[PATCH 6.1] netfilter: nf_tables: fix UAF in nf_tables_netdev_event walker

[Ma Xinmeng]

This is a 6.1-only fix. Mainline already fixed this bug in fc0133428e7a
("netfilter: nf_tables: Tolerate chains with no remaining hooks"), which
6.1 cannot take because it lacks 207296f1a03b ("netfilter: nf_tables:
allow to create netdev chain without device") and b9703ed44ffb
("netfilter: nf_tables: support for adding new devices to an existing
netdev chain"). So 6.1 keeps dropping the chain on the last
NETDEV_UNREGISTER, and this patch only makes the walker safe.

nf_tables_netdev_event() walks table->chains with
list_for_each_entry_safe(). On the last NETDEV_UNREGISTER for a base
chain, nft_netdev_event() calls __nft_release_basechain(), which removes
that base chain (nft_chain_del()) and destroys its rules. A JUMP/GOTO rule
targeting an NFT_CHAIN_BINDING chain deactivates and frees that successor
(nft_immediate_destroy() -> nf_tables_chain_destroy()), so the walker's
saved "nr" iterator can point at freed memory, triggering a
slab-use-after-free.

Fix it by restarting the table->chains walk whenever nft_netdev_event()
released a chain. __nft_release_basechain() removes the base chain from
the list before returning, so a restart cannot revisit the freed chain and
the walk terminates.

Signed-off-by: Ma Xinmeng <1564938642@xxxxxx>
---
 net/netfilter/nft_chain_filter.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/net/netfilter/nft_chain_filter.c b/net/netfilter/nft_chain_filter.c
index d170758..ca6450a 100644
--- a/net/netfilter/nft_chain_filter.c
+++ b/net/netfilter/nft_chain_filter.c
@@ -318,7 +318,7 @@ static const struct nft_chain_type nft_chain_filter_netdev = {
 	},
 };
 
-static void nft_netdev_event(unsigned long event, struct net_device *dev,
+static bool nft_netdev_event(unsigned long event, struct net_device *dev,
 			     struct nft_ctx *ctx)
 {
 	struct nft_base_chain *basechain = nft_base_chain(ctx->chain);
@@ -326,7 +326,7 @@ static void nft_netdev_event(unsigned long event, struct net_device *dev,
 	int n = 0;
 
 	if (event != NETDEV_UNREGISTER)
-		return;
+		return false;
 
 	list_for_each_entry(hook, &basechain->hook_list, list) {
 		if (hook->ops.dev == dev)
@@ -335,7 +335,7 @@ static void nft_netdev_event(unsigned long event, struct net_device *dev,
 		n++;
 	}
 	if (!found)
-		return;
+		return false;
 
 	if (n > 1) {
 		if (!(ctx->chain->table->flags & NFT_TABLE_F_DORMANT))
@@ -343,7 +343,7 @@ static void nft_netdev_event(unsigned long event, struct net_device *dev,
 
 		list_del_rcu(&found->list);
 		kfree_rcu(found, rcu);
-		return;
+		return false;
 	}
 
 	/* UNREGISTER events are also happening on netns exit.
@@ -353,6 +353,8 @@ static void nft_netdev_event(unsigned long event, struct net_device *dev,
 	 * so we cannot skip exiting net namespaces.
 	 */
 	__nft_release_basechain(ctx);
+
+	return true;
 }
 
 static int nf_tables_netdev_event(struct notifier_block *this,
@@ -380,6 +382,7 @@ static int nf_tables_netdev_event(struct notifier_block *this,
 
 		ctx.family = table->family;
 		ctx.table = table;
+restart:
 		list_for_each_entry_safe(chain, nr, &table->chains, list) {
 			if (!nft_is_base_chain(chain))
 				continue;
@@ -390,7 +393,8 @@ static int nf_tables_netdev_event(struct notifier_block *this,
 				continue;
 
 			ctx.chain = chain;
-			nft_netdev_event(event, dev, &ctx);
+			if (nft_netdev_event(event, dev, &ctx))
+				goto restart;
 		}
 	}
 	mutex_unlock(&nft_net->commit_mutex);
-- 
2.49.0.windows.1