TIPC UDP media bearer teardown calls dst_cache_destroy() on its
replicast caches before calling synchronize_net() to wait for
concurrent RCU readers (transmitters) to finish:

static void cleanup_bearer(struct work_struct *work)
{
...
	list_for_each_entry_safe(rcast, tmp, &ub->rcast.list, list) {
		dst_cache_destroy(&rcast->dst_cache);
		list_del_rcu(&rcast->list);
		kfree_rcu(rcast, rcu);
	}
...
	dst_cache_destroy(&ub->rcast.dst_cache);
	udp_tunnel_sock_release(ub->sk);
	synchronize_net();
...
}

This is highly buggy because dst_cache_destroy() immediately frees the
per-CPU cache memory (free_percpu()) and releases the cached dst
entries without any synchronization.

If a concurrent transmitter (e.g., tipc_udp_xmit()) is running on another
CPU under RCU protection, it can call dst_cache_get() concurrently,
leading to:
1. Use-After-Free on the per-CPU cache pointer itself (crash).
2. "rcuref - imbalanced put()" warning if it attempts to release a
   dst that was concurrently released by dst_cache_destroy().

Furthermore, calling kfree(ub) immediately after synchronize_net() without
closing the socket first (or waiting after closing it) leaves a window
where a concurrent receiver (tipc_udp_recv()) could start after
synchronize_net(), access ub, and suffer a UAF when kfree(ub) runs.

To fix this, we must defer dst_cache_destroy() and kfree(ub) until after
we have ensured that no more readers can see the bearer/socket and all
existing readers have finished:

1. Defer rcast entry destruction (both dst_cache_destroy() and kfree())
   to an RCU callback using call_rcu_hurry().
   Using call_rcu_hurry() ensures the dst entries are released quickly.

2. Release the bearer socket using udp_tunnel_sock_release() (stops
   new receive readers).

3. Call synchronize_net() to wait for all outstanding RCU readers
   (both transmit and receive) to finish.

4. Now that it is safe, call dst_cache_destroy() on the main bearer
   cache, and free ub.

Note: 3) and 4) can be changed later in net-next to also use
call_rcu_hurry() and get rid of the synchronize_net() latency.

Fixes: e9c1a793210f ("tipc: add dst_cache support for udp media")
Reported-by: syzbot+e14bc5d4942756023b77@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a396a66.52ae72c2.136ac7.0003.GAE@google.com/T/#u (local)
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: Jon Maloy <jmaloy@redhat.com>
Cc: tipc-discussion@lists.sourceforge.net
---
v2: addressed Xin Long feedback
v1: https://lore.kernel.org/netdev/CANn89i+dkbrSAwvaWXW7yWMfcwUebuTBLG5T7AGZaZcpVYGyfQ@mail.gmail.com/T/#m7bbeedffe3bedb69e33236410e3833c7ce809850 (local)
 net/tipc/udp_media.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/net/tipc/udp_media.c b/net/tipc/udp_media.c
index 988b8a7f953ad6da860e6190f1f244650f121dce..66f3cb87a0aaaac8f40e8f237ab9a44d539b1cd8 100644
--- a/net/tipc/udp_media.c
+++ b/net/tipc/udp_media.c

@@ -803,6 +803,14 @@ static int tipc_udp_enable(struct net *net, struct tipc_bearer *b,
 	return err;
 }
 
+static void rcast_free_rcu(struct rcu_head *rcu)
+{
+	struct udp_replicast *rcast = container_of(rcu, struct udp_replicast, rcu);
+
+	dst_cache_destroy(&rcast->dst_cache);
+	kfree(rcast);
+}
+
 /* cleanup_bearer - break the socket/bearer association */
 static void cleanup_bearer(struct work_struct *work)
 {

@@ -811,18 +819,17 @@ static void cleanup_bearer(struct work_struct *work)
 	struct tipc_net *tn;
 
 	list_for_each_entry_safe(rcast, tmp, &ub->rcast.list, list) {
-		dst_cache_destroy(&rcast->dst_cache);
 		list_del_rcu(&rcast->list);
-		kfree_rcu(rcast, rcu);
+		call_rcu_hurry(&rcast->rcu, rcast_free_rcu);
 	}
 
 	tn = tipc_net(sock_net(ub->sk));
 
-	dst_cache_destroy(&ub->rcast.dst_cache);
 	udp_tunnel_sock_release(ub->sk);
 
-	/* Note: could use a call_rcu() to avoid another synchronize_net() */
 	synchronize_net();
+
+	dst_cache_destroy(&ub->rcast.dst_cache);
 	atomic_dec(&tn->wq_count);
 	kfree(ub);
 }

-- 
2.55.0.rc0.799.gd6f94ed593-goog