[PATCH net-next 12/16] netfilter: nft_connlimit: update the count if add was skipped

[Pablo Neira Ayuso <pablo@netfilter.org>]

From: Fernando Fernandez Mancera <fmancera@suse.de>

Connlimit expression can be used for all kind of packets and not only
for packets with connection state new. See this ruleset as example:

table ip filter {
        chain input {
                type filter hook input priority filter; policy accept;
                tcp dport 22 ct count over 4 counter
        }
}

Currently, if the connection count goes over the limit the counter will
count the packets. When a connection is closed, the connection count
won't decrement as it should because it is only updated for new
connections due to an optimization on __nf_conncount_add() that prevents
updating the list if the connection is duplicated.

To solve this problem, check whether the connection was skipped and if
so, update the list. Adjust count_tree() too so the same fix is applied
for xt_connlimit.

Fixes: 976afca1ceba ("netfilter: nf_conncount: Early exit in nf_conncount_lookup() and cleanup")
Closes: https://lore.kernel.org/netfilter/trinity-85c72a88-d762-46c3-be97-36f10e5d9796-1761173693813@3c-app-mailcom-bs12/
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/nf_conncount.c  | 12 ++++++++----
 net/netfilter/nft_connlimit.c | 13 +++++++++++--
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/nf_conncount.c b/net/netfilter/nf_conncount.c
index eabce7e141f8..81915ef99a83 100644
--- a/net/netfilter/nf_conncount.c
+++ b/net/netfilter/nf_conncount.c
@@ -179,7 +179,7 @@ static int __nf_conncount_add(struct net *net,
 	if (ct && nf_ct_is_confirmed(ct)) {
 		if (refcounted)
 			nf_ct_put(ct);
-		return 0;
+		return -EEXIST;
 	}
 
 	if ((u32)jiffies == list->last_gc)
@@ -408,7 +408,7 @@ insert_tree(struct net *net,
 			int ret;
 
 			ret = nf_conncount_add_skb(net, skb, l3num, &rbconn->list);
-			if (ret)
+			if (ret && ret != -EEXIST)
 				count = 0; /* hotdrop */
 			else
 				count = rbconn->list.count;
@@ -511,10 +511,14 @@ count_tree(struct net *net,
 			/* same source network -> be counted! */
 			ret = __nf_conncount_add(net, skb, l3num, &rbconn->list);
 			spin_unlock_bh(&rbconn->list.list_lock);
-			if (ret)
+			if (ret && ret != -EEXIST) {
 				return 0; /* hotdrop */
-			else
+			} else {
+				/* -EEXIST means add was skipped, update the list */
+				if (ret == -EEXIST)
+					nf_conncount_gc_list(net, &rbconn->list);
 				return rbconn->list.count;
+			}
 		}
 	}
 
diff --git a/net/netfilter/nft_connlimit.c b/net/netfilter/nft_connlimit.c
index 41770bde39d3..714a59485935 100644
--- a/net/netfilter/nft_connlimit.c
+++ b/net/netfilter/nft_connlimit.c
@@ -29,8 +29,17 @@ static inline void nft_connlimit_do_eval(struct nft_connlimit *priv,
 
 	err = nf_conncount_add_skb(nft_net(pkt), pkt->skb, nft_pf(pkt), priv->list);
 	if (err) {
-		regs->verdict.code = NF_DROP;
-		return;
+		if (err == -EEXIST) {
+			/* Call gc to update the list count if any connection has
+			 * been closed already. This is useful for softlimit
+			 * connections like limiting bandwidth based on a number
+			 * of open connections.
+			 */
+			nf_conncount_gc_list(nft_net(pkt), priv->list);
+		} else {
+			regs->verdict.code = NF_DROP;
+			return;
+		}
 	}
 
 	count = READ_ONCE(priv->list->count);
-- 
2.47.3