udp: fix a race in encap_rcv handling

udp_queue_rcv_skb() has a possible race in encap_rcv handling, since
this pointer can be changed anytime.

We should use ACCESS_ONCE() to close the race.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 3 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 131d8a720086..ab0966df1e2a 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1397,6 +1397,8 @@ int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
         nf_reset(skb);
 
         if (up->encap_type) {
+                int (*encap_rcv)(struct sock *sk, struct sk_buff *skb);
+
                 /*
                  * This is an encapsulation socket so pass the skb to
                  * the socket's udp_encap_rcv() hook. Otherwise, just
@@ -1409,11 +1411,11 @@ int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
                  */
 
                 /* if we're overly short, let UDP handle it */
-                if (skb->len > sizeof(struct udphdr) &&
-                    up->encap_rcv != NULL) {
+                encap_rcv = ACCESS_ONCE(up->encap_rcv);
+                if (skb->len > sizeof(struct udphdr) && encap_rcv != NULL) {
                         int ret;
 
-                        ret = (*up->encap_rcv)(sk, skb);
+                        ret = encap_rcv(sk, skb);
                         if (ret <= 0) {
                                 UDP_INC_STATS_BH(sock_net(sk),
                                                  UDP_MIB_INDATAGRAMS,