CVE-2026-99253 in Linux
Summary
by MITRE • 11/03/2026
In the Linux kernel, the following vulnerability has been resolved:
io_uring/cancel: rearm local work wake up before sleep
io_uring_try_cancel_requests() sets ->cq_wait_nr to 1 so that the next io_req_local_work_add() wakes the cancelling task. The add that takes the count to zero is the only one that wakes, later ones see a count <= 0 and return. io_uring_cancel_generic() however only becomes TASK_INTERRUPTIBLE much later, so the one wake up can be spent on a running task. That is fine as long as the entry of that add is still queued, as the io_local_work_pending() check then skips the sleep. It is not if the add was delayed after queueing its entry:
CPU0 (submitter in exec or exit) CPU1 (io_req_local_work_add) mpscq_push(req A) io_uring_try_cancel_requests() io_run_local_work() // runs req A io_uring_try_cancel_requests() atomic_set(&ctx->cq_wait_nr, 1) atomic_read(&ctx->cq_wait_nr) == 1 count taken, wake_up_state() // task is running, no-op prepare_to_wait(TASK_INTERRUPTIBLE) io_local_work_pending() == false schedule() mpscq_push(req B) cq_wait_nr <= 0, return
Only the sleeping task can run req B, so it is stuck in execve() until a signal happens to wake it. An exiting task doesn't even get that.
Arm the count again once the task state is set and before the work list is checked, for the rings whose local work this task is the one to run.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability described involves a race condition within the Linux kernel's io_uring subsystem, specifically affecting the cancellation logic of asynchronous I/O requests. The core issue stems from an incorrect ordering of operations during the process of cancelling pending requests and managing local work items. When io_uring attempts to cancel requests, it sets the cq_wait_nr counter to one with the intention that subsequent additions to the local work queue will trigger a wake-up for the task performing the cancellation. However, due to timing discrepancies between setting this flag and actually putting the task into an interruptible sleep state, the intended wake-up signal is effectively lost if another piece of local work executes in the interim. This results in the cancelling task remaining stuck in a sleeping state even after it should have resumed execution, leading to potential hangs or delays in system operations such as execve calls or process termination sequences.
From a technical perspective, the flaw lies in the synchronization mechanism used by io_uring_try_cancel_requests and io_req_local_work_add functions. The kernel sets cq_wait_nr to one expecting that when another request is added via io_req_local_work_add, it will detect this flag and wake up the waiting task. Yet, because prepare_to_wait which transitions the task state to TASK_INTERRUPTIBLE occurs later in the execution flow, there exists a window where local work items can be processed before the task actually begins sleeping. If such processing happens during this interval, particularly involving operations like mpscq_push that add new requests, the wake-up condition is consumed prematurely or rendered ineffective since no one is yet waiting on that specific wait queue entry. Consequently, subsequent additions see a count less than or equal to zero and do not trigger any further wake-ups, leaving the task indefinitely suspended until an external signal intervenes.
The operational impact of this vulnerability can be significant depending on how it is exploited or encountered in production environments. For processes executing system calls like execve that rely heavily on io_uring for efficient asynchronous handling, being stuck due to missed wake-ups means prolonged latency or complete unresponsiveness until a timeout occurs or an external signal forces termination. In scenarios involving process exit paths where cleanup routines depend upon timely completion of outstanding I/O operations, failure to properly manage these internal states could delay resource release and potentially cause deadlocks if other parts of the system are waiting for those resources. While not directly exploitable by remote attackers without prior access privileges, this bug represents a denial-of-service vector against local users who might trigger conditions leading to such hangs through carefully crafted application behavior leveraging io_uring features.
Mitigation strategies primarily involve applying kernel patches that correct the sequence of operations within the affected code paths. Specifically ensuring that atomic counters indicating pending work are updated only after confirming appropriate wait states have been established helps prevent premature consumption of wake-up signals. Administrators should monitor for updates to their distribution's linux-kernel packages addressing this specific issue related to io_uring cancellation logic improvements. Additionally developers utilizing io_uring APIs must remain aware of potential edge cases around concurrent request submission and cancellation patterns while awaiting official fixes becoming available across supported platforms.
This type of defect aligns closely with CWE-362 which describes concurrent execution issues resulting in race conditions where shared resources are accessed improperly leading to unpredictable behavior. Furthermore within the context of MITRE ATT&CK framework it relates broadly towards techniques involving resource exhaustion or denial-of-service attacks although typically requiring local access rather than remote exploitation capabilities typical more advanced persistent threats might employ against critical infrastructure components relying heavily upon high-performance networking stacks like those provided by modern Linux kernels employing io_uring extensively for scalable network processing tasks.