netfilter: ipset: fix race condition between swap/destroy and kernel side add/del/test

Linkui Xiao reported that there's a race condition when ipset swap and destroy is
called, which can lead to crash in add/del/test element operations. Swap then
destroy are usual operations to replace a set with another one in a production
system. The issue can in some cases be reproduced with the script:

ipset create hash_ip1 hash:net family inet hashsize 1024 maxelem 1048576
ipset add hash_ip1 172.20.0.0/16
ipset add hash_ip1 192.168.0.0/16
iptables -A INPUT -m set --match-set hash_ip1 src -j ACCEPT
while [ 1 ]
do
	# ... Ongoing traffic...
        ipset create hash_ip2 hash:net family inet hashsize 1024 maxelem 1048576
        ipset add hash_ip2 172.20.0.0/16
        ipset swap hash_ip1 hash_ip2
        ipset destroy hash_ip2
        sleep 0.05
done

In the race case the possible order of the operations are

	CPU0			CPU1
	ip_set_test
				ipset swap hash_ip1 hash_ip2
				ipset destroy hash_ip2
	hash_net_kadt

Swap replaces hash_ip1 with hash_ip2 and then destroy removes hash_ip2 which
is the original hash_ip1. ip_set_test was called on hash_ip1 and because destroy
removed it, hash_net_kadt crashes.

The fix is to force ip_set_swap() to wait for all readers to finish accessing the
old set pointers by calling synchronize_rcu().

The first version of the patch was written by Linkui Xiao <[email protected]>.

v2: synchronize_rcu() is moved into ip_set_swap() in order not to burden
    ip_set_destroy() unnecessarily when all sets are destroyed.
v3: Florian Westphal pointed out that all netfilter hooks run with rcu_read_lock() held
    and em_ipset.c wraps the entire ip_set_test() in rcu read lock/unlock pair.
    So there's no need to extend the rcu read locked area in ipset itself.

Closes: https://lore.kernel.org/all/[email protected]/
Reported by: Linkui Xiao <[email protected]>
Signed-off-by: Jozsef Kadlecsik <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>

Author: Jozsef Kadlecsik

net/netfilter/ipset/ip_set_core.c

@@ -61,6 +61,8 @@ MODULE_ALIAS_NFNL_SUBSYS(NFNL_SUBSYS_IPSET);
 	ip_set_dereference((inst)->ip_set_list)[id]
 #define ip_set_ref_netlink(inst,id)	\
 	rcu_dereference_raw((inst)->ip_set_list)[id]
+#define ip_set_dereference_nfnl(p)	\
+	rcu_dereference_check(p, lockdep_nfnl_is_held(NFNL_SUBSYS_IPSET))
 
 /* The set types are implemented in modules and registered set types
  * can be found in ip_set_type_list. Adding/deleting types is
@@ -708,15 +710,10 @@ __ip_set_put_netlink(struct ip_set *set)
 static struct ip_set *
 ip_set_rcu_get(struct net *net, ip_set_id_t index)
 {
-	struct ip_set *set;
 	struct ip_set_net *inst = ip_set_pernet(net);
 
-	rcu_read_lock();
-	/* ip_set_list itself needs to be protected */
-	set = rcu_dereference(inst->ip_set_list)[index];
-	rcu_read_unlock();
-
-	return set;
+	/* ip_set_list and the set pointer need to be protected */
+	return ip_set_dereference_nfnl(inst->ip_set_list)[index];
 }
 
 static inline void
@@ -1397,6 +1394,9 @@ static int ip_set_swap(struct sk_buff *skb, const struct nfnl_info *info,
 	ip_set(inst, to_id) = from;
 	write_unlock_bh(&ip_set_ref_lock);
 
+	/* Make sure all readers of the old set pointers are completed. */
+	synchronize_rcu();
+
 	return 0;
 }