CVE-2026-90097 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
Drivers: hv: vmbus: Skip VMBus module cleanup for non-nested root partition
The VMBus module initialization function, hv_acpi_init(), currently does nothing when running in the root partition and root is not nested in another VM. But the initialization function reports success, so the VMBus module is indeed loaded. VMBus functionality is not actually needed, but the VMBus module must be loaded so that hv_vmbus_exists() can answer correctly. Furthermore, the mshv_root dependency on the VMBus module is needed as described in the commit message for 840b740a35bf ("mshv: Add conditional VMBus dependency").
Loading the VMBus module without actually initializing it causes failures if the module should later be unloaded. The module unload code tries to clean up things that were never initialized, resulting in memory faults and a panic.
Fix this by having VMBus module exit function perform the same check for non-nested root partition, and do nothing in such a case, just like hv_acpi_init().
In the long run, the code that manages the Hyper-V provided SynIC should be refactored to better coordinate the requirements of root partition scenarios and normal VM scenarios, and to hopefully remove the hv_vmbus_exists() dependnecy between mshv_root and VMBus modules. Preventing the current unload failure scenario is an expediency until such a refactoring is done.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel contains a logic flaw within the Hyper-V Virtual Machine Bus (VMBus) subsystem that leads to system instability when the module is unloaded in specific virtualization configurations. The vulnerability arises from an asymmetry between the initialization and cleanup routines of the Vmbus driver, specifically affecting systems running as non-nested root partitions under Microsoft Hyper-V. In these environments, the primary initialization function hv_acpi_init correctly detects that it should perform no operations because the hardware features are not applicable to a non-nested root partition. However, despite performing no actual setup work, this function returns a success status code. This return value signals to the kernel module loader that the VMBus driver has been successfully initialized and is ready for operation. Consequently, the module remains loaded in memory, satisfying dependencies such as those required by the mshv_root subsystem, which relies on hv_vmbus_exists() to determine availability.
The critical failure occurs during the module unloading phase. Because the initialization routine reported success without performing any actual hardware or software setup, the corresponding exit function is not equipped with a matching guard condition. When an administrator attempts to unload the VMBus module using standard kernel utilities like rmmod, the cleanup code executes blindly. It proceeds to deallocate resources and tear down structures that were never created during initialization. This mismatch between state expectation and actual runtime reality results in invalid memory accesses. The attempt to free or access uninitialized data structures triggers a null pointer dereference or similar memory fault within the kernel space.
This technical flaw manifests as a catastrophic system failure, commonly known as a kernel panic. Since the error occurs deep within the core subsystem handling virtual machine communication channels, it compromises the stability of the entire operating environment rather than just an isolated application process. The severity is elevated because this issue can be triggered by routine administrative actions or automated scripts that manage module loading states based on reported success codes. It represents a classic case where API contract adherence fails to account for edge cases in state management, leading to undefined behavior when the system attempts to reverse operations that were never performed.
From a classification perspective, this vulnerability aligns with CWE-416, Use After Free, or more accurately CWE-825, Expired Pointer Dereference, as the cleanup routine accesses memory structures that do not exist in their expected state due to skipped initialization. In terms of attack vectors and defensive mapping, while this is primarily a stability issue rather than an exploit for privilege escalation, it falls under ATT&CK technique T1499, Endpoint Denial of Service, specifically within the context of resource exhaustion or system crash via local execution. An attacker with sufficient privileges to load and unload kernel modules could intentionally trigger this condition to cause a denial of service against the host machine.
The remediation strategy implemented in the fix involves synchronizing the exit function hv_vmbus_exit() with the initialization logic. The patch introduces a check within the cleanup routine that mirrors the conditions used during startup. If the system is identified as a non-nested root partition, the exit function now performs no operations and returns immediately, just as the init function did. This ensures that no invalid memory accesses occur during teardown. While this resolves the immediate crash scenario, it serves as an expedient fix rather than a complete architectural solution. The underlying dependency structure between mshv_root and VMBus remains suboptimal. Future development efforts are directed toward refactoring the SynIC management code to better distinguish between root partition requirements and standard virtual machine scenarios, with the ultimate goal of removing the rigid hv_vmbus_exists() dependency that necessitates this fragile loading behavior in the first place.