packet: in packet_do_bind, test fanout with bind_lock held

Once a socket has po->fanout set, it remains a member of the group
until it is destroyed. The prot_hook must be constant and identical
across sockets in the group.

If fanout_add races with packet_do_bind between the test of po->fanout
and taking the lock, the bind call may make type or dev inconsistent
with that of the fanout group.

Hold po->bind_lock when testing po->fanout to avoid this race.

I had to introduce artificial delay (local_bh_enable) to actually
observe the race.

Fixes: dc99f600698d ("packet: Add fanout support.")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index d288f52c53f7c1b16024e8369663ed4773c517c6..a10c2836465cf499e2b723419da25bc3ad07d69f 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -3069,13 +3069,15 @@ static int packet_do_bind(struct sock *sk, const char *name, int ifindex,
        int ret = 0;
        bool unlisted = false;
 
-       if (po->fanout)
-               return -EINVAL;
-
        lock_sock(sk);
        spin_lock(&po->bind_lock);
        rcu_read_lock();
 
+       if (po->fanout) {
+               ret = -EINVAL;
+               goto out_unlock;
+       }
+
        if (name) {
                dev = dev_get_by_name_rcu(sock_net(sk), name);
                if (!dev) {