udp: fix dst races with multicast early demux

Multicast dst are not cached. They carry DST_NOCACHE.

As mentioned in commit f8864972126899 ("ipv4: fix dst race in
sk_dst_get()"), these dst need special care before caching them
into a socket.

Caching them is allowed only if their refcnt was not 0, ie we
must use atomic_inc_not_zero()

Also, we must use READ_ONCE() to fetch sk->sk_rx_dst, as mentioned
in commit d0c294c53a771 ("tcp: prevent fetching dst twice in early demux
code")

Fixes: 421b3885bf6d ("udp: ipv4: Add udp early demux")
Tested-by: Gregory Hoggarth <Gregory.Hoggarth@alliedtelesis.co.nz>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Gregory Hoggarth <Gregory.Hoggarth@alliedtelesis.co.nz>
Reported-by: Alex Gartrell <agartrell@fb.com>
Cc: Michal Kubeček <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 83aa604..1b8c5ba 100644 (file)
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1995,12 +1995,19 @@ void udp_v4_early_demux(struct sk_buff *skb)
 
        skb->sk = sk;
        skb->destructor = sock_efree;
-       dst = sk->sk_rx_dst;
+       dst = READ_ONCE(sk->sk_rx_dst);
 
        if (dst)
                dst = dst_check(dst, 0);
-       if (dst)
-               skb_dst_set_noref(skb, dst);
+       if (dst) {
+               /* DST_NOCACHE can not be used without taking a reference */
+               if (dst->flags & DST_NOCACHE) {
+                       if (likely(atomic_inc_not_zero(&dst->__refcnt)))
+                               skb_dst_set(skb, dst);
+               } else {
+                       skb_dst_set_noref(skb, dst);
+               }
+       }
 }
 
 int udp_rcv(struct sk_buff *skb)