netfilter: nf_tables: unlink table before deleting it

[ Upstream commit a499b03 ]

syzbot reports following UAF:
BUG: KASAN: use-after-free in memcmp+0x18f/0x1c0 lib/string.c:955
 nla_strcmp+0xf2/0x130 lib/nlattr.c:836
 nft_table_lookup.part.0+0x1a2/0x460 net/netfilter/nf_tables_api.c:570
 nft_table_lookup net/netfilter/nf_tables_api.c:4064 [inline]
 nf_tables_getset+0x1b3/0x860 net/netfilter/nf_tables_api.c:4064
 nfnetlink_rcv_msg+0x659/0x13f0 net/netfilter/nfnetlink.c:285
 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2504

Problem is that all get operations are lockless, so the commit_mutex
held by nft_rcv_nl_event() isn't enough to stop a parallel GET request
from doing read-accesses to the table object even after synchronize_rcu().

To avoid this, unlink the table first and store the table objects in
on-stack scratch space.

Fixes: 6001a93 ("netfilter: nftables: introduce table ownership")
Reported-and-tested-by: syzbot+f31660cf279b0557160c@syzkaller.appspotmail.com
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/netfilter/nf_tables_api.c

@@ -9599,7 +9599,6 @@ static void __nft_release_table(struct net *net, struct nft_table *table)
 		table->use--;
 		nf_tables_chain_destroy(&ctx);
 	}
-	list_del(&table->list);
 	nf_tables_table_destroy(&ctx);
 }
 
@@ -9612,38 +9611,47 @@ static void __nft_release_tables(struct net *net)
 		if (nft_table_has_owner(table))
 			continue;
 
+		list_del(&table->list);
+
 		__nft_release_table(net, table);
 	}
 }
 
 static int nft_rcv_nl_event(struct notifier_block *this, unsigned long event,
 			    void *ptr)
 {
+	struct nft_table *table, *to_delete[8];
 	struct nftables_pernet *nft_net;
 	struct netlink_notify *n = ptr;
-	struct nft_table *table, *nt;
 	struct net *net = n->net;
-	bool release = false;
+	unsigned int deleted;
+	bool restart = false;
 
 	if (event != NETLINK_URELEASE || n->protocol != NETLINK_NETFILTER)
 		return NOTIFY_DONE;
 
 	nft_net = nft_pernet(net);
+	deleted = 0;
 	mutex_lock(&nft_net->commit_mutex);
+again:
 	list_for_each_entry(table, &nft_net->tables, list) {
 		if (nft_table_has_owner(table) &&
 		    n->portid == table->nlpid) {
 			__nft_release_hook(net, table);
-			release = true;
+			list_del_rcu(&table->list);
+			to_delete[deleted++] = table;
+			if (deleted >= ARRAY_SIZE(to_delete))
+				break;
 		}
 	}
-	if (release) {
+	if (deleted) {
+		restart = deleted >= ARRAY_SIZE(to_delete);
 		synchronize_rcu();
-		list_for_each_entry_safe(table, nt, &nft_net->tables, list) {
-			if (nft_table_has_owner(table) &&
-			    n->portid == table->nlpid)
-				__nft_release_table(net, table);
-		}
+		while (deleted)
+			__nft_release_table(net, to_delete[--deleted]);
+
+		if (restart)
+			goto again;
 	}
 	mutex_unlock(&nft_net->commit_mutex);