l2tp: use rcu_dereference_sk_user_data() in l2tp_udp_encap_recv() [Linux 5.1]

l2tp: use rcu_dereference_sk_user_data() in l2tp_udp_encap_recv() [Linux 5.1]

This Linux kernel change "l2tp: use rcu_dereference_sk_user_data() in l2tp_udp_encap_recv()" is included in the Linux 5.1 release. This change is authored by Eric Dumazet <edumazet [at] google.com> on Tue Apr 23 09:43:26 2019 -0700. The commit for this change in Linux stable tree is c1c4772 (patch).

l2tp: use rcu_dereference_sk_user_data() in l2tp_udp_encap_recv()

Canonical way to fetch sk_user_data from an encap_rcv() handler called
from UDP stack in rcu protected section is to use rcu_dereference_sk_user_data(),
otherwise compiler might read it multiple times.

Fixes: d00fa9adc528 ("il2tp: fix races with tunnel socket close")
Signed-off-by: Eric Dumazet <[email protected]>
Cc: James Chapman <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/l2tp/l2tp_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index fed6bec..aee33d1 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -909,7 +909,7 @@ int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
 {
    struct l2tp_tunnel *tunnel;

-   tunnel = l2tp_tunnel(sk);
+   tunnel = rcu_dereference_sk_user_data(sk);
    if (tunnel == NULL)
        goto pass_up;

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