CVE-2026-99251 in Linux
Summary
by MITRE • 11/03/2026
In the Linux kernel, the following vulnerability has been resolved:
Drivers: hv: Avoid infinite retry loop in init_vp_index()
There is a retry loop in init_vp_index() where the CPUs from a certain node are stripped out if they have already been in the allocated cpumask or not in HK_TYPE_MANAGED_IRQ housekeeping cpumask. If there is no CPU left, the allocated cpumask is ignored and the process is retried again. However, if the HK_TYPE_MANAGED_IRQ housekeeping cpumask turns out not to contain any CPU in that particular node, that will become an infinite retry loop. This particular problem was reported by sashiko [1]. This should rarely happen, but we still need to guard against this.
Fix this infinite loop problem by also skipping NUMA node that has no housekeeping CPU in the inner while loop of init_vp_index(). Also update the early abort check to check for the absence of online housekeeping CPUs instead of just the emptiness of the cpumask. As the outer for loop will only be reached if the housekeeping cpumask has at least one online CPU, a NUMA node with housekeeping CPUs will eventually be found.
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Analysis
by VulDB Data Team • 11/03/2026
The Linux kernel hypervisors driver contains a critical logic flaw within the init_vp_index function that can lead to an infinite retry loop under specific hardware configuration scenarios. This vulnerability arises during the initialization of virtual processor indices, where the system attempts to allocate CPU resources from available nodes while adhering to housekeeping constraints. The core issue stems from how the kernel handles situations where no suitable CPUs remain in a given node after filtering out those already allocated or excluded by managed interrupt policies. When this occurs, the current implementation ignores the empty cpumask and retries the allocation process without adequately verifying whether any viable nodes exist that meet the housekeeping criteria. If all NUMA nodes lack online housekeeping CPUs capable of handling interrupts, the loop continues indefinitely because it fails to detect that no further valid candidates are available for selection.
This infinite loop represents a significant denial-of-service risk as it can cause system hangs or kernel panics by consuming CPU cycles in an unproductive cycle. The flaw is particularly insidious because it only manifests under specific conditions where housekeeping CPUs, which are reserved for managing interrupts and other critical background tasks, are absent from certain NUMA nodes. While the report indicates this scenario rarely occurs in typical deployments, its existence violates fundamental principles of robust system design by allowing a control flow path to loop without termination when resource constraints prevent progress. The absence of proper boundary checks allows the kernel to enter an unrecoverable state that requires manual intervention or reboot to resolve, impacting availability and reliability of virtualized environments running on affected Linux kernels.
From a technical classification perspective, this vulnerability aligns with CWE-835, which describes loops that do not terminate due to incorrect loop control logic. The failure lies in the inner while loop's inability to skip NUMA nodes that lack housekeeping CPUs, combined with an insufficient early abort check that relies solely on cpumask emptiness rather than verifying the presence of online housekeeping CPUs. By updating the logic to explicitly skip nodes without housekeeping capabilities and refining the termination condition to account for online status, the kernel ensures that it will eventually find a valid node or terminate gracefully if none exist. This fix prevents the infinite loop by ensuring forward progress in resource allocation attempts or proper failure handling when resources are genuinely unavailable.
Mitigation strategies involve applying the upstream Linux kernel patch that addresses this logic error immediately upon availability for affected distributions. System administrators should monitor for signs of CPU saturation without corresponding workload activity, which may indicate such a hang is occurring. Additionally, ensuring that NUMA configurations include at least one node with online housekeeping CPUs can prevent triggering the vulnerable code path in environments where hardware topology might otherwise expose this flaw. Regular kernel updates and adherence to best practices for hypervisor driver configuration are essential to maintaining system stability against these types of resource management errors. The fix exemplifies the importance of rigorous edge case testing in low-level system software, particularly when dealing with complex interactions between virtualization layers and physical hardware topologies.