netfilter: nf_tables: skip netdev events generated on netns removal

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2028203
Upstream Status: commit 68a3765

commit 68a3765
Author: Florian Westphal <[email protected]>
Date:   Wed Oct 6 16:20:34 2021 +0200

    netfilter: nf_tables: skip netdev events generated on netns removal

    syzbot reported following (harmless) WARN:

     WARNING: CPU: 1 PID: 2648 at net/netfilter/core.c:468
      nft_netdev_unregister_hooks net/netfilter/nf_tables_api.c:230 [inline]
      nf_tables_unregister_hook include/net/netfilter/nf_tables.h:1090 [inline]
      __nft_release_basechain+0x138/0x640 net/netfilter/nf_tables_api.c:9524
      nft_netdev_event net/netfilter/nft_chain_filter.c:351 [inline]
      nf_tables_netdev_event+0x521/0x8a0 net/netfilter/nft_chain_filter.c:382

    reproducer:
    unshare -n bash -c 'ip link add br0 type bridge; nft add table netdev t ; \
     nft add chain netdev t ingress \{ type filter hook ingress device "br0" \
     priority 0\; policy drop\; \}'

    Problem is that when netns device exit hooks create the UNREGISTER
    event, the .pre_exit hook for nf_tables core has already removed the
    base hook.  Notifier attempts to do this again.

    The need to do base hook unregister unconditionally was needed in the past,
    because notifier was last stage where reg->dev dereference was safe.

    Now that nf_tables does the hook removal in .pre_exit, this isn't
    needed anymore.

    Reported-and-tested-by: [email protected]
    Fixes: 767d121 ("netfilter: nftables: fix possible UAF over chains from packet path in netns")
    Signed-off-by: Florian Westphal <[email protected]>
    Signed-off-by: Pablo Neira Ayuso <[email protected]>

Signed-off-by: Florian Westphal <[email protected]>

Author: fw-strlen

net/netfilter/nft_chain_filter.c

@@ -342,12 +342,6 @@ static void nft_netdev_event(unsigned long event, struct net_device *dev,
 		return;
 	}
 
-	/* UNREGISTER events are also happening on netns exit.
-	 *
-	 * Although nf_tables core releases all tables/chains, only this event
-	 * handler provides guarantee that hook->ops.dev is still accessible,
-	 * so we cannot skip exiting net namespaces.
-	 */
 	__nft_release_basechain(ctx);
 }
 
@@ -366,6 +360,9 @@ static int nf_tables_netdev_event(struct notifier_block *this,
 	    event != NETDEV_CHANGENAME)
 		return NOTIFY_DONE;
 
+	if (!check_net(ctx.net))
+		return NOTIFY_DONE;
+
 	nft_net = nft_pernet(ctx.net);
 	mutex_lock(&nft_net->commit_mutex);
 	list_for_each_entry(table, &nft_net->tables, list) {