callback_lock

[Jakub Sitnicki]

On Thu, Aug 25, 2022 at 12:23 PM +02, Paolo Abeni wrote:
> hello,
>
> On Tue, 2022-08-23 at 12:14 +0200, Jakub Sitnicki wrote:
>> sk->sk_user_data has multiple users, which are not compatible with each
>> other. Writers must synchronize by grabbing the sk->sk_callback_lock.
>> 
>> l2tp currently fails to grab the lock when modifying the underlying tunnel
>> socket. Fix it by adding appropriate locking.
>> 
>> We don't to grab the lock when l2tp clears sk_user_data, because it happens
>> only in sk->sk_destruct, when the sock is going away.
>
> l2tp can additionally clears sk_user_data in sk->sk_prot->close() via 
> udp_lib_close() -> sk_common_release() -> sk->sk_prot->destroy() -> 
> udp_destroy_sock() -> up->encap_destroy() -> l2tp_udp_encap_destroy().
>
> That still happens at socket closing time, but when network has still
> access to the sock itself. It should be safe as the other sk_user_data
> users touch it only via fd, but perhaps a 'better safe the sorry'
> approach could be relevant there?

Fair point. Let me add that.