CVE-2026-99216 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
Drivers: hv: Don't reset VMBus connection state on error/exit paths
In two places, current code forces vmbus_connection.conn_state to DISCONNECTED prior to calling vmbus_disconnect(). But vmbus_disconnect() calls vmbus_initiate_unload(), which unloads the VMBus connection only if the conn_state is CONNECTED. Consequently, the connection remains and the Hyper-V host might continue to send messages or post VMBus interrupts.
The problem was introduced in commit 74347a99e73ae ("x86/Hyper-V: Unload vmbus channel in hv panic callback"), in that vmbus_initiate_unload() started checking the current connection state before doing the unload. This commit removed some occurrences where conn_state was forced to DISCONNECTED, but these two remained, apparently due to an oversight at the time.
Fix this by not forcing conn_state to DISCONNECTED during VMBus exit, or if a failure occurs when establishing the connection in the first place. Whatever value conn_state has at the time is then read by vmbus_initiate_unload() to determine if there is a connection to be unloaded.
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Analysis
by VulDB Data Team • 10/28/2026
The Linux kernel Hyper-V driver contains a logic error related to state management during VMBus disconnection and initialization failure scenarios. Specifically, in two distinct code paths, the implementation forces the vmbus_connection.conn_state variable to DISCONNECTED before invoking the vmbus_disconnect function. This action creates a race condition or logical inconsistency because the subsequent call to vmbus_initiate_unload relies on checking whether the connection state is currently CONNECTED to determine if an unload operation should proceed. By artificially setting the state to DISCONNECTed prior to this check, the driver effectively masks the actual status of the VMBus channel from the unloading routine. Consequently, vmbus_initiate_unload determines that no active connection exists and skips the necessary cleanup procedures required to properly terminate the communication link with the Hyper-V host.
This oversight allows the underlying VMBus connection to remain in an ambiguous or lingering state rather than being fully terminated. As a result, the Hyper-V hypervisor may continue to send messages or post interrupts associated with that channel even though the guest operating system has attempted to exit or encountered an error during initialization. This persistence of active channels can lead to resource leaks within the virtualized environment and potentially cause instability in subsequent interactions between the host and guest. The issue stems from a previous commit aimed at unloading vmbus channels during panic callbacks, which introduced stricter state checking but inadvertently left these two specific instances where the connection state was forcibly altered before the check occurred.
From a security perspective, this vulnerability aligns with CWE-401, which describes missing release of memory or other resources after allocation failure, and CWE-362, involving concurrent execution using shared resource without proper synchronization. The improper handling of the VMBus connection state can lead to Denial of Service conditions where system resources are not reclaimed correctly, potentially affecting the stability of the virtual machine. Furthermore, according to MITRE ATT&CK framework techniques related to Defense Evasion or Resource Hijacking, such lingering connections could theoretically be exploited if an attacker gains control over the guest environment to manipulate state variables and prevent proper teardown, although the primary impact here is operational instability rather than direct privilege escalation.
To mitigate this vulnerability, it is essential that drivers do not force the connection state to DISCONNECTED during VMBus exit sequences or when failures occur during initial establishment of the connection. The fix involves allowing vmbus_initiate_unload to read whatever value conn_state holds at the time of invocation, thereby ensuring that if a valid connection exists, it is properly unloaded and cleaned up. This approach ensures that all active channels are correctly terminated regardless of whether the exit path was triggered by an error or normal shutdown procedures. System administrators should apply kernel updates that include this patch to prevent potential resource leaks and maintain system stability in Hyper-V virtualized environments.