[PATCH 3.19.y-ckt 092/102] udp: fix dst races with multicast early demux

[Kamal Mostafa]

3.19.8-ckt7 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet at google.com>

commit 10e2eb878f3ca07ac2f05fa5ca5e6c4c9174a27a upstream.

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 at alliedtelesis.co.nz>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Reported-by: Gregory Hoggarth <Gregory.Hoggarth at alliedtelesis.co.nz>
Reported-by: Alex Gartrell <agartrell at fb.com>
Cc: Michal Kubeček <mkubecek at suse.cz>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 e0737ac..75f76d7 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2001,12 +2001,19 @@ 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)
-- 
1.9.1