netfilter: nat: fix double register in masquerade modules

There is a reference counter to ensure that masquerade modules register
notifiers only once. However, the existing reference counter approach is
not safe, test commands are:

   while :
   do
   	   modprobe ip6t_MASQUERADE &
	   modprobe nft_masq_ipv6 &
	   modprobe -rv ip6t_MASQUERADE &
	   modprobe -rv nft_masq_ipv6 &
   done

numbers below represent the reference counter.
--------------------------------------------------------
CPU0        CPU1        CPU2        CPU3        CPU4
[insmod]    [insmod]    [rmmod]     [rmmod]     [insmod]
--------------------------------------------------------
0->1
register    1->2
            returns     2->1
			returns     1->0
                                                0->1
                                                register <--
                                    unregister
--------------------------------------------------------

The unregistation of CPU3 should be processed before the
registration of CPU4.

In order to fix this, use a mutex instead of reference counter.

splat looks like:
[  323.869557] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [modprobe:1381]
[  323.869574] Modules linked in: nf_tables(+) nf_nat_ipv6(-) nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 n]
[  323.869574] irq event stamp: 194074
[  323.898930] hardirqs last  enabled at (194073): [<ffffffff90004a0d>] trace_hardirqs_on_thunk+0x1a/0x1c
[  323.898930] hardirqs last disabled at (194074): [<ffffffff90004a29>] trace_hardirqs_off_thunk+0x1a/0x1c
[  323.898930] softirqs last  enabled at (182132): [<ffffffff922006ec>] __do_softirq+0x6ec/0xa3b
[  323.898930] softirqs last disabled at (182109): [<ffffffff90193426>] irq_exit+0x1a6/0x1e0
[  323.898930] CPU: 0 PID: 1381 Comm: modprobe Not tainted 4.20.0-rc2+ #27
[  323.898930] RIP: 0010:raw_notifier_chain_register+0xea/0x240
[  323.898930] Code: 3c 03 0f 8e f2 00 00 00 44 3b 6b 10 7f 4d 49 bc 00 00 00 00 00 fc ff df eb 22 48 8d 7b 10 488
[  323.898930] RSP: 0018:ffff888101597218 EFLAGS: 00000206 ORIG_RAX: ffffffffffffff13
[  323.898930] RAX: 0000000000000000 RBX: ffffffffc04361c0 RCX: 0000000000000000
[  323.898930] RDX: 1ffffffff26132ae RSI: ffffffffc04aa3c0 RDI: ffffffffc04361d0
[  323.898930] RBP: ffffffffc04361c8 R08: 0000000000000000 R09: 0000000000000001
[  323.898930] R10: ffff8881015972b0 R11: fffffbfff26132c4 R12: dffffc0000000000
[  323.898930] R13: 0000000000000000 R14: 1ffff110202b2e44 R15: ffffffffc04aa3c0
[  323.898930] FS:  00007f813ed41540(0000) GS:ffff88811ae00000(0000) knlGS:0000000000000000
[  323.898930] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  323.898930] CR2: 0000559bf2c9f120 CR3: 000000010bc80000 CR4: 00000000001006f0
[  323.898930] Call Trace:
[  323.898930]  ? atomic_notifier_chain_register+0x2d0/0x2d0
[  323.898930]  ? down_read+0x150/0x150
[  323.898930]  ? sched_clock_cpu+0x126/0x170
[  323.898930]  ? nf_tables_core_module_init+0xe4/0xe4 [nf_tables]
[  323.898930]  ? nf_tables_core_module_init+0xe4/0xe4 [nf_tables]
[  323.898930]  register_netdevice_notifier+0xbb/0x790
[  323.898930]  ? __dev_close_many+0x2d0/0x2d0
[  323.898930]  ? __mutex_unlock_slowpath+0x17f/0x740
[  323.898930]  ? wait_for_completion+0x710/0x710
[  323.898930]  ? nf_tables_core_module_init+0xe4/0xe4 [nf_tables]
[  323.898930]  ? up_write+0x6c/0x210
[  323.898930]  ? nf_tables_core_module_init+0xe4/0xe4 [nf_tables]
[  324.127073]  ? nf_tables_core_module_init+0xe4/0xe4 [nf_tables]
[  324.127073]  nft_chain_filter_init+0x1e/0xe8a [nf_tables]
[  324.127073]  nf_tables_module_init+0x37/0x92 [nf_tables]
[ ... ]

