From 10e2eb878f3ca07ac2f05fa5ca5e6c4c9174a27a Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Sat, 1 Aug 2015 12:14:33 +0200 Subject: [PATCH] udp: fix dst races with multicast early demux MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Eric Dumazet Reported-by: Gregory Hoggarth Reported-by: Alex Gartrell Cc: Michal Kubeček Signed-off-by: David S. Miller --- 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) -- 2.20.1