[PATCH 3.4 38/65] packet: read num_members once in packet_rcv_fanout()

[lizf@kernel.org]

From: Eric Dumazet <edumazet@google.com>

3.4.110-rc1 review patch.  If anyone has any objections, please let me know.

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


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>
---
 net/packet/af_packet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index dbe1715..af067d7 100644
--- 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;
 
-- 
1.9.1