netdev - [PATCH net] udp: fix dst races with multicast early demux

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Date:	Sat, 01 Aug 2015 12:14:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Gregory Hoggarth <Gregory.Hoggarth@...iedtelesis.co.nz>
Cc:	Shawn Bohrer <sbohrer@...advisors.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"alexgartrell@...il.com" <alexgartrell@...il.com>,
	Michal Kubeček <mkubecek@...e.cz>
Subject: [PATCH net] udp: fix dst races with multicast early demux

From: Eric Dumazet <edumazet@...gle.com>

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")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Gregory Hoggarth <Gregory.Hoggarth@...iedtelesis.co.nz>
Reported-by: Alex Gartrell <agartrell@...com>
Cc: Michal Kubeček <mkubecek@...e.cz>
---
David : I will be on vacation for following 7 days, no internet access.
Please wait for tests done by Gregory & Alex before merging this ?
Thanks !

 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 83aa604f9273..1b8c5ba7d5f7 100644
--- 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)


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