CVE-2026-72404 in Linux
Summary
by MITRE • 08/15/2026
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: 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.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a critical use-after-free condition within the TIPC (Transparent Inter-Process Communication) subsystem of the Linux kernel, specifically affecting the UDP media bearer implementation. This issue arises during the cleanup process of network bearers where improper ordering of resource deallocation operations creates race conditions that can lead to system crashes and potential privilege escalation. The flaw manifests when the cleanup_bearer function attempts to destroy destination caches before ensuring all concurrent readers have completed their operations, creating a window where memory access violations can occur.
The technical root cause stems from a fundamental violation of proper synchronization protocols during resource teardown. When the TIPC UDP media bearer is being torn down, the system calls dst_cache_destroy() on replicate cache structures before invoking synchronize_net() to wait for concurrent RCU readers to complete their operations. This premature destruction triggers immediate per-CPU memory deallocation through free_percpu(), while concurrent transmitter threads (tipc_udp_xmit()) may still be executing under RCU protection. The resulting race condition produces two distinct failure modes: direct use-after-free conditions on the per-CPU cache pointers causing kernel crashes, and "rcuref - imbalanced put()" warnings when readers attempt to release dst entries that have already been freed by the premature destruction.
The vulnerability demonstrates a clear violation of CWE-416, Use After Free, and CWE-362, Concurrent Execution using Shared Resource, as it involves improper handling of shared resources during concurrent access scenarios. From an ATT&CK perspective, this represents a privilege escalation vector through kernel memory corruption, potentially enabling attackers to execute arbitrary code with kernel privileges. The flaw also aligns with CWE-1187, Improper Synchronization, since the proper sequence of operations required for safe resource deallocation has been inverted, creating temporal dependencies that are not properly managed.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable persistent exploitation within network communication contexts. Network attackers could leverage this condition to disrupt TIPC-based communications or potentially gain elevated privileges through controlled memory corruption. The timing window between socket closure and complete resource cleanup creates opportunities for exploitation, particularly in high-traffic network environments where concurrent access patterns are frequent. Systems using TIPC UDP media bearers are particularly vulnerable during periods of active network communication when multiple threads may be accessing the same bearer resources simultaneously.
The proposed fix implements a robust synchronization strategy that reorders the cleanup operations to ensure proper resource lifecycle management. The solution defers the destruction of replicate cache entries through RCU callbacks using call_rcu_hurry() to ensure quick release of dst entries while maintaining safety guarantees. The approach separates socket closure from memory deallocation by first releasing the bearer socket via udp_tunnel_sock_release() to prevent new readers, then waiting for existing readers to complete with synchronize_net(), and finally performing the final cleanup operations. This fix addresses the core synchronization issues identified in the original implementation and aligns with established kernel programming practices for safe resource teardown. The solution also prepares for future optimizations by identifying opportunities to replace synchronize_net() calls with more efficient RCU-based approaches, as noted in the follow-up implementation plan.