CVE-2026-99268 in Linux
Summary
by MITRE • 11/03/2026
In the Linux kernel, the following vulnerability has been resolved:
io_uring: wake ring pollers for local task_work
With IORING_SETUP_DEFER_TASKRUN completions are queued as local work, and only become CQEs when the submitter enters the kernel to wait for events. For an application that finds out via poll() or epoll on the ring fd when it is time to do that, io_uring_poll() reports EPOLLIN if local work is pending.
That only helps if the work is there at the time of the call. When DEFER_TASKRUN was added, io_req_local_work_add() did io_cqring_wake(), which took care of tasks sleeping in poll() as well. Then polling got a wait queue of its own, ->poll_wq, and the wake up in io_req_local_work_add() was replaced by waking the submitter directly. Since then ->poll_wq is only woken when CQEs are committed, which for such a ring is done by the very task that is waiting to be told to:
submitter completion poll(ring_fd) io_uring_poll() // nothing yet schedule() io_req_local_work_add() // ->cq_wait_nr: nobody waits // sleeps on
Wake ->poll_wq when the first entry is put on the work list. Like for the eventfd, that must not be done right away if the work is queued from a wake up handler, as the chain of wake ups could get back to the wait queue whose lock is held. Use an RCU callback in that case, with a reference to the ring.
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Analysis
by VulDB Data Team • 11/03/2026
The Linux kernel's io_uring subsystem implements a mechanism for asynchronous I/O operations where completions can be deferred as local task work rather than being immediately processed by a dedicated completion thread. This design allows applications to utilize standard file descriptor polling mechanisms, such as poll or epoll, on the ring file descriptor to determine when it is appropriate to enter the kernel and process pending completions. The io_uring_poll function reports an EPOLLIN event if there is local work pending, enabling efficient synchronization between user space and kernel space without requiring constant system calls. However, a critical flaw existed in this wake-up logic that prevented applications from being notified when new deferred work was added to the queue while they were already waiting for events.
The vulnerability stems from an inconsistency introduced during the evolution of the io_uring polling infrastructure. Initially, when local task work was queued via io_req_local_work_add, the system would invoke io_cqring_wake to notify tasks sleeping in poll calls. Later architectural changes separated the wait queue used by the submitter from the one used for general event waiting, creating a dedicated ->poll_wq structure. The direct wake-up mechanism in io_req_local_work_add was replaced with logic that only wakes the submitter directly, leaving the ->poll_wq to be woken exclusively when completion queue entries are committed. This created a deadlock-like scenario where an application could poll for readiness, find no work initially, and then sleep on its wait queue. If new deferred work arrived while the task was sleeping, the system failed to wake it because the commit path had not yet been reached, leaving the application unaware that processing should occur.
This operational flaw results in a denial of service condition where applications relying on io_uring with DEFER_TASKRUN completions may hang indefinitely or experience significant latency spikes. The submitter task remains blocked on its wait queue even though there is work ready to be processed, effectively stalling the application's progress. This behavior violates the expected contract of event-driven I/O models and can lead to resource exhaustion if multiple such tasks accumulate without being serviced. From a classification perspective, this issue aligns with CWE-409, which describes improper handling of highly fragmented data or complex state transitions that lead to unexpected system states. In terms of attack vectors, while primarily affecting availability rather than confidentiality or integrity, the potential for application hangs can be leveraged in denial-of-service attacks against services dependent on high-performance asynchronous I/O stacks.
The resolution involves modifying io_req_local_work_add to explicitly wake the ->poll_wq when the first entry is added to the work list. This ensures that any task waiting via poll or epoll is immediately notified of pending local work, allowing it to exit its sleep state and proceed with processing completions. To prevent potential deadlocks arising from recursive wake-up chains, particularly if the work queueing occurs within a context where locks are already held, the fix employs an RCU callback mechanism. This deferred execution strategy ensures that the wake-up signal is delivered safely without holding critical locks for extended periods or creating circular dependencies in the scheduling logic.
Mitigation strategies for systems running affected kernel versions include applying the upstream Linux kernel patch that addresses this specific io_uring polling race condition. Administrators should monitor system logs for signs of application hangs related to I/O operations and ensure that their kernels are updated to a version containing this fix. For developers utilizing io_uring, it is advisable to implement timeout mechanisms in poll or epoll calls as an additional safeguard against potential stalls caused by similar synchronization issues in other parts of the stack. Regularly updating to stable kernel releases ensures that these subtle concurrency bugs are resolved before they can impact production workloads relying on high-throughput asynchronous I/O patterns.