CVE-2026-99054 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
tipc: destroy topsrv workqueues before closing connections
tipc_topsrv_stop() closed subscriber connections while the topology server's send and receive workqueues were still running. Socket callbacks and subscription events could then queue more work, and in-flight send/recv work could drop the last connection reference during the conn_idr walk.
That race produced several teardown failures: refcount_t addition on 0 from conn_get() on a connection whose release was blocked on idr_lock, a subsequent use-after-free in tipc_conn_close(), queue_work() on an already destroyed workqueue from the listener data-ready callback, and an RCU stall in tipc_topsrv_exit_net() while the walk spun under idr_lock.
Clear srv->listener under idr_lock so it acts as a shutdown flag, skip queue_work() once it is NULL, destroy the workqueues to flush in-flight work, and only then close the remaining connections. Refuse tipc_conn_lookup() after that flag is cleared, and drop idr_lock when the teardown walk finds no connection, so an in-flight subscription event cannot pin idr_in_use while the walk holds the lock.
v3 supersedes the narrower in-thread diff Tung Quang Nguyen posted on 2026-09-23. It keeps his teardown order and adds the lookup refusal and the empty-idr unlock.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified within the Linux kernel's Transparent Inter-process Communication (TIPC) subsystem represents a critical race condition during the shutdown sequence of the topology server service. This flaw arises from an improper ordering of resource cleanup operations, specifically involving the interaction between subscriber connection closures and the lifecycle management of send and receive workqueues. When the tipc_topsrv_stop function is invoked to halt the topology server, it proceeds to close active subscriber connections while the underlying asynchronous workqueues remain operational. This temporal overlap creates a window where socket callbacks and subscription events can still queue new work items onto these queues even as the system attempts to dismantle the connection infrastructure. The core technical flaw lies in the fact that in-flight send or receive operations may execute concurrently with the teardown process, potentially dropping the final reference count on a connection object during an idr (ID Radix Tree) walk. This race condition undermines the integrity of reference counting mechanisms and leads to severe stability issues within the kernel networking stack.
The operational impact of this vulnerability is multifaceted and can result in several distinct failure modes depending on the timing of the race. One primary consequence is a refcount_t addition on zero, which occurs when conn_get attempts to access a connection whose release process has been blocked waiting for the idr_lock. This indicates that the object was already freed or being freed while still being accessed. Another significant issue is a use-after-free condition in tipc_conn_close, where memory previously allocated and potentially reclaimed by another thread is accessed again due to stale pointers resulting from the race. Furthermore, the system may attempt to queue work on an already destroyed workqueue via the listener data-ready callback, leading to undefined behavior or kernel panics. In more severe scenarios, particularly under high load or specific timing conditions, this can trigger an RCU (Read-Copy-Update) stall in tipc_topsrv_exit_net because a walk operation spins indefinitely while holding idr_lock, preventing other threads from accessing the data structure and effectively hanging parts of the kernel subsystem.
From a classification perspective, this vulnerability aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). The root cause is the lack of proper synchronization between the shutdown flagging mechanism and the asynchronous workqueue processing logic. Additionally, it relates to CWE-416: Use After Free, as in-flight operations access connection structures that may have been deallocated due to premature reference counting drops during the teardown walk. In terms of ATT&CK mapping, this scenario reflects techniques associated with T1059.008: Scripting via Kernel Modules or internal kernel exploitation paths where race conditions are leveraged to cause denial of service through resource exhaustion or memory corruption. The vulnerability highlights the complexity of managing shared resources in asynchronous environments and underscores the necessity for strict ordering guarantees when tearing down complex subsystems like TIPC.
The resolution implemented addresses these issues by enforcing a stricter shutdown sequence that eliminates the window of opportunity for race conditions. First, the server listener is cleared under the protection of idr_lock to serve as an atomic shutdown flag. This ensures that any subsequent attempt to queue work checks this flag and skips queuing if it is NULL, effectively stopping new asynchronous tasks from entering the system during teardown. Second, the workqueues are destroyed only after ensuring no new work can be queued, which allows for a safe flush of in-flight work items without risking access to freed memory or locked structures. Third, remaining connections are closed only after these steps are complete, ensuring that reference counting is handled safely. Additionally, tipc_conn_lookup is configured to refuse lookups once the shutdown flag is cleared, preventing new connection attempts from proceeding during teardown. Finally, the idr_lock is dropped when the teardown walk finds no active connections, preventing in-flight subscription events from pinning the idr_in_use state and causing deadlocks or stalls. This comprehensive fix ensures that all asynchronous operations are completed or safely abandoned before resources are released, thereby restoring stability to the TIPC subsystem during service termination.