CVE-2026-98858 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
io_uring: fix task_work add use-after-free with SQPOLL
For SQPOLL, the sqpoll thread runs task_work off its own list and doesn't need to be told about it, so it can pop and complete a request as soon as mpscq_push() has published it. If that was the last request holding the ring alive, io_ring_exit_work() then frees the ctx, stops the thread, and with it the tctx, while io_req_normal_work_add() is still looking at both after the push:
BUG: KASAN: slab-use-after-free in io_req_normal_work_add+0x439/0x510 Read of size 4 at addr ff11000000ca0000 by task repro/55 (...) BUG: KASAN: slab-use-after-free in queue_work_on+0x25/0x70 Write of size 8 at addr ff11000000c3bd00 by task repro/55
Hold an RCU read lock across the add, like io_req_local_work_add() already does, and have io_ring_exit_work() wait for a grace period for SQPOLL rings as well before freeing the ctx.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel's io_uring subsystem implements high-performance asynchronous I/O by allowing user-space applications to submit requests that are processed in kernel space. A critical concurrency flaw was identified within the submission queue polling, or SQPOLL, mechanism of this subsystem. The vulnerability arises from a race condition involving memory management and thread synchronization during the lifecycle of an io_uring context. Specifically, when using SQPOLL, a dedicated kernel thread is spawned to poll for new requests on behalf of the application. This design allows the kernel to process submissions immediately after they are published via mpscq_push(), optimizing latency by avoiding immediate user-to-kernel transitions for every request. However, this optimization introduces a window where memory safety can be compromised if the context containing these structures is destroyed while operations are still in flight.
The technical root cause of this vulnerability lies in the lack of proper synchronization between the thread that completes requests and the code path responsible for tearing down the io_uring context. When an SQPOLL thread processes a request, it may invoke task_work functions to handle completion logic. If the processed request was the final reference holding the ring context alive, the exit routine io_ring_exit_work() is triggered. This function proceeds to free the memory associated with the context and stops the SQPOLL thread. However, there exists a race condition where another operation, specifically io_req_normal_work_add(), may still be accessing the same context data structures after they have been published but before the system has fully stabilized around their destruction. The KASAN reports indicate that this results in both read and write operations on freed slab memory, confirming a classic use-after-free scenario.
This vulnerability is classified under CWE-416, Use After Free, which describes situations where software continues to use a pointer after it has been freed, leading to undefined behavior, crashes, or potential code execution. In the context of ATT&CK techniques, this flaw could potentially be leveraged for privilege escalation if an attacker can trigger the race condition with crafted io_uring requests that manipulate reference counts and timing precisely. By exploiting the window between publishing a request and the safe deallocation of resources, a malicious user might corrupt kernel memory structures, leading to system instability or arbitrary code execution within the kernel space. The impact is severe as it affects systems relying on high-throughput asynchronous I/O operations, particularly those utilizing SQPOLL for performance optimization.
The resolution involves implementing proper read-copy-update (RCU) synchronization mechanisms to ensure that memory access remains safe during concurrent modifications and deallocations. Specifically, an RCU read lock is now held across the add operation in io_req_normal_work_add(), mirroring the existing protection already present in io_req_local_work_add(). This ensures that any ongoing reads of the context data are protected from being freed while they are still being accessed. Furthermore, the exit logic for SQPOLL rings has been modified so that io_ring_exit_wait() waits for an RCU grace period before freeing the context. A grace period guarantees that all pre-existing read-side critical sections have completed their execution before the memory is reclaimed, thereby eliminating the race condition entirely and restoring memory safety to the subsystem.