packet: read num_members once in packet_rcv_fanout()

commit f98f4514d07871da7a113dd9e3e330743fd70ae4 upstream.

We need to tell compiler it must not read f->num_members multiple
times. Otherwise testing if num is not zero is flaky, and we could
attempt an invalid divide by 0 in fanout_demux_cpu()

Note bug was present in packet_rcv_fanout_hash() and
packet_rcv_fanout_lb() but final 3.1 had a simple location
after commit 95ec3eb417115fb ("packet: Add 'cpu' fanout policy.")

Fixes: dc99f600698dc ("packet: Add fanout support.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[lizf: Backported to 3.4: use ACCESS_ONCE() instead of READ_ONCE()]
Signed-off-by: Zefan Li <lizefan@huawei.com>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index dbe1715c629fdef551def1be1d227836fef5fd34..af067d72dff3b5cc9f11e9cb22bf714ec1401b14 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1210,7 +1210,7 @@ static int packet_rcv_fanout(struct sk_buff *skb, struct net_device *dev,
                             struct packet_type *pt, struct net_device *orig_dev)
 {
        struct packet_fanout *f = pt->af_packet_priv;
-       unsigned int num = f->num_members;
+       unsigned int num = ACCESS_ONCE(f->num_members);
        struct packet_sock *po;
        struct sock *sk;