Commit 10e2eb87 authored Aug 01, 2015 by Eric Dumazet Committed by David S. Miller Aug 03, 2015

udp: fix dst races with multicast early demux



Multicast dst are not cached. They carry DST_NOCACHE.

As mentioned in commit f8864972 ("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 d0c294c5 ("tcp: prevent fetching dst twice in early demux
code")

Fixes: 421b3885 ("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>

parent 2fc09962

net/ipv4/udp.c

+10 −3

Original line number	Diff line number	Diff line
		@@ -1995,13 +1995,20 @@ 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)
		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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