In the Linux kernel, the following vulnerability has been resolved:
tipc: fix UAF in cleanup_bearer() due to premature dst_cache_destroy()
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:
- Use-After-Free on the per-CPU cache pointer itself (crash).
- 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:
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.
Release the bearer socket using udp_tunnel_sock_release() (stops new receive readers).
Call synchronize_net() to wait for all outstanding RCU readers (both transmit and receive) to finish.
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.