Fix sel_netnode_insert suspicious rcu dereference.

[Dave Jones <davej@redhat.com>]

I reported this a year ago (https://lkml.org/lkml/2011/4/20/308).
It's still a problem apparently ...

===============================
[ INFO: suspicious RCU usage. ]
3.5.0-rc1+ #63 Not tainted
-------------------------------
security/selinux/netnode.c:178 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 1, debug_locks = 0
1 lock held by trinity-child1/8750:
 #0:  (sel_netnode_lock){+.....}, at: [<ffffffff812d8f8a>] sel_netnode_sid+0x16a/0x3e0

stack backtrace:
Pid: 8750, comm: trinity-child1 Not tainted 3.5.0-rc1+ #63
Call Trace:
 [<ffffffff810cec2d>] lockdep_rcu_suspicious+0xfd/0x130
 [<ffffffff812d91d1>] sel_netnode_sid+0x3b1/0x3e0
 [<ffffffff812d8e20>] ? sel_netnode_find+0x1a0/0x1a0
 [<ffffffff812d24a6>] selinux_socket_bind+0xf6/0x2c0
 [<ffffffff810cd1dd>] ? trace_hardirqs_off+0xd/0x10
 [<ffffffff810cdb55>] ? lock_release_holdtime.part.9+0x15/0x1a0
 [<ffffffff81093841>] ? lock_hrtimer_base+0x31/0x60
 [<ffffffff812c9536>] security_socket_bind+0x16/0x20
 [<ffffffff815550ca>] sys_bind+0x7a/0x100
 [<ffffffff816c03d5>] ? sysret_check+0x22/0x5d
 [<ffffffff810d392d>] ? trace_hardirqs_on_caller+0x10d/0x1a0
 [<ffffffff8133b09e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 [<ffffffff816c03a9>] system_call_fastpath+0x16/0x1b

This patch below does what Paul McKenney suggested in the previous thread.

Signed-off-by: Dave Jones <davej@redhat.com>

diff --git a/security/selinux/netnode.c b/security/selinux/netnode.c
index 28f911c..c5454c0 100644
--- a/security/selinux/netnode.c
+++ b/security/selinux/netnode.c
@@ -174,7 +174,8 @@ static void sel_netnode_insert(struct sel_netnode *node)
 	if (sel_netnode_hash[idx].size == SEL_NETNODE_HASH_BKT_LIMIT) {
 		struct sel_netnode *tail;
 		tail = list_entry(
-			rcu_dereference(sel_netnode_hash[idx].list.prev),
+			rcu_dereference_protected(sel_netnode_hash[idx].list.prev,
+						  lockdep_is_held(&sel_netnode_lock)),
 			struct sel_netnode, list);
 		list_del_rcu(&tail->list);
 		kfree_rcu(tail, rcu);