From: Eric Dumazet <edumazet@google.com>
Date: Tue, 16 Jun 2015 14:59:11 +0000 (-0700)
Subject: packet: read num_members once in packet_rcv_fanout()
X-Git-Tag: v3.16.35~1833
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=95e32b4e001e98973d6b2068f91a28ec47e020a1;p=thirdparty%2Fkernel%2Fstable.git

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>
[ luis: backported to 3.16: used davem's backport to 3.14 ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 3eb786fd3f229..5d2e8d6517479 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1346,7 +1346,7 @@ 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;
 	unsigned int idx;