linux-kernel - [PATCH 3.2 137/164] packet: read num_members once in packet_rcv

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Date:	Sun, 02 Aug 2015 01:02:38 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "David S. Miller" <davem@...emloft.net>,
	"Willem de Bruijn" <willemb@...gle.com>,
	"Eric Dumazet" <edumazet@...gle.com>
Subject: [PATCH 3.2 137/164] packet: read num_members once in
 packet_rcv_fanout()

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

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

From: Eric Dumazet <edumazet@...gle.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@...gle.com>
Cc: Willem de Bruijn <willemb@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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
@@ -1211,7 +1211,7 @@ 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 = ACCESS_ONCE(f->num_members);
 	struct packet_sock *po;
 	struct sock *sk;
 

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