netfilter: xt_hashlimit: reduce hashlimit_mutex scope for htable_put()

It is unnecessary to hold hashlimit_mutex for htable_destroy()
as it is already removed from the global hashtable and its
refcount is already zero.

Also, switch hinfo->use to refcount_t so that we don't have
to hold the mutex until it reaches zero in htable_put().

Reported-and-tested-by: syzbot+adf6c6c2be1c3a718121@syzkaller.appspotmail.com
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_hashlimit.c

@@ -36,6 +36,7 @@
 #include <linux/netfilter_ipv6/ip6_tables.h>
 #include <linux/mutex.h>
 #include <linux/kernel.h>
+#include <linux/refcount.h>
 #include <uapi/linux/netfilter/xt_hashlimit.h>
 
 #define XT_HASHLIMIT_ALL (XT_HASHLIMIT_HASH_DIP | XT_HASHLIMIT_HASH_DPT | \
@@ -114,7 +115,7 @@ struct dsthash_ent {
 
 struct xt_hashlimit_htable {
 	struct hlist_node node;		/* global list of all htables */
-	int use;
+	refcount_t use;
 	u_int8_t family;
 	bool rnd_initialized;
 
@@ -315,7 +316,7 @@ static int htable_create(struct net *net, struct hashlimit_cfg3 *cfg,
 	for (i = 0; i < hinfo->cfg.size; i++)
 		INIT_HLIST_HEAD(&hinfo->hash[i]);
 
-	hinfo->use = 1;
+	refcount_set(&hinfo->use, 1);
 	hinfo->count = 0;
 	hinfo->family = family;
 	hinfo->rnd_initialized = false;
@@ -420,7 +421,7 @@ static struct xt_hashlimit_htable *htable_find_get(struct net *net,
 	hlist_for_each_entry(hinfo, &hashlimit_net->htables, node) {
 		if (!strcmp(name, hinfo->name) &&
 		    hinfo->family == family) {
-			hinfo->use++;
+			refcount_inc(&hinfo->use);
 			return hinfo;
 		}
 	}
@@ -429,12 +430,11 @@ static struct xt_hashlimit_htable *htable_find_get(struct net *net,
 
 static void htable_put(struct xt_hashlimit_htable *hinfo)
 {
-	mutex_lock(&hashlimit_mutex);
-	if (--hinfo->use == 0) {
+	if (refcount_dec_and_mutex_lock(&hinfo->use, &hashlimit_mutex)) {
 		hlist_del(&hinfo->node);
+		mutex_unlock(&hashlimit_mutex);
 		htable_destroy(hinfo);
 	}
-	mutex_unlock(&hashlimit_mutex);
 }
 
 /* The algorithm used is the Simple Token Bucket Filter (TBF)