Fixes: 8dd33cc93ec9 ("netfilter: nf_nat: generalize IPv4 masquerading support for nf_tables")
Fixes: be6b635cd674 ("netfilter: nf_nat: generalize IPv6 masquerading support for nf_tables")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_nat_masquerade_ipv4.c

@@ -147,15 +147,17 @@ static struct notifier_block masq_inet_notifier = {
 	.notifier_call	= masq_inet_event,
 };
 
-static atomic_t masquerade_notifier_refcount = ATOMIC_INIT(0);
+static int masq_refcnt;
+static DEFINE_MUTEX(masq_mutex);
 
 int nf_nat_masquerade_ipv4_register_notifier(void)
 {
-	int ret;
+	int ret = 0;
 
+	mutex_lock(&masq_mutex);
 	/* check if the notifier was already set */
-	if (atomic_inc_return(&masquerade_notifier_refcount) > 1)
-		return 0;
+	if (++masq_refcnt > 1)
+		goto out_unlock;
 
 	/* Register for device down reports */
 	ret = register_netdevice_notifier(&masq_dev_notifier);
@@ -166,22 +168,29 @@ int nf_nat_masquerade_ipv4_register_notifier(void)
 	if (ret)
 		goto err_unregister;
 
+	mutex_unlock(&masq_mutex);
 	return ret;
+
 err_unregister:
 	unregister_netdevice_notifier(&masq_dev_notifier);
 err_dec:
-	atomic_dec(&masquerade_notifier_refcount);
+	masq_refcnt--;
+out_unlock:
+	mutex_unlock(&masq_mutex);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(nf_nat_masquerade_ipv4_register_notifier);
 
 void nf_nat_masquerade_ipv4_unregister_notifier(void)
 {
+	mutex_lock(&masq_mutex);
 	/* check if the notifier still has clients */
-	if (atomic_dec_return(&masquerade_notifier_refcount) > 0)
-		return;
+	if (--masq_refcnt > 0)
+		goto out_unlock;
 
 	unregister_netdevice_notifier(&masq_dev_notifier);
 	unregister_inetaddr_notifier(&masq_inet_notifier);
+out_unlock:
+	mutex_unlock(&masq_mutex);
 }
 EXPORT_SYMBOL_GPL(nf_nat_masquerade_ipv4_unregister_notifier);

net/ipv6/netfilter/nf_nat_masquerade_ipv6.c

@@ -175,15 +175,17 @@ static struct notifier_block masq_inet6_notifier = {
 	.notifier_call	= masq_inet6_event,
 };
 
-static atomic_t masquerade_notifier_refcount = ATOMIC_INIT(0);
+static int masq_refcnt;
+static DEFINE_MUTEX(masq_mutex);
 
 int nf_nat_masquerade_ipv6_register_notifier(void)
 {
-	int ret;
+	int ret = 0;
 
+	mutex_lock(&masq_mutex);
 	/* check if the notifier is already set */
-	if (atomic_inc_return(&masquerade_notifier_refcount) > 1)
-		return 0;
+	if (++masq_refcnt > 1)
+		goto out_unlock;
 
 	ret = register_netdevice_notifier(&masq_dev_notifier);
 	if (ret)
@@ -193,22 +195,29 @@ int nf_nat_masquerade_ipv6_register_notifier(void)
 	if (ret)
 		goto err_unregister;
 
+	mutex_unlock(&masq_mutex);
 	return ret;
+
 err_unregister:
 	unregister_netdevice_notifier(&masq_dev_notifier);
 err_dec:
-	atomic_dec(&masquerade_notifier_refcount);
+	masq_refcnt--;
+out_unlock:
+	mutex_unlock(&masq_mutex);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(nf_nat_masquerade_ipv6_register_notifier);
 
 void nf_nat_masquerade_ipv6_unregister_notifier(void)
 {
+	mutex_lock(&masq_mutex);
 	/* check if the notifier still has clients */
-	if (atomic_dec_return(&masquerade_notifier_refcount) > 0)
-		return;
+	if (--masq_refcnt > 0)
+		goto out_unlock;
 
 	unregister_inet6addr_notifier(&masq_inet6_notifier);
 	unregister_netdevice_notifier(&masq_dev_notifier);
+out_unlock:
+	mutex_unlock(&masq_mutex);
 }
 EXPORT_SYMBOL_GPL(nf_nat_masquerade_ipv6_unregister_notifier);