CVE-2026-90206 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
nvmet: fix max_qid race between configfs and controller allocation
The function nvmet_subsys_attr_qid_max_store() can race against nvmet_alloc_ctrl() when a subsystem's max_qid limit is modified.
Suppose max_qid is currently 64. If nvmet_alloc_ctrl() executes: ctrl->sqs = kzalloc_objs(struct nvmet_sq *, subsys->max_qid + 1); and at this exact point, a userspace process changes max_qid to 128, nvmet_subsys_attr_qid_max_store() will set the new max_qid value. It attempts to delete active controllers to force a reconnect, but the new controller won't be deleted because it hasn't been added to the subsys->ctrls list yet.
nvmet_alloc_ctrl() then proceeds and adds the new controller to the subsys->ctrls list. Later, when nvmet_install_queue() is called, it will see max_qid set to 128, but the memory allocated for sqs is only sized for 64 entries. This results in a KASAN out-of-bounds warning and potential memory corruptions.
Fix this by protecting the queue allocations and list insertion in nvmet_alloc_ctrl() with down_read(&nvmet_config_sem). Because nvmet_subsys_attr_qid_max_store() acquires down_write(&nvmet_config_sem) to modify the attribute, this safely prevents the configfs writer from modifying max_qid during controller creation.
Copy the max_qid from the subsystem to the controller's structure during the allocation; ctrl->max_qid never changes as long as the controller remains in LIVE state, so this will prevent similar race conditions.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel NVMe target implementation contains a critical concurrency flaw within its subsystem attribute management and controller initialization routines. Specifically, the function nvmet_subsys_attr_qid_max_store, which handles updates to the maximum queue identifier limit via configfs interfaces, can execute concurrently with nvmet_alloc_ctrl during the allocation of new controllers. This race condition arises because there is insufficient synchronization between the configuration modification path and the dynamic resource allocation path for NVMe controllers. The vulnerability stems from a lack of mutual exclusion that ensures atomicity when transitioning subsystem parameters while simultaneously provisioning hardware resources associated with those parameters.
The operational mechanics of this flaw involve a specific sequence of events where timing discrepancies lead to memory safety violations. When nvmet_alloc_ctrl is invoked, it initially allocates an array for submission queues based on the current value of max_qid held by the subsystem structure. If a userspace process modifies the max_qid attribute via configfs at this precise moment, the system attempts to enforce consistency by deleting active controllers to force reconnection. However, because the newly allocated controller has not yet been added to the subsystem's list of active controllers, it is overlooked during this cleanup phase. Consequently, nvmet_alloc_ctrl proceeds to register the new controller with an updated max_qid value that exceeds the size of the previously allocated queue array.
This discrepancy results in a severe out-of-bounds memory access when subsequent functions such as nvmet_install_queue attempt to utilize the submission queues. The kernel detects this through KASAN, generating warnings for heap-based buffer overflows. Beyond immediate detection mechanisms, this race condition poses significant risks including potential memory corruption, data leakage, and system instability. An attacker with local access capable of triggering rapid configuration changes during controller allocation could exploit this state inconsistency to corrupt kernel memory structures or cause a denial of service through kernel panic conditions triggered by the out-of-bounds writes.
The remediation strategy implemented in the fix focuses on enforcing strict serialization between configuration updates and resource allocation using semaphore-based locking mechanisms. By protecting both the queue allocation logic and the subsequent list insertion within nvmet_alloc_ctrl with down_read(&nvmet_config_sem), the kernel ensures that no write operations to max_qid can occur during this critical section. Since nvmet_subsys_attr_qid_max_store acquires a write lock on the same semaphore, it is effectively blocked until the controller allocation and registration are complete. This guarantees that the subsystem parameter remains static throughout the entire lifecycle of creating a new controller instance.
Furthermore, to prevent future recurrence of similar race conditions related to state consistency, the fix includes copying the max_qid value from the subsystem structure into the individual controller's data structure during initialization. Because this copied value never changes while the controller is in a live operational state, it decouples the runtime queue management from dynamic configuration updates that might occur after allocation. This architectural change ensures that each controller operates with a consistent view of its resource limits regardless of concurrent modifications to global subsystem attributes.
From a classification perspective, this vulnerability aligns with CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, commonly known as a race condition. The specific impact on memory safety corresponds closely to CWE-125 Out-of-bounds Read and CWE-787 Out-of-bounds Write, depending on the exact nature of the corruption observed during exploitation attempts. In terms of adversarial tactics, this flaw could be leveraged within the ATT&CK framework under techniques related to privilege escalation or defense evasion through kernel memory manipulation, although it primarily serves as a vector for denial of service due to its instability-inducing nature. System administrators and developers should ensure that kernel updates incorporating these synchronization fixes are applied promptly to mitigate risks associated with concurrent NVMe target configuration changes.