netfilter: br_netfilter: Use nested-BH locking for brnf_frag_data_storage.

brnf_frag_data_storage is a per-CPU variable and relies on disabled BH
for its locking. Without per-CPU locking in local_bh_disable() on
PREEMPT_RT this data structure requires explicit locking.

Add a local_lock_t to the data structure and use local_lock_nested_bh()
for locking. This change adds only lockdep coverage and does not alter
the functional behaviour for !PREEMPT_RT.

Cc: Florian Westphal <[e-mail]>
Cc: Jozsef Kadlecsik <[e-mail]>
Cc: Nikolay Aleksandrov <[e-mail]>
Cc: Pablo Neira Ayuso <[e-mail]>
Cc: Roopa Prabhu <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/bridge/br_netfilter_hooks.c

···
 #define NF_BRIDGE_MAX_MAC_HEADER_LENGTH (PPPOE_SES_HLEN + ETH_HLEN)
 
 struct brnf_frag_data {
+	local_lock_t bh_lock;
 	char mac[NF_BRIDGE_MAX_MAC_HEADER_LENGTH];
 	u8 encap_size;
 	u8 size;
···
 	__be16 vlan_proto;
 };
 
-static DEFINE_PER_CPU(struct brnf_frag_data, brnf_frag_data_storage);
+static DEFINE_PER_CPU(struct brnf_frag_data, brnf_frag_data_storage) = {
+	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
+};
 
 static void nf_bridge_info_free(struct sk_buff *skb)
 {
···
 {
 	struct nf_bridge_info *nf_bridge = nf_bridge_info_get(skb);
 	unsigned int mtu, mtu_reserved;
+	int ret;
 
 	mtu_reserved = nf_bridge_mtu_reduction(skb);
 	mtu = skb->dev->mtu;
···
 
 		IPCB(skb)->frag_max_size = nf_bridge->frag_max_size;
 
+		local_lock_nested_bh(&brnf_frag_data_storage.bh_lock);
 		data = this_cpu_ptr(&brnf_frag_data_storage);
 
 		if (skb_vlan_tag_present(skb)) {
···
 		skb_copy_from_linear_data_offset(skb, -data->size, data->mac,
 						 data->size);
 
-		return br_nf_ip_fragment(net, sk, skb, br_nf_push_frag_xmit);
+		ret = br_nf_ip_fragment(net, sk, skb, br_nf_push_frag_xmit);
+		local_unlock_nested_bh(&brnf_frag_data_storage.bh_lock);
+		return ret;
 	}
 	if (IS_ENABLED(CONFIG_NF_DEFRAG_IPV6) &&
 	    skb->protocol == htons(ETH_P_IPV6)) {
···
 
 		IP6CB(skb)->frag_max_size = nf_bridge->frag_max_size;
 
+		local_lock_nested_bh(&brnf_frag_data_storage.bh_lock);
 		data = this_cpu_ptr(&brnf_frag_data_storage);
 		data->encap_size = nf_bridge_encap_header_len(skb);
 		data->size = ETH_HLEN + data->encap_size;
···
 		skb_copy_from_linear_data_offset(skb, -data->size, data->mac,
 						 data->size);
 
-		if (v6ops)
-			return v6ops->fragment(net, sk, skb, br_nf_push_frag_xmit);
+		if (v6ops) {
+			ret = v6ops->fragment(net, sk, skb, br_nf_push_frag_xmit);
+			local_unlock_nested_bh(&brnf_frag_data_storage.bh_lock);
+			return ret;
+		}
+		local_unlock_nested_bh(&brnf_frag_data_storage.bh_lock);
 
 		kfree_skb(skb);
 		return -EMSGSIZE;