[PATCH 4.0 11/55] packet: read num_members once in packet_rcv_fanout()

Greg Kroah-Hartman
2015-07-08T08:18:54Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit f98f4514d07871da7a113dd9e3e330743fd70ae4 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/packet/af_packet.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1347,7 +1347,7 @@ static int packet_rcv_fanout(struct sk_b
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 = READ_ONCE(f->num_members);
struct packet_sock *po;
unsigned int idx;