udp: ipv4: must add synchronization in udp_sk_rx_dst_set()

Unlike TCP, UDP input path does not hold the socket lock.

Before messing with sk->sk_rx_dst, we must use a spinlock, otherwise
multiple cpus could leak a refcount.

This patch also takes care of renewing a stale dst entry.
(When the sk->sk_rx_dst would not be used by IP early demux)

Fixes: 421b3885bf6d ("udp: ipv4: Add udp early demux")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 16 insertions, 6 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 16d246a51a02..62c19fdd102d 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1599,12 +1599,21 @@ static void flush_stack(struct sock **stack, unsigned int count,
                 kfree_skb(skb1);
 }
 
-static void udp_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
+/* For TCP sockets, sk_rx_dst is protected by socket lock
+ * For UDP, we use sk_dst_lock to guard against concurrent changes.
+ */
+static void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
 {
-        struct dst_entry *dst = skb_dst(skb);
+        struct dst_entry *old;
 
-        dst_hold(dst);
-        sk->sk_rx_dst = dst;
+        spin_lock(&sk->sk_dst_lock);
+        old = sk->sk_rx_dst;
+        if (likely(old != dst)) {
+                dst_hold(dst);
+                sk->sk_rx_dst = dst;
+                dst_release(old);
+        }
+        spin_unlock(&sk->sk_dst_lock);
 }
 
 /*
@@ -1737,10 +1746,11 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 
         sk = skb_steal_sock(skb);
         if (sk) {
+                struct dst_entry *dst = skb_dst(skb);
                 int ret;
 
-                if (unlikely(sk->sk_rx_dst == NULL))
-                        udp_sk_rx_dst_set(sk, skb);
+                if (unlikely(sk->sk_rx_dst != dst))
+                        udp_sk_rx_dst_set(sk, dst);
 
                 ret = udp_queue_rcv_skb(sk, skb);
                 sock_put(sk);