Subject: [PATCH 23/25] udp: fix dst races with multicast early demux

From: Eric Dumazet <[email-masked]>
Date: Sat, 1 Aug 2015 12:14:33 +0200

[ Upstream commit 10e2eb878f3ca07ac2f05fa5ca5e6c4c9174a27a ]

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 <[email-masked]>
Signed-off-by: Eric Dumazet <[email-masked]>
Reported-by: Gregory Hoggarth <[email-masked]>
Reported-by: Alex Gartrell <[email-masked]>
Cc: Michal Kubeček <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/udp.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 21a3a9e..947dcae 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1957,12 +1957,19 @@ void udp_v4_early_demux(struct sk_buff *skb)
 
 	skb->sk = sk;
 	skb->destructor = sock_edemux;
-	dst = sk->sk_rx_dst;
+	dst = ACCESS_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)
-- 
2.1.0