netfilter: nf_tables: GC transaction race with netns dismantle

jira VULN-430
cve CVE-2023-4244
commit-author Pablo Neira Ayuso <[email protected]>
commit 02c6c24

Use maybe_get_net() since GC workqueue might race with netns exit path.

Fixes: 5f68718 ("netfilter: nf_tables: GC transaction API to avoid race with control plane")
	Signed-off-by: Pablo Neira Ayuso <[email protected]>
	Signed-off-by: Florian Westphal <[email protected]>
(cherry picked from commit 02c6c24)
	Signed-off-by: Marcin Wcisło <[email protected]>

Author: pvts-mat

net/netfilter/nf_tables_api.c

@@ -9170,9 +9170,14 @@ struct nft_trans_gc *nft_trans_gc_alloc(struct nft_set *set,
 	if (!trans)
 		return NULL;
 
+	trans->net = maybe_get_net(net);
+	if (!trans->net) {
+		kfree(trans);
+		return NULL;
+	}
+
 	refcount_inc(&set->refs);
 	trans->set = set;
-	trans->net = get_net(net);
 	trans->seq = gc_seq;
 
 	return trans;