[PATCH 3.2 73/94] l2tp: hold socket before dropping lock in l2tp_ip{, 6}_recv()

[Ben Hutchings]

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

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

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

commit a3c18422a4b4e108bcf6a2328f48867e1003fd95 upstream.

Socket must be held while under the protection of the l2tp lock; there
is no guarantee that sk remains valid after the read_unlock_bh() call.

Same issue for l2tp_ip and l2tp_ip6.

Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
Signed-off-by: David S. Miller <da...@davemloft.net>
[bwh: Backported to 3.2:
 - Drop changes in l2tp_ip6.c
 - Adjust context]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -197,14 +197,15 @@ pass_up:
 
                read_lock_bh(&l2tp_ip_lock);
                sk = __l2tp_ip_bind_lookup(&init_net, iph->daddr, 0, tunnel_id);
+               if (!sk) {
+                       read_unlock_bh(&l2tp_ip_lock);
+                       goto discard;
+               }
+
+               sock_hold(sk);
                read_unlock_bh(&l2tp_ip_lock);
        }
 
-       if (sk == NULL)
-               goto discard;
-
-       sock_hold(sk);
-
        if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
                goto discard_put;