CVE-2026-99229 in Linux
Summary
by MITRE • 11/02/2026
In the Linux kernel, the following vulnerability has been resolved:
io_uring/wait: don't spin on a stuck overflow list
As long as IO_CHECK_CQ_OVERFLOW_BIT is set, io_cqring_wait_schedule() returns right away, before it gets to look at signals or arms the timeout, for io_cqring_wait() to flush the overflow list. If that doesn't empty the list, and the CQ ring doesn't hold the number of events the caller wants, the loop starts over, with the same result. The task spins in the kernel, and can't be killed.
The assumption is that this can't happen: if CQEs can't be flushed, the ring is full, and min_events is capped to the size of the ring. But:
- With IORING_SETUP_CQE_MIXED a 32b CQE needs two contiguous entries. It is stuck on the list if a single one is free, and then the ring holds less than cq_entries events. A single threaded application that asks for as many events as the ring has entries never gets out of io_uring_enter().
- Whether the ring is full is up to the CQ head, which the application can write to. While one thread waits for events on an empty ring, another one can move the head back so that the ring looks full and have a CQE overflow. The waiter can't be stopped any more, not even by SIGKILL.
Only the application can make room in the CQ ring. Return to it if the list could not be flushed.
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Analysis
by VulDB Data Team • 11/02/2026
The vulnerability identified as CVE-2024-something related to io_uring involves a critical flaw within the Linux kernel's asynchronous I/O subsystem, specifically affecting the wait logic for completion queue events. The core issue resides in the io_cqring_wait_schedule function, which is responsible for managing how tasks block and wake up when waiting for I/O operations to complete. Under normal circumstances, this mechanism ensures that a process waits efficiently until requested data or status updates are available from submitted requests. However, a specific edge case allows a task to enter an unrecoverable spinning state within the kernel space, effectively rendering it unresponsive and immune to standard termination signals such as SIGKILL. This behavior represents a severe denial of service condition where system resources are consumed indefinitely by a single stuck process without any possibility of external intervention or graceful shutdown.
The technical root cause stems from an incorrect assumption regarding the state of the Completion Queue (CQ) ring buffer and how overflow events are handled. The kernel logic assumes that if the IO_CHECK_CQE_OVERFLOW_BIT is set, indicating there are pending overflow entries to be flushed, the subsequent wait loop will successfully empty this list or determine that the ring is full. If the CQ ring holds fewer events than requested by the caller after flushing, the loop restarts. Because the condition for returning early based on the overflow bit remains true, and the flush operation fails to reduce the pending count sufficiently due to structural constraints of the ring buffer, the task enters an infinite spin cycle. This creates a situation where the kernel thread consumes CPU cycles continuously without yielding control back to the scheduler or checking for signal interrupts in time to terminate execution.
Two primary scenarios exacerbate this flaw within different operational contexts. First, when using the IORING_SETUP_CQE_MIXED flag, Completion Queue Entries require two contiguous entries in memory even if they are only thirty-two bits wide. If a single entry is free but not part of a pair capable of holding an event, the ring cannot accept new completions despite appearing partially empty to naive checks. An application requesting as many events as there are total entries will find itself stuck because the logical capacity does not match the physical contiguous availability, leading to perpetual spinning. Second, and more dangerously, the CQ head pointer is writable by user-space applications. A malicious or buggy multi-threaded application can manipulate this head pointer while one thread waits for events on an ostensibly empty ring. Another thread can then move the head back in a way that makes the ring appear full from the perspective of the waiter, triggering overflow conditions that cannot be resolved because only the original waiting context has the authority to advance the pointers correctly. This manipulation traps the waiting task indefinitely.
The operational impact of this vulnerability is significant for system stability and security posture. Since the stuck process runs in kernel mode with high priority scheduling characteristics inherent to io_uring wait loops, it can starve other processes of CPU time on shared cores. More critically, because standard kill signals are ignored during this spin phase due to the early return logic bypassing signal checks, administrators cannot easily terminate the offending process using conventional tools like pkill or kill -9. This effectively locks out management capabilities for that specific workload, potentially requiring a full system reboot if multiple critical services depend on the affected resources. In cloud environments or shared hosting platforms, this could lead to cascading failures as other tenants experience resource exhaustion caused by one misbehaving process monopolizing kernel scheduling slots.
From a classification perspective, this vulnerability aligns with CWE-835: Loop with Unreachable Exit Condition, also known as an infinite loop that cannot be exited through normal control flow mechanisms. The failure to check for signals or timeouts before re-evaluating the stuck condition places it squarely within this category of logic errors. Furthermore, in terms of adversarial tactics, this flaw supports ATT&CK technique T1496: Resource Hijacking, where an attacker uses compromised resources such as CPU cycles and memory to perform malicious activities like cryptomining or simply causing denial of service against other users on the same host. The ability to bypass SIGKILL also touches upon aspects of persistence and evasion, although primarily it serves as a powerful Denial of Service vector rather than a privilege escalation path in this specific context.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary solution is the application of kernel patches that modify io_cqring_wait_schedule to properly handle signal checks and timeout arms even when overflow bits are set, ensuring that tasks can be interrupted if they remain stuck for too long or receive termination signals. System administrators should ensure their Linux kernels are updated with the latest stable releases containing these fixes. For applications utilizing io_uring, developers must audit codebases for improper usage of IORING_SETUP_CQE_MIXED and verify that multi-threaded access to CQ head pointers is strictly synchronized using appropriate locking mechanisms or atomic operations to prevent race conditions that exploit this logic flaw. Additionally, implementing resource limits via cgroups can help contain the impact by preventing any single process from consuming excessive CPU time, thereby mitigating the severity of the denial-of-service effect even if the kernel bug persists in older systems.