CVE-2026-80753 in Linux
Summary
by MITRE • 09/03/2026
In the Linux kernel, the following vulnerability has been resolved:
ovpn: run deferred work on a module-owned workqueue
ovpn queues several work items whose callbacks execute module text. These works currently run on the global system workqueues, so module exit has no driver-owned drain point that guarantees the callbacks have fully returned before the module text can be freed.
Object references protect the objects used by the callbacks, but they do not prove that a workqueue function has returned. In particular, a worker can drop the final reference that unblocks device teardown while it is still executing ovpn code.
Add a module-owned workqueue and queue all ovpn work items on it. During module exit, unregister rtnl and netlink first, flush the workqueue so ordinary ovpn workers finish, run the final RCU barrier, and destroy the workqueue last. This keeps the workqueue available for cleanup work queued from RCU callbacks, while ensuring no ovpn work item can outlive the module text.
The per-device delayed keepalive work remains explicitly disabled during netdev teardown (disable_delayed_work_sync in ndo_uninit), since flush_workqueue does not flush delayed work that is still only pending on its timer.
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Analysis
by VulDB Data Team • 09/03/2026
This vulnerability addresses a critical race condition within the OpenVPN over UDP implementation for the Linux kernel, specifically concerning module lifecycle management and concurrency control. The core issue stems from the fact that several deferred work items in the ovpn driver execute code located within the module's text section while being scheduled on global system-wide workqueues. This architectural choice creates a dangerous scenario during module unloading where there is no guaranteed synchronization point to ensure that all callbacks have fully returned before the kernel frees the module memory. Although object references are utilized to protect specific data structures used by these callbacks, reference counting alone does not guarantee execution completion. A worker thread can drop its final reference to an object, thereby triggering device teardown and potentially freeing associated resources, while it is still actively executing ovpn code in another context. This discrepancy between logical ownership via references and actual CPU execution state leads to a use-after-free condition where the kernel attempts to execute instructions from memory that has already been reclaimed or unmapped.
The technical flaw lies in the lack of driver-specific synchronization mechanisms for deferred work items. By relying on global system workqueues, the ovpn module cannot exert control over when its specific tasks are processed relative to its own initialization and cleanup phases. During a normal module exit sequence, if a worker is mid-execution of an ovpn callback that holds no active reference but has not yet returned from the function call stack, the subsequent unloading of the kernel module will invalidate those instructions. This results in undefined behavior, typically manifesting as a kernel panic or system crash due to execution of invalid memory addresses. The problem is exacerbated by the asynchronous nature of workqueues, where scheduling and execution are decoupled, making it difficult for the driver author to predict exactly when their code might be running after cleanup routines have begun.
To resolve this issue, the fix introduces a module-owned dedicated workqueue specifically for ovpn operations. This ensures that all deferred work items related to the openvpn interface are queued on a queue managed by the module itself rather than shared global infrastructure. The initialization and teardown sequences of the module are strictly reordered to enforce proper synchronization. During module exit, the rtnl and netlink interfaces are unregistered first to prevent new operations from being initiated. Subsequently, the dedicated workqueue is flushed, which blocks until all currently queued ovpn workers have completed their execution. This flush operation guarantees that no worker thread remains in a state of executing ovpn code after this point. An RCU barrier is then executed to ensure memory visibility and ordering constraints are met before the final destruction of the workqueue structure itself.
This structured approach ensures that the module text remains valid for as long as any part of it might be actively running on any CPU core. By keeping the workqueue available until after all workers have finished, including those potentially queued from RCU callbacks which require specific cleanup timing, the driver maintains strict coherence between its lifecycle and the execution context of its code. Additionally, special care is taken with per-device delayed keepalive mechanisms; these are explicitly disabled during network device teardown using disable_delayed_work_sync to handle cases where work items are still pending on their timers rather than actively queued for processing. This comprehensive synchronization strategy eliminates the race condition by ensuring that no ovpn work item can outlive the module text, thereby preventing use-after-free vulnerabilities and enhancing overall system stability.
From a classification perspective, this vulnerability aligns with CWE-416: Use After Free, as it involves accessing memory after it has been freed due to improper synchronization of resource lifecycles. It also relates to CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, highlighting the failure to properly coordinate access between different execution contexts within the kernel space. In terms of attack vectors and defensive mapping, this issue corresponds to MITRE ATT&CK technique T1059.004: Unix Shell Commands via Kernel Exploitation or more broadly to privilege escalation paths where a local attacker might exploit such race conditions to gain unauthorized access or cause denial of service by crashing the kernel. Mitigation primarily involves applying the provided kernel patch which implements the dedicated workqueue and correct teardown ordering, ensuring that developers adhere to strict synchronization protocols when managing deferred work in loadable kernel modules.