CVE-2026-98820 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
blk-mq: set RQF_USE_SCHED when the operation is known
The cached requests are allocated for one operation but can be handed out for another. A passthrough command has RQF_USE_SCHED cleared, so using those flags for a subsequent read/write bio will insert it into the scheduler without ->prepare_request() and frees it without ->finish_request(). For kyber, this leaks the domain token acquired at dispatch and stalls the queue.
Don't set RQF_USE_SCHED based on the first operation the batch happened to be allocated for. Instead, set it after the request is claimed by an operation. Introduce a helper function, blk_mq_rq_late_init() for this, and absorb blk_mq_rq_time_init() since the two run together.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel block multiqueue subsystem contains a logic flaw related to the handling of cached request structures during I/O operations. Specifically, the vulnerability arises from how the RQF_USE_SCHED flag is managed for requests that are allocated in batches but subsequently utilized by different types of operations. In this context, cached requests are pre-allocated to optimize performance, yet they may be handed out for an operation type distinct from the one originally intended during allocation. This mismatch leads to incorrect state management within the block layer scheduler logic, particularly affecting passthrough commands which require specific handling due to their direct interaction with hardware devices without standard filesystem mediation.
The core technical flaw involves the premature or misapplied setting of the RQF_USE_SCHED flag based on the initial operation for which a batch was allocated rather than the actual operation claiming the request at runtime. When a passthrough command is processed, it clears this flag to indicate that scheduler preparation and completion hooks should not be invoked in the standard manner. However, if such a cleared-flag request is later reused for a subsequent read or write bio operation, the system incorrectly treats it as needing full scheduling support. This results in the request being inserted into the scheduler without invoking the necessary ->prepare_request() callback, which initializes internal state required for proper queue management. Furthermore, because the flag indicates scheduled handling but the initial setup was skipped, the corresponding ->finish_request() cleanup is also bypassed or mishandled during deallocation.
This inconsistency has severe operational impacts on specific I/O schedulers like Kyber, a latency-based scheduler designed to maintain low and predictable latencies for mixed workloads. The failure to properly invoke preparation and completion hooks causes the scheduler domain token, acquired at dispatch time, to leak rather than being released correctly. This resource leak accumulates over time as more requests are processed under these flawed conditions. Eventually, the exhaustion of available tokens leads to a stall in the queue processing pipeline, effectively halting I/O operations for affected devices and causing system-wide performance degradation or complete unresponsiveness regarding block device access.
From a vulnerability classification perspective, this issue aligns with CWE-362, Concurrent Execution using Shared Resource without Proper Synchronization, as it involves race-like conditions in state management across different operation contexts within the same request structure. It also relates to CWE-401, Missing Release of Memory after Effective Lifetime, due to the domain token leak that prevents proper resource cleanup. In terms of attack vectors and behavioral mapping, this falls under ATT&CK technique T1529, System Shutdown or Reboot, as the resulting queue stall can lead to denial of service conditions where legitimate I/O requests are blocked indefinitely, impacting system availability.
To mitigate this vulnerability, kernel developers have implemented a fix that defers the determination and setting of the RQF_USE_SCHED flag until after the request is explicitly claimed by an operation. This ensures that the flag accurately reflects the nature of the current operation rather than relying on stale allocation data. A new helper function, blk_mq_rq_late_init(), was introduced to encapsulate this logic, absorbing the functionality previously handled by blk_mq_rq_time_init() since both routines execute together during request initialization. System administrators should ensure that their Linux kernels are updated to versions containing these patches. Regular patching and monitoring of block layer performance metrics can help detect early signs of scheduler stalls or token exhaustion before they lead to full system degradation.