[PATCH 3.16 160/294] l2tp: hold tunnel while handling genl TUNNEL_GET commands

[Ben Hutchings]

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

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

From: Guillaume Nault <g.na...@alphalink.fr>

commit 4e4b21da3acc68a7ea55f850cacc13706b7480e9 upstream.

Use l2tp_tunnel_get() instead of l2tp_tunnel_find() so that we get
a reference on the tunnel, preventing l2tp_tunnel_destruct() from
freeing it from under us.

Also move l2tp_tunnel_get() below nlmsg_new() so that we only take
the reference when needed.

Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
Signed-off-by: David S. Miller <da...@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
 net/l2tp/l2tp_netlink.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -349,34 +349,37 @@ static int l2tp_nl_cmd_tunnel_get(struct
 
        if (!info->attrs[L2TP_ATTR_CONN_ID]) {
                ret = -EINVAL;
-               goto out;
+               goto err;
        }
 
        tunnel_id = nla_get_u32(info->attrs[L2TP_ATTR_CONN_ID]);
 
-       tunnel = l2tp_tunnel_find(net, tunnel_id);
-       if (tunnel == NULL) {
-               ret = -ENODEV;
-               goto out;
-       }
-
        msg = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_KERNEL);
        if (!msg) {
                ret = -ENOMEM;
-               goto out;
+               goto err;
+       }
+
+       tunnel = l2tp_tunnel_get(net, tunnel_id);
+       if (!tunnel) {
+               ret = -ENODEV;
+               goto err_nlmsg;
        }
 
        ret = l2tp_nl_tunnel_send(msg, info->snd_portid, info->snd_seq,
                                  NLM_F_ACK, tunnel);
        if (ret < 0)
-               goto err_out;
+               goto err_nlmsg_tunnel;
+
+       l2tp_tunnel_dec_refcount(tunnel);
 
        return genlmsg_unicast(net, msg, info->snd_portid);
 
-err_out:
+err_nlmsg_tunnel:
+       l2tp_tunnel_dec_refcount(tunnel);
+err_nlmsg:
        nlmsg_free(msg);
-
-out:
+err:
        return ret;
 }