From 7116764ca53ff529335d7ab7c364a69f094b23a5 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 23 Jun 2026 17:30:29 +0000 Subject: [PATCH 1/2] 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: 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 Signed-off-by: Eric Dumazet Cc: Jon Maloy Cc: tipc-discussion@lists.sourceforge.net Reviewed-by: Xin Long Link: https://patch.msgid.link/20260623173030.2925059-2-edumazet@google.com Signed-off-by: Jakub Kicinski --- 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 988b8a7f953a..66f3cb87a0aa 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); } From c1481c94e74c955e0448ddf46b8615a44d840c1e Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 23 Jun 2026 17:30:30 +0000 Subject: [PATCH 2/2] tipc: avoid busy looping in tipc_exit_net() Blamed commit introduced a busy-wait loop in tipc_exit_net() to wait for pending UDP bearer cleanup works to complete: while (atomic_read(&tn->wq_count)) cond_resched(); This loop can busy-wait for a long time if cond_resched() is a NOP. This typically happens if the netns exit is executed by a high priority task, or under kernels configured without preemption (CONFIG_PREEMPT_NONE). In such cases, it wastes CPU cycles and can lead to soft lockups. Fix this by replacing the busy loop with wait_var_event(), allowing the thread to sleep properly until the work queue count reaches zero. Accordingly, update cleanup_bearer() to use atomic_dec_and_test() and wake_up_var() to wake up the waiter when the count drops to zero. This uses the global wait queue hash table, avoiding the need to bloat struct tipc_net with a wait_queue_head_t. The atomic_dec_and_test() provides the necessary memory barrier to ensure the wakeup is not missed. Fixes: 04c26faa51d1 ("tipc: wait and exit until all work queues are done") Signed-off-by: Eric Dumazet Cc: Jon Maloy Cc: tipc-discussion@lists.sourceforge.net Reviewed-by: Xin Long Link: https://patch.msgid.link/20260623173030.2925059-3-edumazet@google.com Signed-off-by: Jakub Kicinski --- net/tipc/core.c | 4 ++-- net/tipc/udp_media.c | 4 +++- 2 files changed, 5 insertions(+), 3 deletions(-) diff --git a/net/tipc/core.c b/net/tipc/core.c index 1ddecea1df6e..315975c3be81 100644 --- a/net/tipc/core.c +++ b/net/tipc/core.c @@ -45,6 +45,7 @@ #include "crypto.h" #include +#include /* configurable TIPC parameters */ unsigned int tipc_net_id __read_mostly; @@ -118,8 +119,7 @@ static void __net_exit tipc_exit_net(struct net *net) #ifdef CONFIG_TIPC_CRYPTO tipc_crypto_stop(&tipc_net(net)->crypto_tx); #endif - while (atomic_read(&tn->wq_count)) - cond_resched(); + wait_var_event(&tn->wq_count, atomic_read(&tn->wq_count) == 0); } static void __net_exit tipc_pernet_pre_exit(struct net *net) diff --git a/net/tipc/udp_media.c b/net/tipc/udp_media.c index 66f3cb87a0aa..62ae7f5b5840 100644 --- a/net/tipc/udp_media.c +++ b/net/tipc/udp_media.c @@ -40,6 +40,7 @@ #include #include #include +#include #include #include #include @@ -830,7 +831,8 @@ static void cleanup_bearer(struct work_struct *work) synchronize_net(); dst_cache_destroy(&ub->rcast.dst_cache); - atomic_dec(&tn->wq_count); + if (atomic_dec_and_test(&tn->wq_count)) + wake_up_var(&tn->wq_count); kfree(ub); }