Patch "netfilter: invoke synchronize_rcu after set the _hook_ to NULL" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

netfilter: invoke synchronize_rcu after set the _hook_ to NULL

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
netfilter-invoke-synchronize_rcu-after-set-the-_hook_-to-null.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Liping Zhang <[email protected]>
Date: Sat, 25 Mar 2017 08:53:12 +0800
Subject: netfilter: invoke synchronize_rcu after set the _hook_ to NULL

From: Liping Zhang <[email protected]>

[ Upstream commit 3b7dabf029478bb80507a6c4500ca94132a2bc0b ]

Otherwise, another CPU may access the invalid pointer. For example:
CPU0 CPU1
- rcu_read_lock();
- pfunc = _hook_;
_hook_ = NULL; -
mod unload -
- pfunc(); // invalid, panic
- rcu_read_unlock();

So we must call synchronize_rcu() to wait the rcu reader to finish.

Also note, in nf_nat_snmp_basic_fini, synchronize_rcu() will be invoked by later nf_conntrack_helper_unregister, but I'm inclined to add a explicit synchronize_rcu after set the nf_nat_snmp_hook to NULL. Depend on such obscure assumptions is not a good idea.

Last, in nfnetlink_cttimeout, we use kfree_rcu to free the time object, so in cttimeout_exit, invoking rcu_barrier() is not necessary at all, remove it too.

---
 net/ipv4/netfilter/nf_nat_snmp_basic.c |    1 +
 net/netfilter/nf_conntrack_ecache.c    |    2 ++
 net/netfilter/nf_conntrack_netlink.c   |    1 +
 net/netfilter/nf_nat_core.c            |    2 ++
 net/netfilter/nfnetlink_cttimeout.c    |    2 +-
 5 files changed, 7 insertions(+), 1 deletion(-)

@@ -1304,6 +1304,7 @@ static int __init nf_nat_snmp_basic_init
 static void __exit nf_nat_snmp_basic_fini(void)
 {
 	RCU_INIT_POINTER(nf_nat_snmp_hook, NULL);
+	synchronize_rcu();
 	nf_conntrack_helper_unregister(&snmp_trap_helper);
 }
 

@@ -200,6 +200,7 @@ void nf_conntrack_unregister_notifier(st
 	BUG_ON(notify != new);
 	RCU_INIT_POINTER(net->ct.nf_conntrack_event_cb, NULL);
 	mutex_unlock(&nf_ct_ecache_mutex);
+	/* synchronize_rcu() is called from ctnetlink_exit. */
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_unregister_notifier);
 

@@ -236,6 +237,7 @@ void nf_ct_expect_unregister_notifier(st
 	BUG_ON(notify != new);
 	RCU_INIT_POINTER(net->ct.nf_expect_event_cb, NULL);
 	mutex_unlock(&nf_ct_ecache_mutex);
+	/* synchronize_rcu() is called from ctnetlink_exit. */
 }
 EXPORT_SYMBOL_GPL(nf_ct_expect_unregister_notifier);
 

@@ -3415,6 +3415,7 @@ static void __exit ctnetlink_exit(void)
 #ifdef CONFIG_NETFILTER_NETLINK_GLUE_CT
 	RCU_INIT_POINTER(nfnl_ct_hook, NULL);
 #endif
+	synchronize_rcu();
 }
 
 module_init(ctnetlink_init);

@@ -892,6 +892,8 @@ static void __exit nf_nat_cleanup(void)
 #ifdef CONFIG_XFRM
 	RCU_INIT_POINTER(nf_nat_decode_session_hook, NULL);
 #endif
+	synchronize_rcu();
+
 	for (i = 0; i < NFPROTO_NUMPROTO; i++)
 		kfree(nf_nat_l4protos[i]);
 	synchronize_net();

@@ -611,8 +611,8 @@ static void __exit cttimeout_exit(void)
 #ifdef CONFIG_NF_CONNTRACK_TIMEOUT
 	RCU_INIT_POINTER(nf_ct_timeout_find_get_hook, NULL);
 	RCU_INIT_POINTER(nf_ct_timeout_put_hook, NULL);
+	synchronize_rcu();
 #endif /* CONFIG_NF_CONNTRACK_TIMEOUT */
-	rcu_barrier();
 }
 
 module_init(cttimeout_init);

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/netfilter-invoke-synchronize_rcu-after-set-the-_hook_-to-null.patch queue-4.4/netfilter-nfnl_cthelper-fix-incorrect-helper-expect_class_max.patch